Purpose/Objective:

The UK SABR Consortium QA group conducted a postal dosimetry audit of SABR lung plans at 21 UK centres. The purpose of this was to verify the accuracy of calculated dose distributions, improve confidence of centres in the early stages of implementing lung SABR and to establish a benchmark QA method. Here the results of the GafChromic film relative dosimetry arm of the audit are given.

Materials and Methods:

Individual centres were asked to plan a treatment to a pre-defined PTV in the CIRS Thorax phantom, using their clinical method and prescription dose. EBT3 GafChromic film was used to measure an axial plane of dose. Pins in the phantom facilitated alignment of the film and calculated dose planes. Gantry linac and Cyberknife centres were audited, using a variety of TPS with pencil beam, AAA, CCC, Acuros and Monte Carlo algorithms. Scanned films were compared to dose distributions calculated by the individual centres, using single red-channel dosimetry and a purpose-built Matlab application. Centres were also asked to irradiate additional calibration films to provide output-normalised optical density to dose calibration. Measured and calculated isodoses corresponding to 120, 100, 70 and 50% of prescription dose were compared (figure 1), and conformity and maximum distance to agreement were measured. For the areas bound by the 100, 50 and 30% calculated isodoses, local gamma analysis, mean gamma and gamma pass rate (at 3%, 2mm) and a mean dose comparison was performed. The latter was compared to the alanine dosimetry results.

Results:

The dosimetry of the calibration films was reproducible to ±0.9% (1.S.D), for doses ranging from 4.3 to 26.9 Gy. The audit relative dosimetry results are reported in table 1. Mean dose differences within the 100% calculated isodose line agreed well with alanine dosimetry; -0.1 ± 2.0 % (1.S.D). Gamma pass rates (%) and mean gamma results varied with some outlying measurements, mostly caused by small dose deviations within the PTV or at low doses. Isodose line agreement (figure 1) was generally much closer at the 70 and 100% dose levels, indicated by the lower S.D. (table 1, column 5). The exception was the centre using a pencil beam algorithm, where the measured prescription dose covered a significantly smaller area than that calculated, consistent with the algorithm’s known limitations calculating dose in low density lung surrounding tumour.

Conclusions:

Of the 21 UK centres audited, 74% of measurements were within ±3% agreement compared to calculated doses. Where appropriate, outlying centres have been offered support from the QA Group to bring their results into line. The EBT3 GafChromic film was found to be highly suited to a postal audit, reliably giving detailed information about the geometric and dosimetric accuracy of treatment.