A descriptive analysis of postimplant dosimetric parameters from Radiation Therapy Oncology Group P0019.

ABSTRACT

BACKGROUND: To date, there are few descriptive analyses of postimplant dosimetry from multi-institutional clinical trials. The purpose of this report is to describe the postimplant dosimetry achieved in Radiation Therapy Oncology Group (RTOG) 0019. METHODS AND MATERIALS Patients were treated with external beam radiation therapy (45 Gy/25 fractions) followed by a prostate implant (I-125, prescription dose 108 Gy). Postimplant dosimetric assessment was accomplished by obtaining a CT scan of the prostate 1 month after the date of the implant procedure. Prostate volume was outlined by the first author. Dose-volume histograms were calculated by the Radiologic Physics Center. Four dosimetric quantifiers (DQs) were examined D90 is the dose (reported as percentage of the prescription dose) received by 90% of the prostate; V100, V150, V200 is the percentage of the prostate volume receiving 100%, 150%, and 200% of the prescription dose, respectively. For the purposes of analysis, institutions were divided into three groups according to accrual (<5, 6-9, 10-12). RESULTS: One hundred thirty-eight patients from 27 institutions were registered in the study. Nineteen patients were excluded from this analysis; 14 who had no data and 5 who were ineligible, leaving 119 for analysis. The mean, median, and range of the four DQs are as follows D90 105.6%, 106.0%, 57.6-174.8%; V100 89.8%, 92.6%, 11.2-100%; V150 58.4%, 59.6%, 0.9-93.7%; and V200 27.9%, 25.1%, 0.3-85.2%. Statistically significant differences according to institutional accrual were observed for D90 (p = 0.0283) and V200 (p = 0.0075), but not for V100 (p = 0.1534) and V150 (p = 0.0509). CONCLUSIONS: The DQ observed in this multi-institutional prospective study are roughly comparable to series from single institutions with considerable brachytherapy experience. Differences in DQs were observed according to institutional accrual. These data could be used to determine a community standard with respect to postimplant dosimetry.