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Published online before print August 30, 2002, 10.1148/radiol.2251011627
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(Radiology 2002;225:215-224.)

Musculoskeletal Imaging

Imaging of Soft-Tissue Myxoma with Emphasis on CT and MR and Comparison of Radiologic and Pathologic Findings1

Mark D. Murphey, MD, Gina A. McRae, MD2, Julie C. Fanburg-Smith, MD, H. Thomas Temple, MD3, Alan M. Levine, MD and Albert J. Aboulafia, MD

1 From the Depts of Radiologic Pathology (M.D.M., G.A.M.) and Soft Tissue Pathology (J.F.S.), Armed Forces Inst of Pathology, 6825 16th St NW, Bldg 54, Rm M-127A, Washington, DC 20306; Depts of Radiology and Nuclear Med (M.D.M.) and Surgery (H.T.T.), Uniformed Services Univ of the Health Sciences, Bethesda, Md; Dept of Radiology, Univ of Maryland School of Med, Baltimore (M.D.M.); Dept of Surgery, Orthopedic Service, Walter Reed Army Med Ctr, Washington, DC (H.T.T.); and Sinai Hospital, Cancer Institute, Baltimore, Md (A.M.L., A.J.A.). From the 1997 RSNA scientific assembly. Received Oct 3, 2001; revision requested Dec 18; revision received Mar 27, 2002; accepted Apr 19. Address correspondence to M.D.M. (e-mail: murphey@afip.osd.mil).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 ADDENDUM
 REFERENCES
 
PURPOSE: To determine the imaging characteristics of soft-tissue myxoma, with emphasis on computed tomographic (CT) and magnetic resonance (MR) imaging findings and pathologic comparison.

MATERIALS AND METHODS: Records of 45 pathologically confirmed soft-tissue myxomas in 44 patients were retrospectively reviewed. Patient demographics and radiographs (n = 20), bone scintigrams (n = 2), angiograms (n = 3), and ultrasonographic (US) (n = 6), CT (n = 14), and MR images (n = 33) were evaluated by two musculoskeletal radiologists with agreement by consensus for lesion location, lesion size, and intrinsic characteristics.

RESULTS: Soft-tissue myxoma more commonly affected women (59%; average age 52 years) and manifested as a slowly enlarging (64%) painful (51%) mass. Lesions were most frequently intramuscular (82%) and involved the thigh (51%). An appearance similar to that of a cyst was seen at CT (at which the lesions demonstrated low attenuation) and at MR imaging (at which the lesions demonstrated markedly high signal intensity on T2-weighted images) in all cases because of the high water content of mucin that was seen histologically. The true solid architecture of these lesions was best depicted in all cases at US (at which the lesions were hypoechoic, not anechoic) and on MR images obtained with contrast material (at which the lesions demonstrated internal enhancement). A small amount of tissue similar to fat surrounding these intramuscular myxomas (71% at MR imaging) corresponded histologically (70%) to atrophy of surrounding muscle.

CONCLUSION: Soft-tissue myxoma often demonstrates characteristic US, CT, and MR imaging findings, including intramuscular location, intrinsic high water content, and a surrounding rim of fat.

Index terms: Myxoma, 40.369 • Soft tissues, CT, 40.1211 • Soft tissues, MR, 40.12141, 40, 12143 • Soft tissues, neoplasms, 40.839 • Soft tissues, US, 40.1298


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
 ADDENDUM
 REFERENCES
 
The term myxoma, introduced by Virchow in 1863, describes a tumor that histologically resembles the umbilical cord (1). The initial criteria for diagnosis of myxoma was established by Stout in 1948: Myxoma is a true mesenchymal neoplasm composed of undifferentiated stellate cells in a myxoid stroma (2).

Myxomas most commonly involve the heart, although other areas that are affected (in decreasing order of frequency) include subcutaneous tissue, aponeurotic tissue, bone, the genitourinary system, and the skin (3). Myxomas of the noncardiac soft tissues are described in the pathology literature, which includes reports of 34 cases by Enzinger, 65 cases by Meis and Enzinger, and 58 cases by Ireland et al (4–9). To the best of our knowledge, however, evaluation of the radiologic features of soft-tissue myxoma has been limited to a small number of cases (10–19).

The purpose of our study was to define the imaging characteristics of soft-tissue myxoma, with emphasis on computed tomographic (CT) and magnetic resonance (MR) imaging findings and pathologic comparison.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 ADDENDUM
 REFERENCES
 
Patients
We retrospectively reviewed records of 45 pathologically confirmed soft-tissue myxomas in 44 patients from the archives of our institutions. This study was performed with approval of the Armed Forces Institute of Pathology Human Subjects Committee. Informed consent was not required. Clinical evaluation of patients included recording of age, sex, race, symptoms at presentation (palpable mass, pain, and growth of the mass [slow or fast]), and presence or absence of an underlying diagnosis of fibrous dysplasia (whether mono-ostotic or polyostotic; if fibrous dysplasia was present, the relationship of the soft-tissue myxoma to adjacent affected bone was evaluated). None of the patients had a diagnosis of Carney complex, and patients with superficial dermal angiomyxomas were excluded (20). The radiologic examinations were performed at multiple institutions. All examination results were reviewed by two musculoskeletal radiologists (M.D.M., G.A.M.) with agreement by consensus. Images of the 45 lesions for review included radiographs (n = 20), bone scintigrams (n = 2), angiograms (n = 3), and ultrasonographic (US) (n = 6), CT (n = 14), and MR images (n = 33).

