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(Radiology. 1999;213:61-66.)
© RSNA, 1999

Radiation Oncology

Cosmetic Outcome in Patients Receiving an Interstitial Implant as Part of Breast-Conservation Therapy1

Bradley A. Kramer, MD, Douglas W. Arthur, MD, Kenneth Ulin, PhD, Rupert K. A. Schmidt-Ullrich, MD, Robert D. Zwicker, PhD and David E. Wazer, MD

1 From the Department of Radiation Oncology, New England Medical Center, Tufts University School of Medicine, Boston, Mass (B.A.K., K.U., D.E.W.), and the Department of Radiation Oncology, Massey Cancer Center, Medical College of Virginia of Virginia Commonwealth University, Richmond (D.W.A., R.K.A.S.U., R.D.Z.). From the 1998 RSNA scientific assembly. Received August 26, 1998; revision requested October 22; revision received January 14, 1999; accepted February 23. Address reprint requests to B.A.K., Department of Radiation Oncology, Midwestern Regional Medical Center, 2520 Elisha Ave, Zion, IL 60099.


   Abstract
TOP
 Abstract
Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
PURPOSE: To study factors related to breast cosmetic outcome in patients treated with an interstitial implant as part of breast-conservation therapy.

MATERIALS AND METHODS: One hundred fifty-six patients with stage I or II breast carcinoma who received 50 Gy of external-beam irradiation followed by a 20-Gy interstitial boost were examined. The dose homogeneity index (DHI) was calculated for each evaluable implant and was examined in light of other patient-, treatment-, and tumor-related variables previously demonstrated to affect cosmesis.

RESULTS: Of the variables examined, both the DHI (P = .021) and the total excision volume (P = .019) were significantly related to cosmetic outcome (excellent vs less than excellent) in a univariate model. In the multivariate analysis, only the total excision volume remained significant (P = .032). The mean total excision volume ± SD in patients with excellent cosmetic outcome (81.8 cm3 ± 84.0) was significantly less than that in patients with less than excellent cosmetic outcome (120 cm3 ± 84). The probability of excellent cosmetic outcome linearly increased with an increase in DHI. The mean DHI was 0.74 ± 0.12 for the cases with excellent cosmetic outcome and 0.68 ± 0.10 for those with less than excellent cosmetic outcome.

CONCLUSION: To achieve optimal cosmesis, DHI should be maximized. The volume of tissue removed, however, remains the most significant determinant.

Index terms: Breast neoplasms, therapeutic radiology, 00.1299, 00.32 • Dosimetry, 00.1299 • Iridium, radioactive, 00.1299 • Therapeutic radiology, complications, 00.4532, 00.4533, 00.458, 00.47 • Treatment planning, 00.1299


   Introduction
TOP
 Abstract
 Introduction
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Breast-conservation therapy is the preferred method of treatment for early-stage carcinoma of the breast, with survival and local control rates comparable to those for more radical surgery (1–4). Interstitial implantation has been widely employed to deliver the boost to the resection bed after standard external-beam irradiation of the entire breast as part of breast-conservation therapy (5–11). In addition, the use of interstitial implantation alone without whole-breast irradiation is being studied by the Radiation Therapy Oncology Group, or RTOG, in an ongoing prospective trial (12).

One of the main advantages of breast-conservation therapy is improved psychologic well-being with less disfiguring treatment (13). Hence, optimal cosmetic outcome is critical in this regard. It is incumbent on the oncology team, particularly the radiation oncologist, to apply careful technique to maximize the cosmetic outcome results in breast-conservation therapy.

Interstitial implantation was the preferred method to deliver a boost to the tumor bed at our institutions in those cases requiring high total doses (70 Gy). We (14) recently published a comparison of the results in a cohort treated with interstitial implantation and the results in a cohort that received boosts with external electron-beam irradiation. Cosmetic outcome in the cases with implants was superior to that in cases with electron-beam boosts with the same nominal dose prescribed. In patients with implants, excellent cosmetic outcome was dependent on the technical quality of the source position and the volume of breast tissue removed.

