Regulatory evaluation of prostate volume implants: Pitfalls of a retrospective assessment

Purpose: Evaluate for regulatory compliance the prostate implants from the Philadelphia Veterans Medical Center applying both an activity-based and volume-corrected D ₉₀ (the maximum dose delivered to 90% of the prostate volume) metrics. Methods and Materials: Dosimetry from 107 prostate implants performed at the Philadelphia Veterans Medical Center used immediate postprocedural CT image sets. D ₉₀ values were adjusted for volume differences from planning volumes. Medical events (MEs) determined from the volume-corrected data were compared with an activity-based metric. Results: Examination of images using original and third-party reviewed prostate contours revealed 56 and 62 cases with D ₉₀ values <80% of the prescription dose, respectively. Because postprocedural prostate volumes were on average 55.7% larger than the planned volume, clinical nomogram-based doses using the implanted activity and actual volumes found 34-47 implants failing to achieve doses greater than 80% of the prescription dose.Volume correction identified 20 MEs, 9 cases with D ₉₀ values within 4% of the ME threshold and 11 significantly inferior cases with median D ₉₀ values <52% of the prescribed dose.Eleven implants also had 20% or more seeds beyond the treatment site according to an activity metric recommended by the VHA Blue Ribbon Panel. Ten of these 11 cases were also identified by volume-corrected D ₉₀ metric. The remaining 96 cases, however, had 95% (±6%) of seeds placed within the treatment site. Conclusions: Of the cases reported to the United States Nuclear Regulatory Commission (NRC) on the basis of Day-1 D ₉₀ values, many appear to have been acceptable implants relative to standard-of-practice clinical criteria. The activity-based dose metric, endorsed by the NRC Advisory Committee on the Medical Uses of Isotopes in 2005 and recommended by the VHA Blue Ribbon Panel for Prostate Brachytherapy yields a more robust determination of ME for this population of implants.