Uncertainty in dose per monitor unit estimates for passively scattered proton therapy: The role of compensator and patient scatter in prostate cases

Abstract

Standard methods for determining dose per monitor unit values in a patient do not yet exist for proton therapy. Indeed, some aspects of D/MU estimation remain poorly understood, such as the conversion of absorbed dose in a water phantom to absorbed dose in a patient. This study focused on the water-to-patient absorbed dose conversion factor, F CSPS , which accounts for differences in scatter (from the range compensator and internal patient anatomy) between patient treatments and their corresponding calibration irradiation in a homogeneous water-box-phantom. We estimated F CSPS for 32 prostate fields using a pencil beam dose algorithm in the treatment planning system (TPS). The mean F CSPS value was 1.006; its standard deviation of the mean was ±0.001. The lower bound for uncertainty in F CSPS , μF CSPS , was estimated for a sub-set of fields through comparisons of TPS dose predictions with measurements and Monte Carlo (MC) simulations. Comparison of TPS predictions and measurements yielded μF CSPS of 0.4% - 0.8%. Comparison of TPS predictions and MC simulations yielded μF CSPS < 0.3%. For a prostate treatment, a comparison of F CSPS values from TPS predictions with the historical value of 1.0 yielded μF CSPS < 3% and a mean μF CSPS of 0.6%. Regardless of estimation method, μF CSPS was approximately 1%, suggesting that uncertainty in F CSPS for proton treatments of prostate cancer is clinically acceptable.