Importance of post-implant dosimetry in permanent prostate brachytherapy

Abstract

Objective: Post-implant dosimetry has become the gold standard for implant evaluation and it is recommended that it be performed on all patients undergoing prostate brachytherapy. The technique, results and correlation with clinical outcomes will be presented.

Methods: The method and outcomes of post-implant dosimetry are explored by outlining the experience at the Mount Sinai Medical Center, New York, as well as reviewing the literature. The most accurate time to perform post-implant dosimetry is 1 month after implant. Computed tomography (CT)-based dosimetry is currently the best available technique for performing this analysis. The technique involves taking 3-mm abutting CT slices throughout the implanted area. The prostate and normal structures are outline on the CT slices. These structures are recreated in three dimensions. Dose volume histograms (DVH) are created and allow the dose to these organs to be quantified.

Results: The relationship between dosimetric findings and clinical outcomes has been established. The dose delivered to 90% of the prostate on DVH (D90) has been correlated to prostate-specific antigen (PCA) control and post-treatment biopsy results. D90 values of >or=140Gy have been associated with improved biochemical control and lower positive post-treatment biopsy results. Doses derived from the dosimetric analysis to prostate, urethra and rectum have been correlated with the development of acute and chronic urinary morbidity, sexual potency and rectal morbidity. Future initiatives involve performing dosimetric calculations intraoperatively at the time of the implant.

Conclusions: Post-implant CT-based dosimetry is an essential component of prostate brachytherapy. It is the only method of assessing the actual dose delivered to the prostate and normal surrounding structures. Future development in post-implant and intraoperative dosimetry will continue to improve permanent prostate brachytherapy as a safe an effective treatment for prostate cancer.