Inferring postimplant dose distribution of salvage permanent prostate implant (PPI) after primary PPI on CT images.

PURPOSE: To evaluate the dose distribution of additional radioactive seeds implanted during salvage permanent prostate implant (sPPI) after a primary permanent prostate implant (pPPI).

METHODS AND MATERIALS: Patients with localized prostate cancer were primarily implanted with iodine-125 seeds and had a dosimetric assessment based on day 30 postimplant CT (CT1). After an average of 6 years, these patients underwent sPPI followed by the same CT-based evaluation of dosimetry (CT2). Radioactive seeds on each CT were detected. The detected primary seeds on CT1 and CT2 were registered and then removed from CT2 referred as a modified CT2 (mCT2). Dosimetry evaluations (D90 and V100 ) of sPPI were performed with dedicated planning software on CT2 and mCT2. Indeed, prostate volume, D90 , and V100 differences between CT2 and either CT1 or mCT2 were calculated, and values were expressed as mean (standard deviation).

RESULTS: The mean prostate volume difference between sPPI and pPPI over the 6 patients was 9.85 (7.32) cm3 . The average D90 and V100 assessed on CT2 were 486.5 Gy (58.9) and 100.0% (0.0), respectively, whereas it was 161.3 Gy (47.5) and 77.3% (25.2) on mCT2 (p = 0.031 each time). The average D90 the day of sPPI [145.4 Gy (11.2)] was not significantly different from that observed on mCT2 (p = 0.56).

CONCLUSION: Postimplant D90 and V100 of sPPI after pPPI can be estimated on CT images after removing the primary seeds.