Intensity Modulated Proton Therapy as a Boost Treatment after Prostate Seed Implant: A Treatment Planning Study

Purpose: Combination of Prostate Seed Implant (PSI) with External Beam Radiation Therapy (EBRT) remains as an attractive option for patients with intermediate or high-risk prostate cancer. One of the most widely used approaches is to use external beam radiation therapy (EBRT) to deliver boost doses after permanent prostate seed implant (PSI). In this study, the feasibility of using Intensity Modulated Proton Therapy (IMPT) as an alternative EBRT boost treatment for PSI patients was investigated in the presence of a large number of high Z metallic seeds. The dosimetry of IMPT boost plans was compared with that of conventional Intensity Modulated Radiation Therapy (IMRT) boost plans. Methods: Ten post prostate seed implants with seeds of I-125 were randomly selected for this study. Proton treatment plans were created with two lateral opposed proton beams in Eclipse treatment planning system. IMRT boost plans were generated with seven co-planner beams for comparison. Several plan evaluation parameters such as the planning target volume (PTV) dose homogeneity, dose conformity and dose to surrounding normal tissues were evaluated. Results: Compared to conventional IMRT boost plans, IMPT demonstrated better sparing of normal tissues while providing similar satisfactory PTV coverage. The high Z implanted seeds is not a problem for IMPT as boost treatment. Conclusions: PSI with an IMPT boost can be a valuable option for prostate cancer patient treatment. It delivers comparable or better radiation dose distribution in terms of normal tissue sparing compared to IMRT boost plan.