Image Evaluation
Radiographs were evaluated for the presence of a soft-tissue mass or fullness; if one was present, its radiopacity was characterized as being less than, equal to, or greater than that of muscle. Radiographs were also evaluated for the presence of calcification and bone involvement (ie, extrinsic erosion, invasion, or periosteal reaction). Bone scintigrams were evaluated for the degree of radionuclide uptake in the lesion. Radionuclide uptake less than that of the anterior iliac crest was considered mild, uptake similar to that of the anterior iliac crest was considered moderate, and uptake greater than that of the iliac crest was considered marked. Scintigrams were also evaluated to determine if tracer activity was homogeneous or heterogeneous. Angiograms were evaluated for the presence of neovascularity compared with normal muscle (ie, whether the mass was hypovascular or hypervascular) and evidence of displacement of adjacent native vessels.

US images were evaluated for the predominant type of echogenicity relative to that of skeletal muscle (ie, whether the mass was anechoic, hypoechoic, or hyperechoic), for the presence of through sound transmission, and for whether the mass appeared homogeneous or heterogeneous (heterogeneity was further categorized as mild, moderate, or marked). Additional features evaluated included the lesion margin, which was characterized as a defined capsule (ie, a capsule marked by a rind of defined peripheral echogenicity), as defined but having no capsule or having only a partial capsule, or as infiltrative; the presence of echogenic foci with shadowing, a finding that suggests calcification in the lesion; the presence of an echogenic rind, a finding that suggests the lesion is surrounded by fat; and evidence of decreased echogenicity of the surrounding muscle, a finding that suggests there is edema around the lesion. The presence of cystic (ie, anechoic) foci in the lesion was also evaluated. Doppler US images were evaluated for avascularity, hypovascularity, or hypervascularity in and around the lesion.

CT images obtained before the administration of intravenous contrast material were evaluated for the predominant attenuation (ie, less than that of muscle but greater than that of fat, equal to that of muscle, or greater than that of muscle) and homogeneity or heterogeneity (mild, moderate, or marked) of the lesion. The margin of each lesion evaluated with CT was characterized as a defined capsule (ie, a rim of soft tissue separate from surrounding muscle), as defined but having no capsule or having only a partial capsule, or as infiltrative. Additional factors evaluated included the presence of the following: calcification, a rim of low-attenuating fat, a "cap" of low-attenuating fat at the superior or inferior margin of the lesion, and surrounding edema (indicated by lower attenuation in surrounding muscle with effacement of muscle). CT images obtained after the administration of intravenous contrast material (n = 8) were evaluated for the degree (ie, none, mild, moderate, or marked) and type (ie, peripheral, peripheral and septal, septal, peripheral and nodular, nodular, or diffuse) of enhancement of the lesion.

MR images were evaluated for the predominant signal intensity of the lesion on both T1-weighted images (on which low signal intensity was characterized as being less than that of muscle, intermediate signal intensity was characterized as being similar to or slightly greater than that of muscle, and high signal intensity was characterized as being similar to that of fat) and T2-weighted images (on which low signal intensity was characterized as being similar to that of muscle, intermediate signal intensity was characterized as being similar to that of fat, high signal intensity was characterized as being greater than that of fat, and markedly high signal intensity was characterized as being much greater than that of fat). The degree of homogeneity of the lesion (ie, whether it was homogeneous or mildly, moderately, or markedly heterogeneous) was also determined on both T1- and T2-weighted images.

Additional factors evaluated included the presence of the following: a rim of tissue of increased signal intensity (similar to or approaching that of fat) on T1-weighted images, a cap of adipose tissue at the superior or inferior margin of the lesion, and high signal intensity on T2-weighted, gradient-echo (T2*), or short inversion time inversion-recovery images that represented edema around the lesion.

Lesion margin was categorized as a defined capsule (ie, a rim of low-signal-intensity tissue or tissue that showed enhancement on contrast material–enhanced images), as defined but having no capsule or having only a partial capsule, or as infiltrative.

MR images obtained after the intravenous administration of a gadolinium chelate with T1 weighting (n = 21) were evaluated for the degree (ie, none, mild, moderate, or marked) and type (ie, peripheral, peripheral and septal, septal, peripheral and nodular, nodular, or diffuse) of enhancement of noncystic areas. The peripheral and septal patterns of enhancement were further graded as thick (>2 mm) or thin (<=2 mm). Regions of thin (<=2 mm) peripheral and septal contrast enhancement were considered cystic foci at MR imaging provided that they also showed low signal intensity on T1-weighted images and markedly high signal intensity on T2-weighted images.

All US, CT, and MR images and available clinical reports were also evaluated for evidence of bone, joint, or neurovascular involvement. Lesion size was determined on the basis of images obtained with the imaging modality that best depicted the mass. The location of the lesion was categorized as subcutaneous, intramuscular, or intermuscular. If the site was thought to be juxtaarticular, this was noted. Lesion locations were confirmed by correlating imaging findings with available surgical reports.

