The GLAaS algorithm for portal dosimetry and quality assurance of RapidArc, an intensity modulated rotational therapy

Background: To expand and test the dosimetric procedure, known as GLAaS, for amorphous silicon detectors to the RapidArc intensity modulated arc delivery with Varian infrastructures and to test the RapidArc dosimetric reliability between calculation and delivery. Methods: The GLAaS algorithm was applied and tested on a set of RapidArc fields at both low (6 MV) and high (18 MV) beam energies with a PV-aS1000 detector. Pilot tests for short arcs were performed on a 6 MV beam associated to a PV-aS500. RapidArc is a novel planning and delivery method in the category of intensity modulated arc therapies aiming to deliver highly modulated plans with variable MLC shapes, dose rate and gantry speed during rotation. Tests were repeated for entire (360 degrees) gantry rotations on composite dose plans and for short partial arcs (of similar to 6 or 12 degrees) to assess GLAaS and RapidArc mutual relationships on global and fine delivery scales. The gamma index concept of Low and the Modulation Index concept of Webb were applied to compare quantitatively TPS dose matrices and dose converted PV images. Results: The Gamma Agreement Index computed for a Distance to Agreement of 3 mm and a Dose Difference (Delta D) of 3% was, as mean +/- 1 SD, 96.7 +/- 1.2% at 6 MV and 94.9 +/- 1.3% at 18 MV, over the field area. These findings deteriorated slightly is Delta D was reduced to 2% (93.4 +/- 3.2% and 90.1 +/- 3.1%, respectively) and improved with Delta D = 4% ( 98.3 +/- 0.8% and 97.3 +/- 0.9%, respectively). For all tests a grid of 1 mm and the AAA photon dose calculation algorithm were applied. The spatial resolution of the PV-aS1000 is 0.392 mm/pxl. The Modulation Index for calculations resulted 17.0 +/- 3.2 at 6 MV and 15.3 +/- 2.7 at 18 MV while the corresponding data for measurements were: 18.5 +/- 3.7 and 17.5 +/- 3.7. Partial arcs findings were ( for Delta D = 3%): GAI = 96.7 +/- 0.9% for 6 degrees rotations and 98.0 +/- 1.1% for 12 degrees rotations. Conclusion: The GLAaS method can be considered as a valid Quality Assurance tool for the verification of RapidArc fields. The two implementations ( composite rotation or short arcs) allow the verification of either the entire delivery or of short partial segments to possibly identify local discrepancies between delivery and calculations. RapidArc, according to the findings, appears to be a safe delivery method in terms of dosimetric accuracy allowing its clinical application.