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

Wayne D. Newhauser, Annelise Giebeler, Ronald Zhu, Uwe Titt, Andrew Lee, Rui Zhang


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, FCSPS, 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 FCSPS for 32 prostate fields using a pencil beam dose algorithm in the treatment planning system (TPS). The mean FCSPS value was 1.006; its standard deviation of the mean was ±0.001. The lower bound for uncertainty in FCSPS, μFCSPS, 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 μFCSPS of 0.4% - 0.8%. Comparison of TPS predictions and MC simulations yielded μFCSPS < 0.3%. For a prostate treatment, a comparison of FCSPS values from TPS predictions with the historical value of 1.0 yielded μFCSPS < 3% and a mean μFCSPS of 0.6%. Regardless of estimation method, μFCSPS was approximately 1%, suggesting that uncertainty in FCSPS for proton treatments of prostate cancer is clinically acceptable.



Dose Per Monitor Unit, Prostate Cancer, Proton Therapy, Uncertainty

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DOI: http://dx.doi.org/10.14319/jpt.11.6

Copyright (c) 2015 Wayne D. Newhauser

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