SU‐E‐T‐659: Maximizing the Therapeutic Effect of a Fractionated Treatment with Delivery of Inhomogeneous Daily Dose Distributions

Purpose: To evaluate potential gain in therapeutic effect from delivery of daily inhomogeneous fractional dose distributions in proton therapy, using pencil beam scanning. Methods: For a sample case of prostate cancer, we consider a standard course of 39 fractions of 2 Gy (78 Gy total), and a hypofractionated course of 20 fractions of 3 Gy (assuming alpha/beta=1.5, the equivalent dose in 2 Gy fractions (ED2Gy) is 77.1 Gy). Two sets of dose distributions were planned: (1) the standard approach, with a uniform dose (100% of prescription) delivered in every fraction to the entire target; (2) delivery of inhomogeneous fractional dose (IFD) distributions, which varied within the target volume between 65% and 135% of the nominal prescription. For IFD, the daily distributions were optimized in such a way that the two hemispheres of the prostate (split sagittally through the urethra) received, in alternating fractions, either 65% or 135% of the nominal fractional dose (e.g., 1.3 and 2.7 Gy), while the urethra received approximately 100% (e.g., 2 Gy) daily. The equivalent uniform dose (EUD), and ED2Gy were compared for different plans. Results: In the IFD course, the whole prostate received a nearly uniform dose over every 2 fractions, however ED2Gy was higher than with standard uniform dose: 81.9 Gy (5% increase) for the conventional, and 82.6 Gy (7% increase) for the hypofractionated course. Rectal and bladder EUD increased by