Iori Sumida MS, Presenter: Nothing to Disclose

Hiroya Shiomi, Abstract Co-Author: Nothing to Disclose

Etienne Lessard PhD, Abstract Co-Author: Nothing to Disclose

I-Chow Hsu MD, Abstract Co-Author: Nothing to Disclose

Jean Pouliot PhD, Abstract Co-Author: Nothing to Disclose

To optimize dose distribution for prostate cancer in high dose rate interstitial brachytherapy, we have developed a new algorithm named Attraction-Repulsion Model (ARM). In this study, we compared ARM with geometric optimization (GO) for prostate brachytherapy.

ARM was used to optimize the dose distribution by finding the best dwell time combination. We established grids at intervals of a certain distance inside the clinical target volume (CTV) and critical organs. We then assigned attributes based and specific dose constraints for the grids. Attraction or repulsion occurred depending on dose delivered from the source to the grids and its assigned attributes. Dwell time was changed by the forces that the grids impose to the sources. Calculations were done repeatedly until the attraction and repulsion forces reach equilibrium. The optimal dwell time distribution was established once the equilibrium is achieved. Two optimization plans were generated for ten patients using GO and ARM. The same contours, and the same dwell positions were used in both optimizations. All dwell positions outside of the CTV were turned off. No manual adjustment of the dwell weight was used in GO. The prescription dose was normalized to the isodose volume that covers 95% of the CTV. Dose volume histograms (DVH) from all plans were analyzed, and multiple volume indices were calculated from DVH.

The CTV ranged from 23-48 cc (median: 33 cc). The CTV V150 ranged from 63-81% (median: 71%), and 27-44% (median: 38%) using GO and ARM respectively. Therefore, the calculated homogeneity indices (HI), defined as (V100-V150) / V100, ranged from 0.15-0.34 (median: 0.25), and 0.54-0.72 (median: 0.61) using GO and ARM respectively. The conformal indices (COIN), defined as (CTVref / CTV)*(CTVref / Vref), ranged from 0.57-0.74 (median: 0.67), and 0.80-0.87 (median: 0.85) using GO and ARM respectively. The urethra V150 ranged from 14-95% (median: 70%), and 0-1% (median: 0%) using GO and ARM respectively.

ARM can be used to optimize the dose distribution. ARM is superior to GO in minimize dose to normal structures and improve dose homogeneity of the target while sparing the dose to normal tissues.

Sumida, I, Shiomi, H, Lessard, E, Hsu, I, Pouliot, J, Optimization of Dose Distribution for HDR Brachytherapy of the Prostate Using Attraction-Repulsion Model.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4406970.html