Influence of Organ Motion on Conformal vs. Intensity-Modulated Pelvic Radiotherapy for Prostate Cancer

Purpose: To compare an intensity-modulated radiotherapy (IMRT) planning approach for prostate pelvic RT with a conformal RT (CRT) approach taking into account the influence of organ-at-risk (OAR) motion. Methods and Materials: A total of 20 male patients, each with one planning computed tomography scan and five to eight treatment computed tomography scans, were used for simulation of IMRT and CRT for delivery of a prescribed dose of 50 Gy to the prostate, seminal vesicles, and pelvic lymph nodes. Planning was done in Eclipse without correcting for OAR motion. Evaluation was performed using the CRT and IMRT dose matrices and the planning and treatment OAR outlines. The generalized equivalent uniform dose (gEUD) was calculated for 894 OAR volumes using a volume-effect parameter of 4, 12, and 8 for bowel, rectum and bladder, respectively. For the bowel, the gEUD was normalized to a reference volume of 200 cm3. For each patient and each OAR, an average of the treatment gEUDs (gEUDtreat) was calculated for CRT and IMRT. The paired t test was used to compare IMRT with CRT and gEUDtreat with gEUDplan. Results: The mean gEUDtreat was reduced from 43 to 40 Gy, 47 to 46 Gy, and 48 to 45 Gy with IMRT for the bowel, rectum, and bladder, respectively (p < 0.001). Differences between the gEUDplan and gEUDtreat were not significant (p > 0.05) for any OAR but was >6% for the bowel in 6 of 20 patients. Conclusion: Intensity-modulated RT reduced the bowel, rectum, and bladder gEUDs also under influence of OAR motion. Neither CRT nor IMRT was robust against bowel motion, but IMRT was not less robust than CRT