Evaluation criteria for film based intensity modulated radiation therapy quality assurance

Abstract 

The aim of this study was to use different gamma histogram criteria for the comparison of planned dose with irradiated dose distribution and find that what percent of pixels passing a certain criteria imitate a good quality plan. The dose was calculated for 156 patients by inverse planning optimization using the Corvus treatment planning system. Gafchromic films in combination with 2571 0.6cm³ Farmer type ionization chamber and Farmer 2570/1 electrometer from NE Technology were used to measure the delivered dose in solid water phantom. All the measurements were performed on Varian CL21EX linear accelerator (Varian Medical Systems, Palo Alto, CA) fitted with a Millennium 120 leaf collimator. In this study the mean value of the percent of passing pixels within the region of interest under the criterion of 3% DD and 3mm DTA is 90.2±7.1% for head and neck cases and 92.2±5.8% for non-head and neck cases. If we choose the criteria of 3% DD and 3mm DTA then 96.3% head and neck plans have the percent of passing pixels≥75% and 95.1% non-head and neck plans have the percent of passing pixels≥80%. It is evident from the results of this study that the criterion of 5% DD and 3mm DTA with the percent of passing pixels≥90 for non-head and neck cases while the percent of passing pixels≥85 for head and neck cases endorse that a plan is good. The results of this study may be useful for other institutions which use verification software and EBT films for patient specific IMRT QA.

Keywords: IMRT, Gafchromic™ EBT, γ-Histograms, Film dosimetry