Agreement between institutional measurements and treatment planning system calculations for basic dosimetric parameters as measured by IROC-Houston

Publication date: Available online 2 April 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): James R. Kerns, David Followill, Jessica Lowenstein, Andrea Molineu, Paola Alvarez, Paige A. Taylor, Stephen F. Kry
PurposeWe compared radiation machine measurement data collected by the Imaging and Radiation Oncology Core at Houston (IROC-H) with institutional treatment planning system (TPS) values to identify parameters with large differences in agreement. The findings will help institutions focus their efforts to improve the accuracy of their TPS models.MethodsBetween 2000 and 2014, IROC-H visited more than 250 institutions and conducted independent measurements of machine dosimetric data points, including percentage depth dose, output factors, off-axis factors, multileaf collimator small fields, and wedge data. We compared these data with the institutional TPS values for the same points by energy, class, and parameter to identify differences and similarities using criteria involving both the medians and standard deviations for Varian linear accelerators. Distributions of differences between machine measurements and institutional TPS values were generated for basic dosimetric parameters.ResultsOn average, intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT)-style output factors and upper physical wedge output factors were the most problematic. Percentage depth dose, jaw output factors, and enhanced dynamic wedge output factors agreed best between the IROC-H measurements and the TPS values. Although small differences were shown between two common TPS systems, neither was superior to the other. Parameter agreement was constant over time from 2000 to 2014.ConclusionDifferences in basic dosimetric parameters between machine measurements and TPS values vary widely depending on the parameter, although agreement does not appear to vary by TPS and has not changed over time. IMRT-style output factors, SBRT-style output factors, and upper physical wedge output factors had the largest disagreement and should be carefully modeled to ensure accuracy.

Teaser

Measurements of more than 500 Varian linear accelerators were compared to their respective treatment planning system calculations (TPS) for simple dosimetric conditions. Analysis compared various parameters to identify those that, on average, performed poorly compared to the TPS. Troublesome parameters included small MLC fields and physical wedges. To improve accuracy, these parameters should be given special attention during TPS commissioning.


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