[en] PURPOSE/OBJECTIVE: Although some controversy remains, most authors agree that post-treatment prostatic biopsy is the best measure of local control in prostate cancer. Brachytherapy series reporting post-implant biopsy results have been few in number, limited in size, and involving older open or combined external beam techniques. The present study was undertaken to assess local control rates as determined by post-implant prostate biopsy in a large series of consecutive patients who have received permanent interstitial brachytherapy using a contemporary transrectal ultrasound directed, transperineal, computer generated, volume technique. METHOD/MATERIALS: From January 1988 to January 1994, 402 patients received permanent I-125 (285, 71%) or Pd-103 (117, 29%) interstitial brachytherapy as primary treatment for prostatic carcinoma at the Northwest Tumor Institute. Of these, 201 have consented to prostatic biopsy at least 12 months post-implant with a median follow-up of 40 months (range of 12 to 83 months). None had received hormone manipulation. A total of 361 biopsies were performed on 201 patients with a range of 1 to 6 yearly biopsies per patient; 91 receiving multiple biopsies. The other 201 patients were either unable (for geographic reasons) or unwilling to submit for biopsy. However, all patients with a rising PSA or clinical suspicion of recurrence underwent biopsy when possible. The 201 biopsy patients presented with a median age of 69 (range 47 to 89). Stages included 51 T1 (25%), 125 T2a (62%), 22 T2b (11%), and 3 T2c (1%). Gleason sums included 69 2-4 (34%), 117 5-6 (58%), 15 7-10 (7%), and 2 ungraded (1%). The initial PSA was 6.6 (range 0.7 to 74.6). There was no significant difference in the presenting characteristics or implant parameters between those patients biopsied and those that were not. 143 received I-125 (71%) prescribed to a minimum peripheral dose of 160 Gy with a median activity of 35.5 mCi, and 58 (29%) received Pd-103 prescribed to a minimum peripheral dose of 115 Gy with a median activity of 123 mCi. Sextant biopsies were performed under transrectal ultrasound guidance as well as any focal hypoechoic areas on ultrasound. All biopsy specimens were examined by a single, genitourinary pathologist and classified as either 'negative', 'indeterminate', or 'positive' depending on cellular, architectural, and immunohistochemical criteria. RESULTS: At the time of last biopsy, 161 (80%) have achieved negative pathology (median follow-up 30.8 months). 34 patients (17%) had indeterminate biopsies (median follow-up 21.0 months), and 6 (3%) were positive (median follow-up 30.0 months). Positive, indeterminate, and negative biopsy results by stage were respectively: 51 T1 - 1 (2%), 11 (22%), and 39 (76%); 125 T2a - 3 (2%), 23 (18%), and 99 (79%); 25 T2b/c - 2 (8%), 0, and 23 (92%). Excluding 6 patients with distant relapse proven on bone scan, median PSA values at the time of post-implant biopsy according to biopsy category were: negative 0.2 (range 0.1-7.4), indeterminate 0.5 (range 0.1-2.2), and positive 8.1 (range 2.6-26) (p < 0.005). Only 2 of the 186 patients with a PSA <4.0 at the time of biopsy were positive. All 6 of the positive, 26 indeterminate, and 111 negative biopsies were treated with I-125. Among the 58 Pd-103 patients, none have had positive biopsies, 8 have been indeterminate, and 50 negative. Among those 33 patients with a biopsy interpreted as 'indeterminate' who had serial biopsies, 28 subsequently converted to negative, 1 to positive, and 4 have remained indeterminate. The number of biopsies classified as indeterminate were time-dependent comprising 33% (48/144) of biopsies taken between 12 and 18 months, decreasing to 14.2% (15/106) at 18-30 months, and 5% (1/19) for biopsies taken 54 or more months post-implant. CONCLUSION: These data demonstrate an 80% biopsy confirmed local control rate following permanent prostate implant. In addition, a higher local control rate will likely be attained with further follow-up as the majority of 'indeterminate' biopsies seem to convert to negative over time. Although this series represents a selected group of favorable patients, the results surpass those published in the teletherapy literature and support the use of modern interstitial brachytherapy techniques for selected patients with early stage disease