Treatment Planning System Calculation Errors Are Present in the Majority of IROC-Houston Phantom Failures

Publication date: Available online 4 April 2017
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): James R. Kerns, Francesco Stingo, David Followill, Rebecca Howell, Adam Melancon, Stephen F. Kry
PurposeThe anthropomorphic phantom program at the Houston branch of the Imaging and Radiation Oncology Core (IROC-Houston) is an end-to-end test that can be used to determine whether an institution can accurately model, calculate, and deliver an intensity-modulated radiation therapy (IMRT) dose distribution. Currently, institutions that do not meet IROC-Houston's criteria have no specific information with which to identify and correct problems. In this work an independent recalculation system is developed that can identify treatment planning system (TPS) calculation errors.MethodsA recalculation system was commissioned and customized using IROC-Houston measurement reference dosimetry data for common linear accelerator classes. Using this system, 259 head and neck phantom irradiations were recalculated. Both the recalculation and the institution's TPS calculation were compared with the delivered dose that was measured. In cases where the recalculation was statistically more accurate by 2% on average or 3% at a single measurement location than was the institution's TPS, the irradiation was flagged as having a "considerable" institutional calculation error. Error rates were also examined according to the linac vendor and delivery technique.ResultsSurprisingly, on average, the reference recalculation system had better accuracy than the institution's TPS. Considerable TPS errors were found in 17% (45) of head and neck irradiations. 68% (13) of irradiations that failed to meet IROC-Houston criteria were found to have calculation errors.ConclusionNearly 1 in 5 institutions were found to have TPS errors in their IMRT calculations, highlighting the need for careful beam modeling and calculation in the TPS. An independent recalculation system can help identify the presence of TPS errors and pass on knowledge to the institution.

Teaser

IROC-Houston recalculated over 250 head and neck phantom irradiations performed between 2012 and 2016 using a customized independent calculation system that was modeled after IROC-Houston linear accelerator measurement reference data. Dose recalculations that were substantially more accurate than the institution's calculations suggested that the institution had treatment planning system calculation errors. Among all recalculations, 17% revealed such errors; among irradiations that failed IROC-Houston criteria, 68% had such errors.


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