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The incidence of residual neoplastic cells on prostatic biopsy following conventional external beam radiotherapy is reported to range from 40-90%. As a result, it has been stated that current modalities of radiotherapy may carry an unacceptable local failure rate even in patients irradiated for low stage disease. In order to assess the potential benefits of three-dimensional (3-D) treatment planning, an unselected, consecutive group of patients with localized adenocarcinoma of the prostate was evaluated. This study was designed to determine the frequency of residual cancer in the prostate two years following definitive external beam radiotherapy designed, using a 3-D planning system. Between February 1988 and February 1989, 30 consecutive patients with localized (Stage T1-T3NxMo) adenocarcinoma of the prostate received definitive external beam radiotherapy. All treatment fields were designed with a computed tomography (CT)-based 3-D treatment planning system, resulting in a static conformal radiotherapy plan. The minimum dose delivered to the target volume, which included the prostate, periprostatic tissues, and a 1 cm margin, was between 65 and 69 cGy. Twenty-six patients had Stage T1, T2NxMo primary tumors and four were T3NxMo. Two years following the completion of treatment, all patients underwent digital rectal examination, transrectal ultrasound examination of the prostate with multiple biopsies, bone scan, and serum prostate specific antigen (PSA) determinations. Residual prostate cancer was proven by biopsy in six of 30 patients (20%). Four of 26 (15%) with Stage T1 and T2 tumors had a positive biopsy. However, two of the four Stage T3 tumors had postradiation biopsies positive for cancer (50%). Only one patient with a positive biopsy had an abnormal rectal examination. Five of the eight patients with elevated serum PSA levels after two years had residual neoplasia identified on biopsy. One of six patients with an abnormal postradiation ultrasound had residual tumor. Only one of the 22 patients (5%) with a normal serum PSA at two years had a positive postradiation biopsy. In patients with localized prostate cancer, the use of 3-D static conformal radiotherapy followed by multiple ultrasound guided biopsies confirmed the efficacy of external beam radiotherapy in low stage disease. We believe that the low incidence of positive biopsies in this study resulted from the benefits of 3-D treatment planning as well as the fact that all patients were evaluated, whereas past studies have been in selected patient groups when suspicion of residual disease existed prior to biopsy.(ABSTRACT TRUNCATED AT 400 WORDS)