The Regression of Rectal Tumors During Preoperative Chemoradiation Therapy: A Prospective Study With Weekly MRI

Purpose/Objective(s)
Preoperative chemoradiotherapy (CRT) decreases the local recurrence rate in patients with locally advanced rectal cancer. To enhance response, multiple centers are investigating the possibility of a boost dose to the gross tumor volume (GTV), either sequentially or simultaneously. Few data is available on GTV regression during CRT. We aimed to determine the pattern in order to optimize timing of dose escalation on the GTV and assess the need for adaptive planning.
Materials/Methods
MRI was obtained before, weekly during, and 4 and 7 weeks after a five-week course of concomitant CRT in sixteen patients with locally advanced rectal cancer (T3-4 and/or N1-2). A dose of 50 Gy was administered during CRT, combined with daily capecitabine. GTV was contoured on high resolution axial T2 images, aided by diffusion weighted imaging (DWI) and the apparent diffusion coefficient map (ADC). Contouring was done independently by two experienced radiation oncologists, and resulting contours were compared with Dice similarity coefficients. In case of a dissimilarity (Dice <0.7), the contours were discussed and a consensus contouring reached.
Results
Overall, 120 usable MRIs were acquired in 15 patients, after exclusion of one patient without MRI baseline. GTV volume measured on average 53.6 cc (SD 32.8 cc) at baseline. In most patients, tumor shrinkage started in the first two weeks of CRT, amounting to an average absolute reduction of 10.7% per week during CRT. One patient showed no response during CRT and suffered local progression before planned surgery. Mean GTV volume remained 25.1 cc (± 16.9 cc) during the last week of CRT, an average reduction of 53.6 % (p<0.001). Volume reduction continues up to seven weeks post-CRT, resulting in a mean tumor volume of 17.7 cc (± 14.5 cc), corresponding to a reduction of 67.6% (p<0.001) compared to baseline, and a difference of 7.4 cc with the end of CRT (p<0.001).
Conclusion
GTV shrinkage of rectal tumors, treated with standard dose of neoadjuvant chemoradiotherapy, is seen from the beginning of treatment and continues up to the last observation 7 weeks post-treatment. The observed volume reduction during treatment suggests that a simultaneous integrated boost strategy could benefit from adaptive planning during the course. Future work will quantify this benefit in terms of organs at risk exposure.