THE EFFECT OF TREATMENT POSITIONING ON NORMAL TISSUE DOSE IN PATIENTSWITH PROSTATE-CANCER TREATED WITH 3-DIMENSIONAL CONFORMAL RADIOTHERAPY

Purpose: To prospectively assess the effect of supine vs, prone treatm ent position on the dose to normal tissues in prostate cancer patients treated with the three-dimensional conformal technique. Methods and M aterials: Twenty-six patients underwent three-dimensional treatment pl anning in both the supine and prone treatment positions, The planning target volume and normal tissue structures were outlined on each CAT s can slice, and treatment plans were compared to assess the effect of t reatment position on the volume of rectum, bladder, and bowel exposed to the high doses of irradiation. Results: The average dose to the rec tal wall and the V95 (volume of rectal wall receiving at least 95% of the prescription dose) for the prone position were 64 and 24% of the p rescription dose, respectively, compared to 72 and 29%, respectively, for the supine position (p < 0.05). When the average rectal wall dose was used as an endpoint, 14 of the 26 patients (54%) had an advantage for the prone position compared to 1 (4%) who demonstrated an advantag e for the supine position (p < 0.0002), Similarly, when V95 of the rec tal wall was used as a measure of comparison, 15 patients (58%) had an advantage for the prone position compared to 1 (4%) who demonstrated an advantage for the supine position (p < 0.0002), In 13 patients (50% ), a change from supine to the prone position was associated with redu ction of the V95 to levels < 30% of the prescription dose compared to 3 patients (11%) in whom such an advantage resulted from change of the prone to the supine position (p < 0.005), The effect of treatment pos ition on the rectal wall dose was most pronounced in the region of the seminal vesicles, An increased volume of bowel was also noted in the supine position, The treatment position, however, had no significant i mpact on the dose to the bladder wall. Conclusions: Three-dimensional conformal radiotherapy for prostate cancer in the prone position is as sociated with significant reduction of the dose to the rectum and bowe l resulting in an improving in the therapeutic ratio. Copyright (C) El sevier Science Inc.