Commissioning small fields in lung using Monte Carlo corrected film measurements

Current commissioning of radiotherapy treatment planning systems with heterogeneous phantoms generally measures the dose delivered in solid water downstream of the lung material. The dose delivered directly in lung material is important to characterise the uncertainty of lung stereotactic body radiotherapy (SBRT) treatments, but film measurements require a correction to accurately measure dose to lung.

Monte Carlo (MC) modelled corrections were applied to film measurements used for commissioning of lung SBRT. Medium dependent correction factors, k_med, were established using 6 and 10 MV simulations to account for film being calibrated in water but measuring in regions of lung material. The correction factors are dependent on energy, field size, and position. To avoid the onerous requirement of modelling each individual beam to correctly match penumbra an alternative approach is presented where the correction is applied as a function of isodose level for a nominal field size.

Improvement in central axis dose agreements was seen for all field sizes, with the largest improvement of 7 % observed for 6MV 1 × 1 cm² field. Application of position dependent corrections improved the percentage of points passing a 5 %/1 mm or 3 %/1 mm gamma assessment in all cases, whilst a uniform central axis correction did not improve the passing rate in most cases.

MC simulations provide a method for correcting dose measured in lung materials allowing more accurate comparison with treatment planning system doses. In this work a generic approach to correct small field film lung measurements as a function of isodose levels is presented.