SU-E-T-378: IMRT Severity Scoring for TG-100: Do You Really Know?

PURPOSE Failure modes and effects analysis (FMEA) as defined in TG-100 has become a popular concept throughout the radiotherapy community. This risk mitigation technique involves detailed process mapping, analysis, and ranking of potential errors by means of a subjective, ordinal scoring system. This study aims to reduce the subjectivity of FMEA severity scoring for IMRT delivery by providing quantitative values. METHODS First we created an IMRT delivery process map for physics-applicable processes and identified 11 physical failure modes (FMs). To determine the magnitude of dose delivery errors for several of the physical FMs (i.e., the severity of the FM), FMs were induced and dosimetry measurements were performed on a Varian Clinac accelerator going out of clinical service. Treatment planning studies to simulate remaining FMs are to follow. The quantitative severity scores will be compared to recommended FMEA subjective scores. RESULTS We identified the following physical FMs to investigate: photon beam energy, symmetry, MLC position, MLC transmission and leakage, MLC rounded-end and leaf offset, MLC tongue-and-groove, CT look-up table, gantry angle, collimator angle, couch angle and displacement, and MU linearity. Within the PTV of a H&N IMRT phantom, the maximum dose delivery error of 3% absolute dose was seen for 2 mm systematic MLC offsets, 4% for 1.1% increased energy, and 4% for 3.5% symmetry error. Measurements will be compared to computational study results and then used for quantitative severity scoring determination. CONCLUSION Current FMEA practice for radiotherapy requires quantitative data in order to make accurate assessments associated with clinical QA programs. This study has shown examples of error magnitudes induced by IMRT physical FMs that can be used to quantitate and rank FMEA severity scoring. Work supported by grants CA10953 and CA81647 (NCI, DHHS).