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Feasibility Study of Prostate Seed Implant with Intensity Modulated Proton Boost

J Wu

J Wu1*, C Cheng2, H Wu3, (1) Purdue University, West Lafayette, IN, (2) Indiana University- School of Medicine, Bloomington, IN, (3) IUPUI, Indianapolis, IN

SU-E-T-668 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: External beam radiation therapy (EBRT) has been widely used to deliver boost doses after permanent prostate seed implant (PSI). In this study, we tested the feasibility of EBRT boost with Intensity Modulated Proton Therapy (IMPT) in the presence of a large number of high Z metallic seeds. The dosimetric effect caused by implanted high density prostate seeds in spot scanned proton beam boost treatment was analyzed. IMPT boost plans were also compared with conventional Intensity Modulated Radiation Therapy (IMRT) boost plans.

Method: Post prostate seed implants with seeds of I-125 and Pd-103 were tested in this study. The post PSI CT scans were processed at a slice thickness of 1mm. Proton treatment plans were created with two lateral beams in Eclipse treatment planning system. IMRT boost plans were generated with seven co-planner beams. The calculation grids were set up at 1mm. Extreme cases with prostate seeds close to each other and prostate seed drift were also analyzed.

Results: Higher density PSI seeds did not show significant effect on dose distributions in IMPT plans. Compared to conventional IMRT boost plans, IMPT has better sparing of rectum and bladder, while providing the same PTV coverage. The mean rectal dose decreases from 32.5% for IMRT to 22.4% for IMPT of the prescribed doses. The mean bladder dose decreases from 28.3% (IMRT) to 24.9% (IMPT) of the prescribed doses. The spot scanned proton beam provides an improved boost plan to different cold spots of suboptimal dose coverage PSI.

Conclusion: PSI with an IMPT boost can be a valuable tool for prostate cancer patient treatment. It delivers better plan in terms of normal tissue sparing compared to IMRT boost plan. The high density seeds have no effect on the proton distribution.

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