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Purpose A Preliminary report of clinical and treatment factors associated with toxicity in men receiving high dose radiation (RT) on a phase III dose escalation trial. Methods and Materials Trial was initiated with 3 dimensional RT (3DCRT) and amended after 1 year to allow intensity modulated RT (IMRT). Patients treated with 3DCRT received 55.8Gy to a planning target volume that included the prostate and seminal vesicles then 23.4Gy to prostate only. IMRT patients were treated to the prostate and proximal seminal vesicles to 79.2Gy. CTC v2.0 and RTOG/EORTC late morbidity scores were used for acute and late effects. Results 748 of 763 patients randomized to the 79.2 Gy arm of RTOG 0126 were eligible and evaluable. 491 and 257 were treated with 3DCRT and IMRT, respectively. For both bladder and rectum, the volumes receiving 65, 70, and 75Gy were significantly lower with IMRT (all p<0.0001). For G2+ acute GI/GU toxicity, both univariate and multivariate analyses show a statistically significant decrease in G2+ acute collective GI/GU toxicity for IMRT. There are no significant differences with 3DCRT or IMRT for acute or late, G2+ or 3+ GU toxicities. Univariate analysis shows a statistically significant decrease in late G2+ GI toxicity for IMRT (p=0.039). On multivariate analysis, IMRT shows a 26% reduction in G2+ late GI toxicity (p=0.099). Acute G3+ toxicity was associated with late G3+ toxicity (p=0.005). With DVH data in the multivariate analysis, RT modality is not significant whereas white race (p=0.001) and rectal V70 >=15% are associated with G2+ rectal toxicity (p=0.034). Conclusions IMRT is associated with a significant reduction in acute G2+ GI/GU toxicity. There is a trend for a clinically meaningful reduction in late G2+ GI toxicity with IMRT. The occurrence of acute GI toxicity and large (>15%) volumes of rectum >70Gy are associated with late rectal toxicity.

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