Evaluation of Pathologic Reports
Reports of pathologic examination were reviewed (in all cases) to confirm the diagnosis of myxoma and the site (subcutaneous, intramuscular, intermuscular, and/or juxtaarticular) of the lesion. In addition, pathologic tissue, when available (n = 24), was examined a second time for the following information about each lesion: microscopic location (subcutaneous, intramuscular, intermuscular, and/or juxtaarticular); margin (ie, circumscribed or infiltrative); degree of cellularity (grade 1, <=100 nuclei per high-power field; grade 2, >100 nuclei per high-power field; grade 3, cells touching and crowded nuclei); degree of vascularity (grade 1, inconspicuous capillaries; grade 2, many capillaries; grade 3, dilated sinusoidal vessels); presence or absence of cyst formation; cyst size (small, <1 mm; medium, 1-2 mm; large, >2 mm); myxoid content (grade 1, inconspicuous; grade 2, moderate; grade 3, abundant); collagen content (grade 1, none or inconspicuous in stroma; grade 2, intermediate in stroma; grade 3, marked in stroma); presence (whether complete or incomplete) or absence of stroma; and presence or absence and extent (mild, moderate, or marked) of surrounding muscle atrophy and edema.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 ADDENDUM
 REFERENCES
 
Demographic and Clinical Information
The 44 patients evaluated in the study included 26 (59%) female and 18 (41%) male patients with an age range of 14–84 years (average age, 52 years). Of patients of known race (n = 39), 24 were white (62%), 11 were black (28%), three were Hispanic (8%), and one was Asian (3%). The most common clinical manifestation (clinical history was unknown for six lesions, leaving 39 for evaluation of symptoms) was a slowly enlarging (25 lesions, 64%), painful (20 lesions, 51%) soft-tissue mass. Masses were painless in 19 (49%) cases, no growth of the mass was seen in seven (18%) cases, and one (3%) mass was discovered incidentally. However, in six (15%) cases, rapid enlargement of the mass occurred. Patients had concurrent polyostotic fibrous dysplasia (Mazabraud syndrome) in three cases (7%). The myxoma in these cases was in soft tissue, and the adjacent bone was involved by fibrous dysplasia. Lesions ranged in size from 1.5 x 2 x 2 cm to 11 x 15 x 17 cm, with an average size of 4 x 5 x 7 cm. Location of the 45 lesions was intramuscular in 37 (82%), intermuscular in four (9%), and subcutaneous in four (9%). Three lesions (7%) were located in a juxtaarticular region. The single most common location was the thigh, in which 23 (51%) lesions were observed. Other sites included the upper arm (four cases, 9%); the calf or buttock (three cases each, 7%); the hand, forearm, chest wall, or paraspinal region (two cases each, 4%); and the groin, knee, toe, or retroperitoneum (one case each, 2%).

Radiographic, Scintigraphic, and Angiographic Findings
Radiographs (n = 20) revealed a soft-tissue fullness or mass in nine cases (45%) (Fig 1). All soft-tissue masses were of equal opacity to muscle. There was no evidence of calcification, bone involvement (by the myxoma), or periosteal reaction in any case. Delayed bone scintigrams (n = 2) showed mild homogeneous uptake of radionuclide (Fig 2) in one patient (50%); the other patient demonstrated no tracer activity. The blood pool images in the former patient revealed moderate homogeneous radionuclide uptake. Angiograms (n = 3) demonstrated two lesions to be avascular and one to be hypovascular (Fig 3). All lesions displaced adjacent vessels, which appeared to be "draped" over the mass (Fig 3).



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  		Figure 1. Intramuscular myxoma of the vastus medialis muscle in an 84-year-old woman with a rapidly enlarging distal thigh mass. Anteroposterior radiograph of the distal thigh shows a nonspecific soft-tissue mass (arrowheads).

 


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  		Figure 2. Intramuscular myxoma of the vastus intermedius muscle in a 69-year-old man with a painful slow-growing mass of the upper thigh. Anterior delayed bone scintigram of the upper thigh shows mild homogeneous radionuclide uptake (arrow).

 


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  		Figure 3. Intramuscular myxoma of the vastus medialis muscle in a 62-year-old man with a painless mass not changing in size. Anteroposterior arteriogram of the distal thigh shows no tumor staining but reveals displacement of vessels (arrowheads) around the soft-tissue mass.

 
US Findings
US images (n = 6) revealed all lesions to be hypoechoic relative to skeletal muscle, heterogeneous (two lesions were mildly heterogeneous and four were moderately heterogeneous), and subject to through sound transmission (Fig 4). All lesions had a defined margin. No capsule was seen in four lesions (67%), while a partial capsule was observed in two lesions (33%). No evidence of calcification in the lesions or fat or edema surrounding the lesions was seen. Anechoic cystic foci were present in five (83%) of the lesions (Fig 4). Doppler US images were available in three patients; two lesions (67%) were avascular, one (33%) was hypovascular, and all three were surrounded by vessels (Fig 4).



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  		Figure 4a. Intramuscular myxoma of the deltoid muscle in a 52-year-old man with a painless mass not changing in size. (a) Transverse gradient-echo (repetition time msec/echo time msec, 130/1.7; 60° flip angle) MR image obtained after intravenous administration of gadolinium chelate shows peripheral (thick wall) and nodular (arrowheads) enhancement posteriomedially and a small cystic focus (arrows) anterolaterally with a thin rim of enhancement. (b) Transverse Doppler US image shows a hypoechoic solid heterogeneous mass (arrows) with posterior acoustic enhancement and a small anechoic cyst component (*). Vessels (arrowheads) are seen prominently at the lesion margin; some are also seen within the lesion. (c) Photomicrograph reveals a cystic component (c), solid myxomatous component (m), pseudocapsule (arrow), and mild fat atrophy (*) in the surrounding muscle. (Hematoxylin-eosin stain; original magnification, x150.)

