SU-E-T-826: Dosimetric Comparison of Intensity-Modulated Proton Therapy and 3D Proton Radiotherapy with Passively Scattered Beams for Primary Prostate Cancer

Abstract

Purpose: To evaluate the dose distributions for treatment of primary prostate tumor using plans with intensity modulated proton therapy (IMPT) and plans with 3D conformal proton beam using passive scattering beams. Methods: We generated treatment plans for five patients with two different proton treatment planning technique (IMPT and 3D conformal) using opposed lateral beams. The 3D conformal plans included passively scattered beams with appropriate beam compensators to conform the dose along the distal region of each beam. IMPT planning applies the spot scanning beam and uses the dose optimization process to reduce the dose to normal tissue and provide adequate prescription dose to prostate. The planning target volume (PTV) included prostate and margins to account for dosimetric and setup uncertainty. The total prescription dose (PD) for each plan is 79.2 Gy. We require that 99% of the clinical target volume (prostate) is covered by 98% of PD (D99%=78.4 Gy) and the PTV dose criteria is D98%=75.2 Gy. The dose for rectum and bladder for volumes of 20% and 40% (D20%, D40%) was optimized such that D20% and D40% are less than or equal to 60 Gy and 40 Gy respectively. The index of conformality (CI) was evaluated using the ratio of PTV volume to that of volume covered by 95% of PD. Results: The PTV dose coverage is achieved for both techniques but IMPT plans showing better dose homogeneity and lower maximum dose. Overall, the D20% and D40% slightly decreased for rectum for all plans. However, for bladder, there is higher dose reduction for IMPT plans as compared to 3D proton plans. The CI was improved for IMPT plans. Conclusions: The IMPT technique results in plans with better PTV coverage and lower dose to organ at risk as compared to plans with passively scattered beams.