[en] Purpose: To assess the impact of treatment technique (whole pelvic treatment vs. prostate only, 3-Dimensional conformal (3-DCRT) vs. non-conformal (NCRT), total dose and age on the acute toxicity experienced by patients undergoing external beam radiation for adenocarcinoma of the prostate. Material and Methods: All patients with clinically localized prostate cancer who had pre-treatment PSAs obtained prior to treatment in the Department of Radiation Oncology between July, of 1987 and February, 1993 were eligible for this analysis. One hundred and thirty seven pts received XRT via either NCRT (n=33) or 3-DCRT (n=104) techniques. The median age was 71 (range 48 - 85). Fourteen pts were staged clinically as T1 (1 pt T1a, 8 pts T1b, 5 pts T1c), 59, T2 pts (22=T2a, 34=T2b, and 3=T2c) and 64 pts had T3 tumors (27=T3a, 15=T3b, and 22=T3c). The maximum tumor doses ranged from 57.9Gy to 87.3Gy with a median dose of 73.5Gy. Results: The major toxicities were gastrointestinal and urinary. Overall, 68 pts had grade 1 (gr. 1), 32 pts gr. 2 and 10 pts gr. 3 toxicity. Twenty seven pts had no significant acute toxicity. Twenty of those were treated to higher dose via 3-DCRT. No pts experienced Gr. 4 (severe) or Gr. 5 (death) acute toxicities. Conformal RT was associated with a lower incidence of gr. 2 - 3 toxicity. Only 22% of pts. in the 3-DCRT group experienced gr. 2 - 3 toxicity as compared with 58% in the non-conformal group (p < 0.01 Mann-Whitney test). Age was associated with increased toxicity. Thirty eight percent of pts > 71 years of age experienced gr. 2 - 3 toxicity compared to 24% of pts ≤ 71 years (p < 0.06). Forty four percent of pts treated with 3-DCRT who were > 71 years of age and who received doses > 73.5Gy experienced gr. 2 - 3 toxicity compared with 6% treated to a lower dose (p=0.01). No pt. treated with non-conformal RT received > 70.1Gy but 53% experienced gr. 2 - 3 acute toxicity. There was a trend for increased toxicity in pts receiving whole pelvic RT with (28(78)) (36%) experiencing gr. 2 - 3 acute toxicity compared to (14(59)) (24%) treated with prostate only irradiation (p=0.09 Fisher exact test). Conclusion: Age greater than 71 was associated with an increased incidence of acute toxicity. In addition, pts treated with 3-DCRT received a higher dose with a lower incidence of acute toxicity when compared with non-conformal RT. Although, previous studies have reported decreased toxicity with 3-DCRT, lower doses were than than in our series. Prospective randomized trials will be required to prove the value of 3-DCRT conclusively