 


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  		Figure 4b. Intramuscular myxoma of the deltoid muscle in a 52-year-old man with a painless mass not changing in size. (a) Transverse gradient-echo (repetition time msec/echo time msec, 130/1.7; 60° flip angle) MR image obtained after intravenous administration of gadolinium chelate shows peripheral (thick wall) and nodular (arrowheads) enhancement posteriomedially and a small cystic focus (arrows) anterolaterally with a thin rim of enhancement. (b) Transverse Doppler US image shows a hypoechoic solid heterogeneous mass (arrows) with posterior acoustic enhancement and a small anechoic cyst component (*). Vessels (arrowheads) are seen prominently at the lesion margin; some are also seen within the lesion. (c) Photomicrograph reveals a cystic component (c), solid myxomatous component (m), pseudocapsule (arrow), and mild fat atrophy (*) in the surrounding muscle. (Hematoxylin-eosin stain; original magnification, x150.)

 


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  		Figure 4c. Intramuscular myxoma of the deltoid muscle in a 52-year-old man with a painless mass not changing in size. (a) Transverse gradient-echo (repetition time msec/echo time msec, 130/1.7; 60° flip angle) MR image obtained after intravenous administration of gadolinium chelate shows peripheral (thick wall) and nodular (arrowheads) enhancement posteriomedially and a small cystic focus (arrows) anterolaterally with a thin rim of enhancement. (b) Transverse Doppler US image shows a hypoechoic solid heterogeneous mass (arrows) with posterior acoustic enhancement and a small anechoic cyst component (*). Vessels (arrowheads) are seen prominently at the lesion margin; some are also seen within the lesion. (c) Photomicrograph reveals a cystic component (c), solid myxomatous component (m), pseudocapsule (arrow), and mild fat atrophy (*) in the surrounding muscle. (Hematoxylin-eosin stain; original magnification, x150.)

 
CT Findings
CT images (n = 14) revealed that all lesions (n = 10) evaluated prior to the administration of intravenous contrast material were of low attenuation (ie, less than the attenuation of muscle); six (60%) of these lesions demonstrated homogeneity (Figs 5, 6), while four (40%) demonstrated heterogeneity, which was mild in three lesions and moderate in one. Lesion margin was most frequently defined without a capsule; this was observed in 11 lesions (79%) at CT (Figs 5, 6). Two lesions (14%) were defined with a partial capsule, and one lesion (7%) was defined with a complete capsule. Areas of low-attenuating fat around the rim of intramuscular lesions (n = 12) could be seen in three (25%) cases, although these fatty areas were always incomplete and mild (Fig 5). A small amount of fat (fat cap) around the lesion was seen at the superior or inferior margin of five (42%) intramuscular lesions (Fig 5). No lesion demonstrated surrounding edema or internal calcification. Enhancement on CT images obtained after the administration of intravenous contrast material was evaluated in eight cases. There was mild enhancement in four cases (50%) and no enhancement in four cases (50%) (Fig 6). The pattern of enhancement was heterogeneously diffuse in two (50%) of the four cases and thick, peripheral, and septal with some nodularity in the other two (50%) cases (Fig 6).



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  		Figure 5a. Intramuscular myxoma of the vastus intermedius muscle in a 66-year-old man with a painless slow-growing mass. (a) Transverse CT image shows a homogeneous low-attenuating intramuscular mass (arrow) with a small rim of surrounding fat (arrowheads). (b) Transverse CT image obtained at the superior margin of the myxoma reveals a larger cap of fat (arrowhead). (c) Sectioned gross pathologic specimen shows that the myxoma has a surrounding pseudocapsule (arrow), a cap of fat (*) superiorly and inferiorly, and a small amount of fat (arrowhead) in the surrounding rim of muscle.

 


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  		Figure 5b. Intramuscular myxoma of the vastus intermedius muscle in a 66-year-old man with a painless slow-growing mass. (a) Transverse CT image shows a homogeneous low-attenuating intramuscular mass (arrow) with a small rim of surrounding fat (arrowheads). (b) Transverse CT image obtained at the superior margin of the myxoma reveals a larger cap of fat (arrowhead). (c) Sectioned gross pathologic specimen shows that the myxoma has a surrounding pseudocapsule (arrow), a cap of fat (*) superiorly and inferiorly, and a small amount of fat (arrowhead) in the surrounding rim of muscle.

 


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  		Figure 5c. Intramuscular myxoma of the vastus intermedius muscle in a 66-year-old man with a painless slow-growing mass. (a) Transverse CT image shows a homogeneous low-attenuating intramuscular mass (arrow) with a small rim of surrounding fat (arrowheads). (b) Transverse CT image obtained at the superior margin of the myxoma reveals a larger cap of fat (arrowhead). (c) Sectioned gross pathologic specimen shows that the myxoma has a surrounding pseudocapsule (arrow), a cap of fat (*) superiorly and inferiorly, and a small amount of fat (arrowhead) in the surrounding rim of muscle.

 


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  		Figure 6a. Intramuscular myxoma of the gluteal muscles in a 14-year-old boy. Transverse CT images obtained (a) before and (b) after intravenous administration of contrast material show a low-attenuating intramuscular mass (*) with faint peripheral and septal enhancement (arrowheads) and a small focus of nodularity (arrow). No pseudocapsule is seen.

 


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  		Figure 6b. Intramuscular myxoma of the gluteal muscles in a 14-year-old boy. Transverse CT images obtained (a) before and (b) after intravenous administration of contrast material show a low-attenuating intramuscular mass (*) with faint peripheral and septal enhancement (arrowheads) and a small focus of nodularity (arrow). No pseudocapsule is seen.