There are numerous dosimetric systems in use for performing interstitial implantation (15–20). The implant configuration, the distribution of source activity, and the choice of dose rate vary from system to system. These variables, along with the quality of the final actual implant, determine the homogeneity of the dose distribution within as well as outside the target volume. We hypothesize that a homogeneous dose distribution is important in breast irradiation to minimize long-term fibrosis and thereby achieve optimal cosmesis. Maximizing the dose homogeneity has been shown to be important in external-beam technique (ie, use of compensators, wedges, etc) with regard to cosmetic outcome (21). The lack of appropriate, uniformly accepted dosimetric criteria to evaluate the quality of an implant has made comparison and evaluation of clinical outcome difficult.

Previously described quantitative measures of the quality of an implant include the dose homogeneity index (22), the uniformity index (23), the ratio of two volumes (24), and the width of the enhancement in a modified volume-versus-dose curve (25). The dose homogeneity index has been previously defined as a method for evaluating the dosimetric quality of an implant (26–29). The International Commission on Radiation Units and Measurements, or ICRU, has established the standard definition for the use of the dose homogeneity index as a ratio of the peripheral dose to the mean central dose (12). In brief, the mean central dose is the mean of all the local dose minima, or geometric center doses, in the central plane of the implant. The peripheral dose is the minimum dose at the periphery of the clinical target volume. For the purpose of this study, the peripheral dose was equivalent to the prescription dose.

In a previous publication, Wazer et al (14) found the dose homogeneity index to be the most significant variable with regard to cosmetic outcome in a univariate analysis. To our knowledge, prior to that publication, the utility of the dose homogeneity index had not been established in relation to cosmesis. Hence, our objective in this retrospective study was to further examine the relationship between patient-, treatment-, and tumor-related variables and cosmetic outcome by using additional data from patients treated similarly at another institution. In particular, we sought to further explore the dose homogeneity index to more aptly define its use in the evaluation and planning of implantation and its predictive value for cosmetic outcome.


   MATERIALS AND METHODS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Between 1982 and 1994, 156 patients with stage I or II breast carcinoma treated in one of two institutions and deemed to have a high risk of recurrence (<=2-mm final surgical margin) received an interstitial implant as a part of breast-conservation therapy (30). All patients underwent initial excision with the goal of tumor removal with a 0.5–1.0-cm gross margin. The details of histopathologic assessment have been previously described (30). The excision volume was determined by multiplying the measured lengths in three dimensions of the removed breast tissue. The total excision volume was the sum of the excised volumes. Repeat excision was performed in 102 (65%) of 156 patients. This series included only those patients with final margins of 2 mm or less. This group of patients was thought to be at higher risk of local recurrence and received a boost to the tumor bed of 20 Gy. This was preferentially done by using brachytherapy.

All patients received 50 Gy of external-beam irradiation to the entire breast through parallel opposed tangential portals, with wedges used to limit the inhomogeneity to 7% or less. The daily fraction was not greater than 2 Gy and was almost exclusively 1.8 Gy. An interstitial iridium 192 implant boost was performed after completion of external-beam irradiation.

Nodal irradiation also was performed with careful technique to minimize overlap in 90 (58%) of the 156 patients. Axillary nodal dissection, generally level I or II, was performed in 139 (89%) of the 156 patients. Adjuvant tamoxifen citrate or chemotherapeutic agents were administered in 126 (81%) or 98 (63%) of the 156 patients, respectively. It was typical that patients received one to two cycles of chemotherapy before radiation and completed the course after radiation therapy was completed.

We have previously described our implantation procedure (14). The target volume was defined by means of a clinical assessment of the biopsy cavity, with a 2-cm margin. Catheters were placed in the breast in a tangential direction. The implant volume was calculated by using the size of the excisional biopsy specimen, the location of the tumor within the removed tissue, the orientation of positive margins within the specimen, and clinical evaluation of the scar location, tissue induration, and postexcisional mammograms.

