Postimplant analysis of transperineal interstitial permanent prostate brachytherapy: Evidence for a learning curve in the first year at a single institution

Purpose: The utilization of transperineal interstitial permanent prostate b rachytherapy (TIPPB) is increasing in the United States. Quality assessment of TIPPB is in its infancy, and to date, dosimetric analyses have only bee n reported from centers with a large experience in prostate brachytherapy. The purpose of this report is to critically analyze the dosimetric coverage achieved following TIPPB in the first 63 cases performed by a multidiscipl inary group of investigators with no prior experience with TIPPB, Methods and Materials: The information in this report concerns the first 63 men treated with TIPPB alone at our institution between September 1997 and September 1998, All men were treated similarly, adapting the methods descr ibed by Blasko and Grimm. All men were treated with I-125. The prescription dose was 144 Gy according to the TG43 formalism, TIPPB was performed joint ly by a radiation oncologist and a urologist. One month following TIPPB, al l men underwent a computed tomography (CT) scan of the pelvis according to a protocol using 3-mm abutting slices. CT images were transferred by a loca l area network to a commercially available treatment planning system and do se-volume histograms were calculated with 0.5-mm pixel spacing. A variety o f dosimetric endpoints were examined. A single measure of dose homogeneity, the dose-homogeneity index (DHI), is defined as the volume within the pros tate that receives 100-150% of the prescription dose (144-216 Gy) divided b y the volume within the prostate that receives 100% of the prescription dos e (144 Gy), Three measures of target (prostate) dosimetric coverage are pro vided. C100 is defined as the percentage of the prostate volume defined on postimplant CT that receives at least 100% of the prescription dose. C90 an d C80 are similar but represent the percentage of the prostate volume that receive 90% and 80% of the prescription dose, respectively. Statistical ana lyses were performed using commercially available computer software, To inv estigate any changes with time the first 30 cases (group 1) are compared to cases 31-63 (group 2). All p-values are two-sided. Results: The mean C100, C90, and C80 for all 63 patients were 80.7% (SD 10. 1), 85.1% (SD 10.2), and 89.3% (SD 9.5), The quantifiers of implant adequac y were all improved in the most recent 33 patients compared to the first 30 patients, (group 1: C100, 75.8% [SD 12.2], C90 79.9% [SD 11.4], C80 84.3% [SD 11.1]; group 2: C100, 85.2 [SD 7.0], C90 89.9% [SD 5.8], C80 93.8% [SD 4.2]; p < 0.001). The mean DHI was 0.538 SD (0.124). A multivariate model i ncorporating a number of variables (ultrasound volume, CT volume, total act ivity, activity/seed, implant number) with C100 as the dependent variable f ound that the implant number was the only statistically significant predict or of C100 (p = 0.0001), Using C90 and C80 as the dependent variable produc ed similar results (C90, p = 0.0001; C80, p = 0.0001). Conclusion: In this single institution experience with the first 63 men rec eiving TIPPB by a multidisciplinary group of investigators, there is eviden ce for a learning curve. All quantifiers of implant adequacy improved as cl inicians gained experience. In the most recent group of patients, quantifie rs of implant adequacy are similar to those reported from other groups with significantly more experience with TIPPB, (C) 2000 Elsevier Science Inc.