Determination of the P rp and radial dose correction factor in reference dosimetry

Abstract

Objectives. In an addendum to AAPM TG-51 protocol, McEwen et al, (DOI: 10.1118/1.4866223) introduced a new factor P rp to account for the radial dose distribution of the photon beam over the detector volume mainly in flattening filter free (FFF) beams. P rp and its extension to non-FFF beam reference dosimetry is investigated to see its impact in a clinical situation. Approches. The P rp was measured using simplified version of Sudhyadhom et al (DOI: 10.1118/1.4941691) for Elekta and Varian FFF beams with two commonly used calibration detectors; PTW-30013 and Exradin-A12 ion chambers after acquiring high resolution profiles in detectors cardinal coordinates. For radial dose correction factor, the ion chambers were placed in a small water phantom and the central axis position was set to center of the sensitive volume on the treatment table and was studied by rotating the table by 15-degree interval from −90 to +90 degrees with respect to the initial (zero) position. Main results. The magnitude of P rp varies very little with machine, detector and beam energies to a value of 1.003 ± 0.0005 and 1.005 ± 0.0005 for 6FFF and 10FFF, respectively. The radial anisotropy for the Elekta machine with Exradin-A12 and PTW-30013 detector the magnitudes are in the range of (0.9995±0.0011 to 1.0015±0.0010) and (0.9998±0.0007 to 1.0015±0.0010), respectively. Similarly, for the Varian machine with Exradin-A12 and PTW-30013 ion chambers, the magnitudes are in the range of (1.0004±0.0010 to 1.0018±0.0018) and (1.0006±0.0009 to 1.0027±0.0007), respectively. Significance. The P rp is ≤ 0.3% and 0.5% for 6FFF and 10FFF, respectively. The radial dose correction factor in regular beams also does not impact the dosimetry where the maximum magnitude is ±0.2% which is within experimental uncertainty.