 
MR Imaging Findings
MR images (n = 33; T1-weighted, n = 32) revealed low signal intensity in 26 (81%) lesions and homogeneity in 19 (59%) lesions and heterogeneity in 13 (41%) lesions (12 lesions had mild heterogeneity and one lesion had moderate heterogeneity) (Figs 6, 7). Six (19%) lesions had intermediate signal intensity on T1-weighted MR images (Figs 8, 9). All lesions demonstrated markedly high signal intensity on T2-weighted, gradient-echo (T2*), or short inversion time inversion-recovery MR images (n = 32); 12 (38%) lesions showed homogeneity and 20 (62%) showed heterogeneity, which was mild in 19 lesions and moderate in one (Figs 7–9). This heterogeneity was caused by low-signal-intensity septae in seven cases (Figs 6, 7). A thin rim of higher signal intensity approaching that of fat was seen around the lesion on T1-weighted MR images in 20 (71%) of the 28 intramuscular lesions (Figs 6 –9). This rim was usually incomplete (15 cases, 75%). A more prominent cap of fat superior and/or inferior to the lesion was seen in 18 (64%) of the intramuscular lesions (Figs 6, 8, 9). Lesion margins were well defined with a capsule in seven (21%) of the 33 lesions evaluated with MR imaging, were best seen on images obtained after administration of a gadolinium chelate in six cases (Figs 6, 7), were defined with no capsule in eight (24%) cases, and were defined with a partial capsule in 18 (55%) cases (Figs 6, 7, 9). Edema around the lesion was seen in 26 (79%) of 33 lesions on T2-weighted MR images, gradient-echo (T2*) MR images, short inversion time inversion-recovery MR images, or images obtained after the administration of a gadolinium chelate (Fig 8). MR images obtained after administration of an intravenous gadolinium chelate (n = 21) revealed a mild (16 cases, 76%) to moderate (five cases, 24%) degree of enhancement (Figs 6, 7). Cystic components (ie, thin peripheral and septal areas of enhancement) were seen on contrast-enhanced MR images in 11 (52%) cases (Figs 6, 7). The pattern of enhancement of the noncystic regions was always heterogeneous and was most commonly diffuse (12 cases, 57%) (Figs 6, 7). Nodular and thick peripheral and septal patterns of enhancement were seen in nine (43%) cases. Advanced imaging (ie, US, CT, or MR imaging) did not reveal any cases of bone, joint, or neurovascular involvement.



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  		Figure 7a. Intramuscular myxoma of the gluteal muscles in a 40-year-old man with a painless slowly enlarging mass. (a, b) Transverse T1-weighted (416/17) MR images obtained (a) before and (b) after intravenous administration of gadolinium chelate reveal a low-signal-intensity intramuscular mass (* in a) with an incomplete rim of fat laterally (arrowheads in a). The contrast-enhanced image shows two distinct areas, with moderate diffuse enhancement of the myxoma medially (black *) and thin peripheral enhancement of the cyst laterally (white *). The peripheral enhancement around the entire lesion represents the pseudocapsule (arrows). (c) Transverse T2-weighted (3,850/135) MR image reveals diffuse high signal intensity in both components separated by a septum of low signal intensity (arrow). (d) Sectioned gross pathologic specimen shows the myxoma (*), the cystic component (arrowheads), and the pseudocapsule (arrows); this appearance corresponds to the imaging findings.

 


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  		Figure 7b. Intramuscular myxoma of the gluteal muscles in a 40-year-old man with a painless slowly enlarging mass. (a, b) Transverse T1-weighted (416/17) MR images obtained (a) before and (b) after intravenous administration of gadolinium chelate reveal a low-signal-intensity intramuscular mass (* in a) with an incomplete rim of fat laterally (arrowheads in a). The contrast-enhanced image shows two distinct areas, with moderate diffuse enhancement of the myxoma medially (black *) and thin peripheral enhancement of the cyst laterally (white *). The peripheral enhancement around the entire lesion represents the pseudocapsule (arrows). (c) Transverse T2-weighted (3,850/135) MR image reveals diffuse high signal intensity in both components separated by a septum of low signal intensity (arrow). (d) Sectioned gross pathologic specimen shows the myxoma (*), the cystic component (arrowheads), and the pseudocapsule (arrows); this appearance corresponds to the imaging findings.

 


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  		Figure 7c. Intramuscular myxoma of the gluteal muscles in a 40-year-old man with a painless slowly enlarging mass. (a, b) Transverse T1-weighted (416/17) MR images obtained (a) before and (b) after intravenous administration of gadolinium chelate reveal a low-signal-intensity intramuscular mass (* in a) with an incomplete rim of fat laterally (arrowheads in a). The contrast-enhanced image shows two distinct areas, with moderate diffuse enhancement of the myxoma medially (black *) and thin peripheral enhancement of the cyst laterally (white *). The peripheral enhancement around the entire lesion represents the pseudocapsule (arrows). (c) Transverse T2-weighted (3,850/135) MR image reveals diffuse high signal intensity in both components separated by a septum of low signal intensity (arrow). (d) Sectioned gross pathologic specimen shows the myxoma (*), the cystic component (arrowheads), and the pseudocapsule (arrows); this appearance corresponds to the imaging findings.