All implants were constructed in accordance with a preplanning algorithm designed to maximize homogeneity within the prescription isodose of 0.5 Gy/h for 40 hours (20-Gy implant dose), as previously described (28,29). In brief, in all cases a preplanning algorithm was used that assumes a strand spacing of 1 cm but allows the interplanar spacing to vary to accommodate target thickness. The planar spacing in each case is chosen to equate the maximum target and treatment volume dimensions in the direction perpendicular to the plane of the implant. By using the interplanar spacing as a variable, this system guaranteed precise coverage of the target volume by the reference isodose along the three implant symmetry axes. Matching the target and treatment boundaries in the three dimensions may be expected to minimize the dose unnecessarily delivered to normal tissues as a result of mismatched boundaries along some edges. This system provided dose-rate tables that include the required interplanar separation to achieve optimal target coverage according to the geometric constraints imposed. The skin dose was monitored with thermoluminescent dosimeters and was maintained below 12 Gy.

For each patient, a preplan was generated with optimal source positions and seed strengths. After insertion, the implant catheters were loaded with dummy sources, and orthogonal radiographs were obtained to determine the actual dosimetry.

Cosmetic outcome was scored as a single event at the last follow-up examination for each patient, with an assessment performed by two independent examiners (including D.E.W., R.K.A.S.U.); the lowest score was retained. The four-tiered cosmetic outcome scale used has been published previously (31). Cosmetic scoring was as follows: "excellent" indicated perfect symmetry, with no visible distortion or skin changes; "good" indicated slight skin distortion, retraction or edema, any mild telangectasia, mild hyperpigmentation, or an absent nipple-areolar complex; "fair" indicated moderate distortion of the nipple or breast symmetry, moderate hyperpigmentation, prominent skin retraction, edema, or telangectasia; and "poor" indicated marked distortion, edema or fibrosis, or severe hyperpigmentation.

The dose homogeneity index was calculated retrospectively for each evaluable implant case. The relationship between cosmetic outcome and dose homogeneity index was examined. Other variables reported to affect cosmetic outcome were also examined (Table 1).


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  		TABLE 1. Univariate Logistic Regression Analysis Models for Excellent Cosmesis
 
We evaluated the effects of individual factors promoting excellent cosmetic outcome scores by using logistic regression, the odds ratio, a 95% confidence interval, and a P value from the Wald {chi}2 test. The logistic regression model was used to perform univariate and multivariate analyses to test the relationship between the dose homogeneity index; patient-, tumor-, and treatment-related variables; and cosmesis.


   RESULTS
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The clinical and therapy-related features of our cohort of patients are listed in Table 2. The mean implant dose ± SD was 19.95 Gy ± 1.38. The mean implant volume was 48.3 cm3 ± 20.4. The median follow-up was 76 months.


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  		TABLE 2. Clinical and Therapy-related Features in the 156 Patients
 
Cosmetic outcome was scored as excellent in 75 (48%) of 156 patients, as good in 61 (39%), as fair in 16 (10%), and as poor in four (3%).

The univariate logistic regression in which 17 variables were examined revealed that only the dose homogeneity index (P = .021) and total excision volume (P = .019) were significantly associated with the probability of excellent cosmetic outcome (Table 1). The mean total excision volume in those patients with excellent cosmetic outcome (81.8 cm3 ± 84.0; range, 7.6–400.0 cm3) was significantly less than that in patients with less than excellent cosmetic outcome (120 cm3 ± 84; range, 6–375 cm3) P = .019).

In some cases, we were unable to find detailed pathologic and dosimetric information to calculate the total excision volume and dose homogeneity index. As such, 121 patients had a calculated total excision volume and 91 patients had a dose homogeneity index for analysis. A multivariate logistic regression analysis was performed to determine if these two variables (total excision volume and dose homogeneity index) were independently associated with excellent cosmesis. There were only 68 patients with both total excision volume and dose homogeneity index data available.

The total excision volume retained significance in the multivariate model (Table 3). The dose homogeneity index did not retain statistical significance, although the odds ratio, 1.37, was high and was close to the odds ratio in the univariate model, 1.63, which suggests a lack of power due to an insufficient number of patients.


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  		TABLE 3. Logistic Regression Models for Excellent Cosmesis
 
We also compared the other variables in the 68 patients who had both the dose homogeneity index and the total excision volume with the other variables in the patients without data on either variable to determine if the sample was biased. There were no significant differences between the two groups, with the exception of median age (49.5 years vs 53.7 years, P = .03), which was slightly lower in the 68 patients in the multivariate analysis. As age was not a significant variable related to cosmetic outcome in this cohort, this difference was not thought to be relevant.

