Radiobiological plan optimization in Proton therapy for Prostate tumors using a Patched Integrated Edge [PIE] technique

Abstract: Purpose: A novel treatment planning technique using proton pencil beam scanning (PBS) is proposed that takes advantage of the increased Linear Energy Transfer (LET) at the distal edge of proton beams to deposit the increased biological effective dose related to the elevated LET protons within prostate tumors. Background: The availability of proton treatment for cancer has increased the latest decade and will continue to increase rapidly in the coming decade. The Relative Biological Effectiveness (RBE) of protons earlier considered to be 1.10 has started to be questioned in the latest decade. This thesis investigates what would be the effect of a variable RBE on the effective dose to the target. Method: Uniform dose distributions were planned using two different beam arrangements: (1) Full-Target Plans (FTP), with two lateral fields, each field targeting the entire target; (2) Patched Integrated Edge (PIE) plans, with 2, 4, and 7 fields, each field targeting only the respective proximal segment of the target. Dose distributions were calculated and optimized with Eclipse in order to deliver the same dose to the target as well as to maintain the same OARs dose constrains for all the plans. Beam properties (range, modulation, spot map and weights) were used to calculate dose and dose averaged LET distributions with Monte Carlo. The RBE for each plan was calculated using radiobiological models taking into account the dose and LETd distribution as well as published values of  for the irradiated tissues as input parameters. The RBE weighted dose (DRBE) was calculated for each planning approach and evaluated with respect to three different aims. Results: An increase of the number of fields using PIE increased the LETd within the target. The increased LETd resulted in an increase of the RBE weighted dose, DRBE, of up to 12.7 Gy (RBE) to the target, which is a 14% increase. However, if the same DRBE is to be delivered to the target with FTP and PIE the increase of LETd in the target implied a decrease of dose per fraction, d, of up to 0.21 Gy, a decrease of 13 %. Conclusions: A modified distribution of proton’s linear energy transfer in PBS allows to deposit highly effective biological dose by the elevated-LET protons within the target, which might help to increase the effectiveness of prostate radiotherapy. This might also serve as a platform to investigate how the physical prescribed dose can be reduced by increasing the LETd in the target in order to maintain a constant DRBE in the prostate.