 


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  		Figure 7d. Intramuscular myxoma of the gluteal muscles in a 40-year-old man with a painless slowly enlarging mass. (a, b) Transverse T1-weighted (416/17) MR images obtained (a) before and (b) after intravenous administration of gadolinium chelate reveal a low-signal-intensity intramuscular mass (* in a) with an incomplete rim of fat laterally (arrowheads in a). The contrast-enhanced image shows two distinct areas, with moderate diffuse enhancement of the myxoma medially (black *) and thin peripheral enhancement of the cyst laterally (white *). The peripheral enhancement around the entire lesion represents the pseudocapsule (arrows). (c) Transverse T2-weighted (3,850/135) MR image reveals diffuse high signal intensity in both components separated by a septum of low signal intensity (arrow). (d) Sectioned gross pathologic specimen shows the myxoma (*), the cystic component (arrowheads), and the pseudocapsule (arrows); this appearance corresponds to the imaging findings.

 


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  		Figure 8. Intramuscular myxoma of the peroneus longus muscle in a 33-year-old woman with a painful slow-growing mass. Sagittal T1-weighted (500/14) MR image shows a homogeneous low-signal-intensity soft-tissue mass (*) with focal areas of tissue around the lesion rim (arrows) similar in appearance to fat. Caps of more prominent fat (arrowheads) are also seen at the superior and inferior margins.

 


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  		Figure 9a. Intramuscular myxoma of the flexor carpi radialis muscle in a 34-year-old woman with a painless slowly enlarging forearm mass. (a) Sagittal T1-weighted (500/20) MR image shows a mass (*) of intermediate signal intensity (similar to that of muscle) with a rim (straight arrows) of tissue of higher signal intensity (approaching that of fat) and thicker caps of this tissue at the superior and inferior margins (curved arrows). (b) Sagittal gradient-echo (620/25, 25° flip angle) T2*-weighted MR image reveals homogeneous high signal intensity in the myxoma (*) and surrounding edema (arrowheads). (c) Photomicrograph shows the myxoma (*), pseudocapsule (solid arrows), and surrounding edema and muscle atrophy (area between open arrows). (Hematoxylin-eosin stain; original magnification, x150.)

 


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  		Figure 9b. Intramuscular myxoma of the flexor carpi radialis muscle in a 34-year-old woman with a painless slowly enlarging forearm mass. (a) Sagittal T1-weighted (500/20) MR image shows a mass (*) of intermediate signal intensity (similar to that of muscle) with a rim (straight arrows) of tissue of higher signal intensity (approaching that of fat) and thicker caps of this tissue at the superior and inferior margins (curved arrows). (b) Sagittal gradient-echo (620/25, 25° flip angle) T2*-weighted MR image reveals homogeneous high signal intensity in the myxoma (*) and surrounding edema (arrowheads). (c) Photomicrograph shows the myxoma (*), pseudocapsule (solid arrows), and surrounding edema and muscle atrophy (area between open arrows). (Hematoxylin-eosin stain; original magnification, x150.)

 


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  		Figure 9c. Intramuscular myxoma of the flexor carpi radialis muscle in a 34-year-old woman with a painless slowly enlarging forearm mass. (a) Sagittal T1-weighted (500/20) MR image shows a mass (*) of intermediate signal intensity (similar to that of muscle) with a rim (straight arrows) of tissue of higher signal intensity (approaching that of fat) and thicker caps of this tissue at the superior and inferior margins (curved arrows). (b) Sagittal gradient-echo (620/25, 25° flip angle) T2*-weighted MR image reveals homogeneous high signal intensity in the myxoma (*) and surrounding edema (arrowheads). (c) Photomicrograph shows the myxoma (*), pseudocapsule (solid arrows), and surrounding edema and muscle atrophy (area between open arrows). (Hematoxylin-eosin stain; original magnification, x150.)

 
Pathologic Findings
The diagnosis of myxoma was confirmed at pathologic examination in all cases. Pathologic tissue was available for reevaluation in 24 cases. The location of the lesion was intramuscular in 21 (88%) cases and juxtaarticular in three (12%). Lesion location at pathologic examination was identical to that observed at imaging for all lesions except one, which was believed to be intermuscular at MR imaging but intramuscular at pathologic examination. The surgical report in this case indicated that an intermuscular location was confirmed at surgery; the discrepancy likely arose because the lesion abutted muscle at the margin. Lesion margin was circumscribed in 13 cases (59%) and infiltrative in nine cases (41%) (two cases could not be evaluated for this feature). Table 1 shows the results of histologic evaluation in the 24 lesions that were reevaluated. Gadolinium enhancement was graded as mild to moderate and vascularity was graded as 1 or 2 in all 10 lesions for which contrast-enhanced MR images could be compared with reevaluated pathologic reports.


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  		TABLE 1. Pathologic Findings in Soft-Tissue Myxomas

 
Cyst formation was observed in 15 (62%) cases. Cyst size was most commonly small (10 cases, 67%) and less frequently moderate (three cases, 20%) or large (two cases, 13%) (Figs 4, 6). Pathologic samples for reevaluation and images adequate for evaluating for the presence of cysts (ie, contrast-enhanced MR images or US images) were available for 12 lesions. In nine (75%) of these lesions, there was complete agreement between imaging and pathologic findings: Six lesions with cysts were identified (two were identified at US and five were identified at MR imaging; one lesion was imaged with both modalities) (Figs 4, 7), and three lesions were identified as not having cysts at MR. In two lesions, no cysts were seen during pathologic reevaluation, but cysts were detected at US in one case and at contrast-enhanced MR imaging in the other. In one case, cysts were seen at pathologic reevaluation but not at US.

