Poster - Thur Eve - 24: Clinical application of the new dosimetry formalism for composite nonstandard beams.

The IAEA-AAPM new dosimetry formalism [Med. Phys. 35, 5179 (2008)] was applied to clinical IMRT quality assurance (QA). Twenty different IMRT QA fields were created on the CT images of a 30×30×17 cm Solid Water™ phantom. Two Farmer-type chambers, Exradin A12 and NE2571, and a smaller Exradin A1SL ionization chamber were cross-calibrated against a reference detector, the PTW micro liquid ion chamber (microLion), in the lowest dose gradient region in each IMRT QA field delivery. Based on the new dosimetry formalism, the clinical correction factor was measured in a fully-rotated delivery and a delivery at a single gantry angle, a collapsed delivery. For the calibrated Exradin A12, the measured dose with the clinical correction factor was compared with a calculated dose using Monte Carlo (MC) methods. The clinical correction factor deviated from unity by up to 2.4% and 3.7% in the fully-rotated and collapsed deliveries, respectively, depending on the dose distribution in the chamber collecting volume. For the Exradin A1SL, the correction factor was generally closer to unity due to the reduced dose gradient on the smaller collecting volume. In the fully-rotated delivery, the measured dose with the clinical correction factor is different from the MC-calculated dose to within 4%; while the discrepancy was greater, up to 8%, in the collapsed delivery due to the much heterogeneous dose distribution in the chamber collecting volume. This work proves that the suggested dosimetry technique is effective to improve the dosimetric consistency of clinical IMRT QA.