Comparison of action levels for patient-specific quality assurance of intensity modulated radiation therapy and volumetric modulated arc therapy treatments.

Purpose: To perform a comprehensive and systematic comparison of fixed-beam IMRT and volumetric modulated arc therapy (VMAT) patient-specific QA measurements for a common set of geometries using typical measurement methods.

Methods: Fixed-beam IMRT and VMAT plans were constructed for structure set geometries provided by AAPM Task Group 119. The plans were repeatedly delivered across multiple measurement sessions, and the resulting dose distributions were measured with (1) radiochromic film and ionization chamber and (2) a commercial two-dimensional diode array. The resulting QA measurements from each delivery technique were then analyzed, compared, and tested for statistically significant differences.

Results: Although differences were noted between QA results for some plans, neither modality showed consistently better agreement of measured and planned doses: of the 22 comparisons, IMRT showed better QA results in 11 cases, and VMAT showed better QA results in 11 cases. No statistically significant differences (p < 0.05) between IMRT and VMAT QA results were found for point doses measured with an ionization chamber, planar doses measured with radiochromic film, or planar doses measured with a two-dimensional diode array.

Conclusions: These results suggest that it is appropriate to apply patient-specific QA action levels derived from fixed-beam IMRT to VMAT.