Partial irradiation of the rectum

Data gathered from dose escalation protocols for the treatment of prostate cancers conducted in the past 10 years have shown that rectal toxicity can be controlled by the use of careful conformal techniques. The most severe complications of rectal irradiation (obstruction and fistula requiring colostomy) have been essentially eliminated. The most frequent gastrointestinal complications of conformal radiotherapy of prostate cancer are now rectal bleeding associated with telangiectatic changes to the vasculature of the submucosa, and in severe cases, ulceration requiring cautery procedures and or transfusion. The benefits of 3-dimensional conformal radiotherapy (3D-CRT) are strongly technique dependent, with a strong dose response for single techniques for prescription doses over 70 Gy. Studies of rectal motion show that the anterior wall can move approximately 1 cm during treatment, so portions of the anterior rectal wall will regularly receive the full prescription dose if posterior margin sizes >/= 1 cm are used in designing the planning target volume (PTV). There is strong evidence that increased rectal shielding and posterior PTV margin sizes approximately 0.6 cm reduce rectal complication rates. Despite uncertainties due to rectal motion, studies of dose-volume histograms (DVHs) show that rectal toxicity is strongly influenced by the percent volumes of rectal wall exposed to doses approximately 70 Gy and higher. Recent data suggests that percent volumes of rectal wall exposed doses between 40 to 50 Gy, and the existence of a reserve of unexposed tissue may also play a role in determining rectal bleeding rates.