The mean dose homogeneity index was 0.71 ± 0.11 for the entire group. Logistic regression analysis of the probability of excellent versus less than excellent cosmetic outcome failed to produce a cutoff value below which cosmetic outcome was overwhelmingly less than excellent. Instead, the probability of excellent cosmetic outcome linearly increased with an increase in the dose homogeneity index (Figure).



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  		Figure 1. Graph shows the probability of excellent cosmesis versus the dose homogeneity index (DHI).

 
An attempt was made to determine the best cutoff value for the prediction of excellent cosmesis. The value of 0.7 or less had the highest specificity (65% [28 of 43 patients]) and sensitivity (54% [26 of 48 patients]). The logistic regression model with this value produced an odds ratio of 2.21 (95% CI: 0.95, 5.14; P = .067).

A univariate logistic regression model for a 0.1-magnitude change in the dose homogeneity index versus cosmetic outcome (excellent or less than excellent) produced an odds ratio of 1.63 (95% CI: 1.08, 2.46). The mean dose homogeneity index for the patients with excellent cosmetic outcome was 0.74 ± 0.12 and was significantly different from the mean dose homogeneity index of 0.68 ± 0.10 for those with less than excellent cosmetic outcome (P = .021).

When we used a logistic regression model to evaluate data in the groups with excellent or good cosmetic outcome versus the groups with fair or poor cosmesis, we found no difference in the mean dose homogeneity index values. The plot of the probability of excellent or good cosmetic outcome versus the dose homogeneity index produced a straight, nearly horizontal line; hence, no relationship could be defined. The lack of an identifiable relationship is probably related to a lack of power, since only 20 (13%) of the 156 patients had fair or poor cosmesis.


   DISCUSSION
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
The ultimate goal of breast-conservation therapy is to achieve local control and survival rates equal to those for mastectomy while providing improved cosmetic outcome and functional results. Review of the literature reveals cosmetic outcome in breast-conservation therapy is a complicated end point with multiple factors related to achieving optimal cosmesis. Published patient-, tumor-, and treatment-related variables and their correlations with cosmetic outcome are listed in Table 4 by reference number. The data are occasionally conflicting, and confounding variables are not consistently accounted for in each study. The present study further examines variables, particularly treatment-related variables, related to cosmetic outcome in patients receiving interstitial therapy as part of breast-conservation therapy.


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  		TABLE 4. References with Correlation between Patient-, Tumor-, and Treatment-related Factors and Cosmetic Outcome
 
An important goal of radiation treatment planning is to deliver a uniform dose to the target volume while minimizing the dose to the surrounding tissues. This is inherently difficult in brachytherapy since this method of irradiation is by default inhomogeneous. However, careful planning can minimize the magnitude of inhomogeneity. Dose uniformity or the lack thereof may have important consequences with regard to tumor control and late tissue complications. It has been hypothesized that an increase in the central dose to a target volume may lead to increased tumor necrosis and local control rates. However, this area of increased dose may also contribute to an increased number of late complications, particularly in the case of breast implantation, where normal breast tissue is innately part of the target volume. An area of inhomogeneity can result in what has been termed "double trouble." Both the total dose and the dose per unit time are increased. Increasing the dose per unit time and the total dose have been shown to have consequences with regard to late normal tissue effects (ie, cosmetic outcome) (39).

Radiation therapeutic factors have been found by other investigators to be related to breast cosmetic outcome and late tissue effects. A high dose per fraction (particularly > 2.5 Gy/d), the use of a boost, a high total target dose, and a total dose to the entire breast of more than 50 Gy have been shown by some investigators (31, 32, 38, 41, 42, 49) to negatively affect cosmetic outcome. These variables were not significantly correlated with cosmetic outcome in our group, since it was a homogeneous population and all patients received 50 Gy to the entire breast and a boost of 20 Gy. Also, no patients were treated with a fraction greater than 2 Gy (in most, 1.8 Gy).

