Is IMRT needed to spare the rectum when pelvic lymph nodes are part of the initial treatment volume for prostate cancer?

PURPOSE: To assess whether a 4-field box technique (4FBT), along with its technical refinements, is an adequate approach in terms of rectal sparing and target coverage for patients with localized prostate cancer undergoing whole-pelvic radiotherapy followed by a prostate boost and whether or not intensity-modulated radiotherapy (IMRT) is needed.

METHODS AND MATERIALS: For 8 patients, 31 plans were generated, each of them differing in one or more features, including prescription (dose/volume) and/or technical factors. For the latter, several "solutions" to try to reduce the amount of irradiated rectal volume were addressed, including modifications of the 4FBT and the use of sequential IMRT. We constructed a database with 248 plans that were tested for their ability to meet a series of rectal dose-volume constraints at V50, V60, V65, V70, V75, and V75.6. Multivariate logistic regression was used to identify factors independently associated with the end point. Successful solutions were also compared in terms of coverage of both pelvic node and prostate planning target volume (PTV) by isodose 95%.

RESULTS: At multivariate logistic regression, both rectal blocking and IMRT were independent predictors of the probability of meeting rectal dose-volume constraints during the pelvic and boost phases of treatment with close relative risks. However, on average, partial rectal blocking on lateral fields of 4FBT during whole-pelvic radiotherapy resulted in about 3% of pelvic node PTV being outside isodose 95%; only 2 of 8 patients had the pelvic nodal PTV covered similarly to what was achieved by whole-pelvis IMRT. Conversely, blocking the rectum during the last 3 fractions of the conformal boost showed a dosimetric coverage of prostate PTV similar to that achieved by IMRT boost. Interestingly, patient anatomic configuration was the strongest predictor of rectal sparing. Finally, the size of prostate margins to generate PTV was also independently associated with the probability of meeting rectal dose-volume constraints.

CONCLUSION: In the dose range of 70-76 Gy to the prostate, IMRT and standard techniques are equally effective in meeting rectal dose-volume constraints. However, whole-pelvis IMRT might be preferable to standard techniques for its slightly superior PTV coverage.