Regulatory evaluation of prostate volume implants: pitfalls of a retrospective assessment.
ABSTRACT 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.
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.
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.
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.
- [show abstract] [hide abstract]
ABSTRACT: The Nuclear Regulatory Commission deems it to be a medical event (ME) if the total dose delivered differs from the prescribed dose by 20% or more. A dose-based definition of ME is not appropriate for permanent prostate brachytherapy as it generates too many spurious MEs and thereby creates unnecessary apprehension in patients, and ties up regulatory bodies and the licensees in unnecessary and burdensome investigations. A more suitable definition of ME is required for permanent prostate brachytherapy. The American Society for Radiation Oncology (ASTRO) formed a working group of experienced clinicians to review the literature, assess the validity of current regulations, and make specific recommendations about the definition of an ME in permanent prostate brachytherapy. The working group found that the current definition of ME in §35.3045 as "the total dose delivered differs from the prescribed dose by 20 percent or more" was not suitable for permanent prostate brachytherapy since the prostate volume (and hence the resultant calculated prostate dose) is dependent on the timing of the imaging, the imaging modality used, the observer variability in prostate contouring, the planning margins used, inadequacies of brachytherapy treatment planning systems to calculate tissue doses, and seed migration within and outside the prostate. If a dose-based definition for permanent implants is applied strictly, many properly executed implants would be improperly classified as an ME leading to a detrimental effect on brachytherapy. The working group found that a source strength-based criterion, of >20% of source strength prescribed in the post-procedure written directive being implanted outside the planning target volume is more appropriate for defining ME in permanent prostate brachytherapy. ASTRO recommends that the definition of ME for permanent prostate brachytherapy should not be dose based but should be based upon the source strength (air-kerma strength) administered.Practical radiation oncology. 10/2011; 1(4):218-223.
Conference Proceeding: Reducing cache misses by application-specific re-configurable indexing[show abstract] [hide abstract]
ABSTRACT: The predictability of memory access patterns in embedded systems can be successfully exploited to devise effective application-specific cache optimizations. In this work, we propose an improved indexing scheme for direct-mapped caches, which drastically reduces the number of conflict misses by using application-specific information; the scheme is based on the selection of a subset of the address bits. With respect to similar approaches, our solution has two main strengths. First, it models the misses analytically by building a miss equation, and exploits a symbolic algorithm to compute the exact optimum solution (i.e., the subset of address bits to be used as cache index that minimizes conflict misses). Second, we designed a re-configurable bit selector, which can be programmed at run-time to fit the optimal cache indexing to a given application. Results show an average reduction of conflict misses of 24%, measured over a set of standard benchmarks, and for different cache configurations.Computer Aided Design, 2004. ICCAD-2004. IEEE/ACM International Conference on; 12/2004