A pseudocapsule was identified in 18 (78%) lesions and was complete in 11 (61%) and incomplete in seven (39%) (Figs 4–9). No pseudocapsule was seen in five (22%) cases, and one case could not be evaluated for this feature.

Atrophy of surrounding muscle was seen in 14 (70%) of the 20 cases in which this feature could be evaluated histologically (Figs 4, 5, 7–9). The extent of muscle atrophy was mild in three cases (21%), moderate in four cases (29%), and marked in seven cases (50%). All nine patients with histologic evidence of muscle atrophy also had similar MR imaging features (100% correlation) (Figs 4, 5, 7–9). However, of six patients without muscle atrophy at histologic examination, four had MR imaging features of muscle atrophy.

Surrounding muscle edema was seen in 16 (84%) of the 19 cases that could be evaluated for this feature (Fig 9). The extent of muscle edema was mild in four cases (25%), moderate in seven (44%), and marked in five (31%). Thirteen patients were also found to have muscle edema at histologic examination and on available MR images. Of these patients, 12 showed MR imaging evidence of muscle edema. Three patients with MR imaging features of muscle edema had no evidence of this finding at pathologic examination. Table 2 shows the comparison of imaging and pathologic findings in soft-tissue myxoma.


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  		TABLE 2. Imaging and Pathologic Findings in Soft-Tissue Myxoma

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADDENDUM
 REFERENCES
 
Myxoma of the soft tissue is a benign neoplasm that may arise from fibroblasts that produce an excessive amount of mucopolysaccharide. The association of myxomas with fibrous dysplasia has led to the proposal that there is an underlying localized error in connective tissue metabolism (6,21–32). The pathologic appearance of soft-tissue myxomas has been well described by Enzinger and Weiss, Enzinger, and Ireland et al (3,4,6). However, to our knowledge, the radiologic appearance and pathologic correlation for these manifestations have been described in only a small number of patients (10–19).

Patient demographics and lesion location in our series are similar those observed in previous studies. However, the higher incidence in blacks (28% of our patients) compared with the general population observed in our study has not been reported previously, to the best of our knowledge. Women were more frequently affected, by a 1.4:1 ratio, and the average patient age was 52 years. The most common clinical history was that of a slowly enlarging soft-tissue mass. The vast majority of cases arose in the intramuscular compartment (82%) of the thigh (51%), upper arm (9%), calf (7%), or buttock (7%). Only a small number of cases were intermuscular (9%), subcutaneous (9%), or juxtaarticular (7%). We believe previous descriptions of juxtaarticular myxomas overestimate their occurrence and reflect their occasional histologic similarity to ganglia, meniscal cysts, and perilabral cysts. We believe this occurs particularly when a small sample size is available for pathologic evaluation, which is a common situation (5,12).

The intrinsic characteristics of soft-tissue myxoma were similar to those of a cyst in all of our cases at CT and MR imaging. This is a direct reflection of the high mucin and low collagen content in the lesion, representing a lesion composed of a large amount of water, as seen histologically (4–9), which accounts for its hypoechoic appearance at US, low attenuation at CT, low signal intensity on T1-weighted MR images, and markedly high signal intensity on T2-weighted MR images. Contrast-enhanced imaging (particularly MR imaging) more accurately reflects the truly solid (although usually hypocellular) consistency of the myxoma because it shows internal enhancement. Cystic components were frequently identified at US (83%), at MR imaging after administration of a gadolinium chelate (52%), and at pathologic examination (62%).

Complete agreement between imaging and pathologic examination in cyst identification was seen in 75% of cases. Discrepant cases were likely due to small tissue sample size (in which case cysts would be seen at imaging but not at histologic examination) or minute cyst size (in which case cysts would be seen at histologic examination but not at imaging). The degree of contrast enhancement of the noncystic areas was always mild to moderate and heterogeneous. The pattern of enhancement of the noncystic areas was diffuse (57%) or of a nodular or thick peripheral and septal (43%) type, allowing them to be differentiated from the cystic regions, which showed a thin peripheral septal pattern of enhancement. This distinguishing pattern of enhancement is best seen at MR imaging rather than at CT; this reflects the superior contrast resolution of MR imaging. As it is in other organ systems, US is also useful in the distinction of a cyst from a solid lesion such as myxoma. Specifically, it is important to detect the small areas of internal echoes seen in all our cases (n = 6) in soft-tissue myxoma at US. This feature is particularly useful in distinguishing myxomas from synovial cysts or ganglia, although a more complex appearance is occasionally seen in these lesions, obscuring this point of differentiation (33–39).

The detection of a small rim of tissue similar to or approaching the appearance of fat around intramuscular myxomas is an important feature that has not been emphasized in the radiology literature. This feature is best seen at MR imaging (71% of lesions)—again a reflection of the superior contrast resolution of this modality. The cause of this imaging finding has been described in the pathology literature and was confirmed in our histologic evaluation. Myxomas often lack a complete pseudocapsule, as was seen in 52% of our cases at pathologic examination and 79% of our cases at MR imaging; the disparity was likely caused by incomplete pathologic samples for review. This lack of a complete pseudocapsule may allow mucoid material from the lesion to extend into the immediately adjacent muscle. We believe this material both leads to muscle atrophy with increased fat deposition and causes the surrounding edema (as seen on water-sensitive MR imaging sequences in 79% of cases), either as a result of mucin from the lesion or tissue irritation.

