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Letters to the Editor |
Division of Abdominal Imaging, Department of Radiology, Emory University Hospital, 1364 Clifton Road NE, Atlanta, GA 30322
e-mail: mkalra{at}emory.edu
Editor:
We read with great interest the original research article by Dr Sahani and colleagues in the March 2005 issue of Radiology (1), in which they reported initial observations of perfusion computed tomography (CT) in rectal cancer with regard to tumor perfusion and treatment response. We congratulate the authors on their impressive and innovative pilot study. We agree with Dr Sahani and colleagues that their study on perfusion CT of rectal cancer is the first report on the use of tumor perfusion for assessing tumor response to nonsurgical treatment. The authors used a 45-second acquisition duration to perform perfusion CT studies in order to limit the radiation dose to their subjects, avoid motion-related artifacts, and obtain an acceptable tracer kinetics model with an estimated level of precision of 5%–15%. Although blood flow and mean transit time helped predict tumor response to treatment, blood volume and capillary permeability–surface area product did not help differentiate tumor from the normal rectal wall or predict tumor response in their study. Importantly, tumors with initial high blood flow and short mean transit time responded poorly to chemotherapy and radiation therapy.
We wish to inform the authors and readers about a recent study (2) that has been performed to assess the effect of acquisition time and its implications on CT perfusion protocols for quantitative colorectal cancer perfusion measurement with use of dynamic contrast material–enhanced multi–detector row CT. In this original article, Goh et al (2) reported that scanning acquisition times of 45 seconds are too short for reliable permeability measurement in the abdomen. A restraining band was used to minimize movement, and hyoscine-N-butylbromide, a spasmolytic drug, was administered to minimize peristalsis. Although no differences for blood volume, blood flow, and mean transit time were noted between 45-second and 65- and 130-second acquisitions, there was significant underestimation of permeability–surface area product values with 45-second acquisitions when compared with 65- and 130-second acquisitions. Interestingly, Goh et al (2) and Dr Sahani and colleagues (1) used four–detector row CT scanners from same vendor (GE Medical Systems, Milwaukee, Wis) and employed similar scanning protocols, with the exception of acquisition times and initiation time for scanning following contrast medium injection. Furthermore, other authors have also recommended use of longer scanning acquisitions (65 seconds and longer) for CT perfusion in the abdomen to include the period when contrast media is exchanged between intravascular and extravascular compartments (3). In contradiction, 45–50-second acquisitions have been found to be acceptable for CT perfusion of the brain, as well as squamous cell carcinoma of the upper aerodigestive tract (4).
Thus, there is scientific evidence to believe that a 45-second acquisition duration for CT perfusion of rectal cancer does not provide a reliable estimate of permeability–surface area product values (2,3). We commend the authors on their use of a lower peak kilovoltage (100–120 kVp) and shorter acquisition duration to reduce radiation dose associated with CT perfusion studies. However, we also believe that use of a lower peak kilovoltage (80 kVp, compared with 100–120 kVp), lower tube current (much less than 200–240 mA), and three-dimensional optimized reconstruction algorithms can also enable substantial reduction in radiation dose associated with longer acquisition duration of CT perfusion studies in the abdomen.
In conclusion, we agree with the authors that a 45-second acquisition duration CT perfusion protocol for rectal cancer can help in the reliable estimation of blood volume, blood flow, and mean transit time values. However, further studies must be performed to assess the value of permeability–surface area product values in the prediction of rectal cancer response with longer acquisition durations.
References
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