Multisector dosimetry in the immediate post-implant period: significant under dosage of the prostate base

PURPOSE: While there are several reports of prostate multisector dosimetry data obtained from CT or MRI scans performed at intervals ranging from 14-70 days after prostate brachytherapy (PB), there are no reports on multisector dosimetry performed in the immediate post-implant period. This study was undertaken to determine the results of prostate multisector dosimetry performed in the immediate post-implant period on day 1 post-implant dosimetry after (125)I PB. MATERIAL AND METHODS: The day 1 post-implant CT-based V(100) and D(90) were determined for the prostate base (PGB) and compared to doses to the entire gland (PG), mid-gland (PMG), and apex (PA) in 75 patients who underwent (125)I PB to a dose of 144 Gy. Similar multisector dosimetry was also performed on the pre-implant ultrasound volume study scans of these patients. RESULTS: All patients had good quality implants. On day 1 post-implant multisector dosimetry there was significant under dosage of the PGB for both V(100) and D(90). The average magnitude of under dosage of PGB compared to PMG and PA was 17.2% and 22.7% for V(100) and 44.6 Gy and 31.7 Gy for D(90), respectively. On pre-implant multisector dosimetry there was no statistically significant under dosage of the PGB for V(100), but the PGB D(90) was significantly lower compared to PMG and PA, however, the average magnitude of under dosage was small at 12.6 Gy and 4.2 Gy, respectively. CONCLUSIONS: This report demonstrates that similar to other reports on more delayed post-implant multisector dosimetry data, there is significant under dosage of the prostate base in the immediate post-implant period based on day 1 post-implant dosimetry. The clinical significance of this under dosage remains to be defined and further studies are warranted.