What Are the Dose-Volume Constraints for Long-Course Radiochemotherapy to Apply for IMRT?

Abstract

Preoperative radiochemotherapy (RCT) has been successfully adopted to reduce local recurrence rates in locally advanced rectal cancer. This benefit however has to be balanced against the acute and late side effects, of which radiation enteritis due to radiotherapy (RT) induced small bowel injury is the major source. A multitude of factors are influencing the tolerance of small bowel to radiation, including diabetes mellitus, pelvic inflammatory disease, prior abdominal or pelvic surgery, and the administration of concurrent chemotherapy. The major factors however are RT related and include mainly the total radiation dose and the volume of irradiated small bowel. Despite well-accepted and implemented measures such as distention of the urinary bladder, the use of the prone position, and a belly-board technique, the irradiated volume of small bowel can be reduced significantly only by using sophisticated forms of conformal RT such as intensity-modulated RT (IMRT), where geometrically shaped fields and varying intensities within the shaped field create a conformal dose distribution that tightly matches the target volume. Considering the concave shaped form of the planning target volume (PTV) with the small bowel and bladder lying in the middle, preoperative RT of rectal cancer represents a real challenge for IMRT. Dose-volume constraints have been of little relevance for 3D-conformal RT (3D-CRT) as the planner is configuring a variety of beams, wedges, and beamweights to end up in a suitable plan (called “forward planning”). In contrast, the complexity of IMRT requires the inverse and allows clinicians to specify dose-volume constraints to PTV as well as critical organs at risk (OAR) before the optimization is initiated (“inverse planning”). The following section proposes dose-volume constraints for the OARs in preoperative RT of rectal cancer based on available dose-response relationships (Table 24.1).