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 brachytherapy (TIPPB) is increasing in the United States. Quality assessment of TIPPB is in its infancy, and to date, dosimetric analyses have only been 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 multidisciplinary 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 described by Blasko and Grimm. All men were treated with 125I. The prescription dose was 144 Gy according to the TG43 formalism. TIPPB was performed jointly by a radiation oncologist and a urologist. One month following TIPPB, all 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 local area network to a commercially available treatment planning system and dose-volume histograms were calculated with 0.5-mm pixel spacing. A variety of dosimetric endpoints were examined. A single measure of dose homogeneity, the dose-homogeneity index (DHI), is defined as the volume within the prostate that receives 100-150% of the prescription dose (144-216 Gy) divided by the volume within the prostate that receives 100% of the prescription dose (144 Gy). Three measures of target (prostate) dosimetric coverage are provided. C100 is defined as the percentage of the prostate volume defined on postimplant CT that receives at least 100% of the prescription dose. C90 and C80 are similar but represent the percentage of the prostate volume that receive 90% and 80% of the prescription dose, respectively. Statistical analyses were performed using commercially available computer software. To investigate 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 adequacy 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 incorporating a number of variables (ultrasound volume, CT volume, total activity, activity/ seed, implant number) with C100 as the dependent variable found that the implant number was the only statistically significant predictor of C100 (p = 0.0001). Using C90 and C80 as the dependent variable produced similar results (C90, p = 0.0001; C80, p = 0.0001).

CONCLUSION

In this single institution experience with the first 63 men receiving TIPPB by a multidisciplinary group of investigators, there is evidence for a learning curve. All quantifiers of implant adequacy improved as clinicians gained experience. In the most recent group of patients, quantifiers of implant adequacy are similar to those reported from other groups with significantly more experience with TIPPB.