SU‐E‐T‐687: Experimental Assessment of the Accuracy of a Semi‐Empirical Model and a Monte Carlo System for Proton Dose Calculations for Highly Inhomogeneous Media

Purpose: To experimentally assess the accuracy of proton dose distributions, computed with a semi‐empirical algorithm and a Monte Carlo (MC) system, for inhomogeneous media. Methods: A complex range compensator for a lung patient, being treated with passively‐scattered proton therapy (PSPT) was used to represent the inhomogeneity. Planar dose distributions for a PSPT beam of exactly the same characteristics as the one used to treat the patient, incident normally on a water equivalent phantom, were measured at a series of depths using a calibrated chamber array detector. Semi‐empirical algorithm and MC (MCNPX) calculations were performed with exactly the same geometry. Absolute depth doses and lateral profiles at multiple depths were compared. Results: Agreement between MC and measured depth dose data was found to be within 2% or 2 mm except at the distal edge where the dose gradient was the highest. The corresponding differences between measurements and the semi‐empirical model were up to 5% in the regions of low gradients and distance to agreement in the distal edge region exceeded 1 cm. Comparison of measured vs. MC vs. se‐empirical profiles showed that, in some regions measured data agreed better with MC, whereas in other regions the agreement was better with semi‐empirical model. Generally, the agreement within the beam boundary was superior for measured vs. MC. The gamma analysis (points within 3% and 3 mm) indicated that the percentage of points passing was comparable for both modes of calculations. Conclusions: The differences between measured and semi‐empirical model dose distributions might be explainable based on the approximations in the latter. However, differences between measurements and MC require further examination of numerous sources of uncertainties, e.g., in modeling of beams and compensators, and the measurements (e.g., the depth of sensitive region of the chamber).