Room: ePoster Forums
Purpose: To evaluate the accuracy and sensitivity of portal dosimetry (PD) for pre-treatment quality assurance of multi-target volumetric arc therapy (VMAT) intracranial stereotactic radiosurgery (SRS) plans.
Methods: Output factors of MLC shaped square fields ranging from 0.5x0.5 cm2 to 10x10 cm2 were measured for 6MV and 6MV flattening filter free (6FFF) beams using an on-board imager (EPID) and a plastic scintillator. Five sample SRS VMAT 6FFF plans were also measured with the EPID and radiochromic film. Standard clinically commissioned procedures were employed for all planar radiochromic film measurements which were analyzed with 4%/1mm gamma criteria. The plans were recalculated with different sets of MLC dosimetric leaf gap and transmission values to assess the dependence of predicted dose on these parameter settings compared to measurements.
Results: For the output factors, EPID and scintillator measurements showed good agreement (within 4%) for field sizes of 1x1 cm2 and greater on central axis for 6MV and 6FFF beams, and off-axis for the 6MV beam. Larger discrepancies were seen between the EPID and scintillator for the 0.5x0.5 cm2 fields and the 6FFF off-axis fields. For one set of clinical MLC parameters, all sample plans showed film agreement over 95% while PD results were below 90%. For other sets of non-clinical MLC parameters, film results demonstrated multiple PTVs failing QA while all PD results erroneously increased to be above 95% of points passing gamma analysis.
Conclusion: Despite accurate output factor measurements of fields as small as 1x1 cm2, the algorithm for calculating the EPID predicted response led to failing PD results using the clinical beam model parameters. When additional beam parameters were evaluated, the results were not robust when the EPID was compared to film as a reference dosimeter. Ultimately, a more robust QA method for VMAT SRS plans is required.