Measures of external-beam inhomogeneity (ie, failure to use wedges, the use of cobalt, increased chest wall separation, match line hot spots) have been shown to negatively affect cosmetic outcome (Table 4). In addition, Touboul et al (11) found superior cosmetic outcome in patients treated with electron-beam boosts as compared with patients treated with interstitial implants. They hypothesized that this may have been a result of the inherent inhomogeneity of their implants. In their study, implantation was performed by using the Pierquin technique and was prescribed at 85% of the basal dose at the central plane per Paris dosimetry (20). An implant dose rate greater than 100 cGy/h was found to adversely affect cosmetic outcome in the study by Deore et al (51). The range of dose rate used in our series is small (30–74 cGy/h); thus, a correlation was not found.

The dose homogeneity index has been formulated to quantify the amount of inhomogeneity in an interstitial implant. If relevant, the dose homogeneity index would provide a physical parameter with which to correlate outcome. We have previously found the dose homogeneity index to be significantly correlated with cosmesis, such that the smaller the dose homogeneity index (more inhomogeneity), the more likely an adverse cosmetic outcome (14). In addition, the total excision volume was negatively associated with cosmetic outcome in a univariate model.

In a multivariate model, only total excision volume remained significantly associated with cosmesis. Given that the odds ratio of 1.37 was near the odds ratio of 1.63 seen in the univariate model, we thought the lack of significance of the dose homogeneity index in the multivariate model was related to a lack of power due to an insufficient number of patients with both total excision volume and dose homogeneity index data available for multivariate analysis. The mean total excision volume in those patients with excellent cosmetic outcome (81.8 cm3 ± 84.0) was significantly less than that in patients with less than excellent cosmetic outcome (120 cm3 ± 84).

No meaningful cutoff volume was found in our series. Others have reported cutoff values above which cosmetic outcome was poor. Mills et al (47) and Olivotto et al (7) have reported worsening cosmetic outcome with a total excision volume greater than 70 cm3. de la Rochefordiere et al (44) noted a decline with greater than 86 cm3, and Taylor et al (21) noted a decline with volumes greater than 100 cm3.

In this study, we further examined the relationship of the dose homogeneity index to cosmesis. We had theorized there might be a cutoff dose homogeneity index below which the probability of less than excellent cosmetic outcome would be highly likely. However, there was no clear cutoff dose homogeneity index; instead, the probability of excellent cosmetic outcome linearly increased with an increase in the dose homogeneity index. This is not entirely surprising since the end point of cosmetic outcome is related to biologic response, which is probably not an all-or-none phenomenon. Using a dose homogeneity index of 0.7 or less as a cutoff value produced unacceptable sensitivity (54%) and specificity (65%). However, the mean dose homogeneity index for those with excellent cosmetic outcome (0.74 ± 0.12) was significantly different from the index in those with less than excellent cosmetic outcome (0.68 ± 0.10).

No relationship was found between the probability of excellent or good cosmesis and the dose homogeneity index. This is likely due to the lack of a sufficient number of patients with less than good cosmetic outcome, and little can be inferred from this observation.

Optimal cosmetic outcome depends on many variables, not the least of which is the irradiation technique. The dose uniformity of any implant is contingent on the design of that implant. We have found that cosmetic outcome is negatively affected by increased inhomogeneity and inversely related to the dose homogeneity index. The dose homogeneity index can be easily calculated for pre- and postimplantation assessment. The goal of brachytherapeutic treatment planning in the breast should be to maximize the dose homogeneity index and hence maximize the probability of excellent cosmesis. Careful attention to other treatment variables, including the amount of breast tissue removed, remains very important.


   Acknowledgments
 
The authors thank Robin Ruthhazer, MPH, from the Division of Clinical Care Research and Biostatistics, New England Medical Center, for performing the statistical analysis.


   Footnotes
 
Author contributions: Guarantor of integrity of entire study, B.A.K.; study concepts, K.U., R.D.Z., D.E.W.; study design, B.A.K., D.E.W.; definition of intellectual content, B.A.K., D.E.W.; literature research, B.A.K.; clinical studies, R.K.A.S.U., D.E.W., B.A.K.; experimental studies, D.E.W., B.A.K.; data acquisition, K.U., D.W.A., B.A.K.; data analysis, D.E.W., B.A.K.; statistical analysis, D.E.W., B.A.K.; manuscript preparation and editing, B.A.K.; manuscript review, D.E.W., D.W.A., B.A.K.


   References
TOP
 Abstract
 Introduction
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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