Algorithms Used in Heterogeneous Dose Calculations Show Systematic Error as Measured With the Radiological Physics Center's Anthropomorphic Thorax Phantom Used for RTOG Credentialing

Abstract

To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for RTOG clinical trials; the irradiations were all done using 6-MV beams. Heterogeneous treatment plans were developed using Monte Carlo algorithms (MC), convolution/superposition (CS) algorithms and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target, as well as planar dose measurements throughout the target, were compared to the calculated dose. The difference between measured and calculated dose was examined as a function of planning algorithm, as well as use of IMRT versus 3D conformal therapy, use of a motion platform versus static irradiation, and date of irradiation. Only minor differences were observed in the planar agreement between different algorithms. Absolute dose agreement showed sizeable and significant variation between algorithms. PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no significant difference observed between IMRT and 3D CRT calculation accuracy, between moving or static irradiations, or as a function of time (i.e., agreement has not improved in recent years). Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This large dosimetric difference requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.Oral Scientific Abstract 319; TableMean ratio and standard deviation of measured to calculated doseAlgorithmMonte CarloAAAPinnacleXioTomotherapyPBMean0.9940.9590.9680.9550.9710.951Standard Deviation0.0300.0270.0240.0250.0240.027 Open table in a new tab