Imaging and histologic findings of the presence of a rim of fat and/or surrounding edema had a close correlation (Table 2), with discrepant cases (changes seen at MR imaging but not at pathologic examination) probably resulting from the existence of incomplete tissue for reevaluation. Both of these features are unusual in other intramuscular soft-tissue neoplasms, and, in combination with intrinsic imaging characteristics of high water content, strongly suggest the diagnosis of myxoma. The rim of higher signal intensity is probably not identical to fat because of the small amount of adipose tissue and volume averaging with intervening muscle.

At the poles of the myxoma, there were frequently (64%) areas of more prominent adipose tissue that were isointense to fat during all MR pulse sequences. The cause of this cap of fat is uncertain. The presence of a rim of fat around soft-tissue masses (ie, the split fat sign) typically suggests an intermuscular mass and, to our knowledge, has not been described in other common intramuscular soft-tissue masses such as lipoma, well-differentiated liposarcoma, malignant fibrous histiocytoma (MFH), or hemangioma (40–42).

The differential diagnosis of a lesion with imaging findings of a soft-tissue mass and high water content that mimics a cyst includes the following: synovial cyst, bursa, ganglion, neurogenic neoplasms, myxoid liposarcomas, and myxoid MFH. The vast majority of synovial cysts, bursae, and ganglia occur in well-recognized locations (ie, popliteal region, dorsum of the wrist), and are intermuscular masses, whereas most soft-tissue myxomas are intramuscular (33–39). In addition, in contradistinction to myxoma, these lesions represent truly cystic masses that can be recognized by the enhancement of a thin rim and delicate septa on CT or MR images obtained after administration of intravenous contrast material or by their anechoic appearance at US.

Neurogenic neoplasms, both benign (eg, neurofibroma, neurilemoma) and malignant (eg, malignant peripheral nerve sheath tumor) are also typically intermuscular lesions (43). An entering and exiting nerve is often seen at imaging, and the "target sign" may be apparent on T2-weighted MR images (43). These features are not seen with soft-tissue myxoma. Unlike soft-tissue myxoma, myxoid liposarcoma is also usually an intermuscular lesion and often contains a small amount of intrinsic fat (observed in 42%–92% of cases at MR imaging) (40,44–46). Myxoid MFH, like soft-tissue myxoma, is typically an intramuscular lesion. However, unlike soft-tissue myxoma, MFH usually has a far more heterogeneous appearance at imaging, with areas of hemorrhage and solid nodular regions that may reveal prominent contrast enhancement (41).

We acknowledge the limitations of our study. A relatively small number of patients underwent US, CT, and MR imaging, although, to our knowledge, this remains the largest reported group of patients with soft-tissue myxoma evaluated radiologically. The imaging techniques and parameters were not standardized because patients were referred from multiple centers. Pathologic material was often limited and could not be mapped so that it could be directly compared with images. However, imaging studies that were available and considered adequate for evaluation included US, CT, and MR. Finally, no follow-up imaging or clinical data were available. Despite these limitations, we believe our findings add information to the understanding of soft-tissue myxoma.

In summary, soft-tissue myxoma often displays characteristic US, CT, and MR imaging findings, including an intramuscular location, a surrounding rim of tissue similar to fat, and high water content (resulting in low attenuation at CT and low signal intensity on T1-weighted MR images and markedly high signal intensity on T2-weighted MR images). These features correspond to the histologic appearance of a highly mucinous, nonencapsulated lesion that infiltrates muscle and causes atrophy. US and contrast-enhanced imaging (particularly MR) allow detection of cystic components and distinction of myxomas from truly cystic lesions (synovial cysts, bursae, or ganglia) by demonstrating the characteristics of low-level internal echoes and mild enhancement. We believe these imaging features should suggest the diagnosis of soft-tissue myxoma and allow distinction of myxoma from more ominous neoplasms such as myxoid liposarcoma and myxoid MFH.


   ADDENDUM
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 ADDENDUM
REFERENCES
 
Since the acceptance of the manuscript of this article, two additional articles dealing with soft-tissue myxoma have been published (47,48).


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the support of Christopher R. Buchanan and Linda C. Wilkins for manuscript preparation and Janeth Amarillo for photographic preparation. In addition, we thank the residents—without whom this project would not have been possible—who attend the Armed Forces Institute of Pathology radiologic-pathology courses (past, present, and future) for their contribution to our series of patients.


   FOOTNOTES
 
2 Current address: Georgia Radiologists, Macon, Ga. Back

3 Current address: Department of Orthopedic Surgery, University of Miami School of Medicine, Fla. Back

The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Army, Navy, Air Force, or the Department of Defense.

Abbreviation: MFH = malignant fibrous histiocytoma

Author contributions: Guarantors of integrity of entire study, M.D.M., G.A.M., J.C.F.S.; study concepts and design, M.D.M.; literature research, M.D.M., G.A.M., J.C.F.S.; clinical studies, M.D.M., G.A.M., J.C.F.S.; data acquisition, all authors; data analysis/interpretation, M.D.M., G.A.M., J.C.F.S., A.M.L., A.J.A.; statistical analysis, M.D.M., G.A.M., J.C.F.S., A.M.L., A.J.A.; manuscript preparation, M.D.M., J.C.F.S., A.M.L., A.J.A.; manuscript definition of intellectual content, M.D.M., J.C.F.S., H.T.T.; manuscript editing, all authors; manuscript revision/review and final version approval, M.D.M.


   REFERENCES
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DISCUSSION
 ADDENDUM
 REFERENCES
 

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