Comparison of percentage depth dose measurements obtained with three detector technologies and Monaco TPS calculations in a 1 × 1 cm²small photon field.

Abstract

The increasing clinical use of stereotactic radiosurgery, stereotactic body radiotherapy and intensity-modulated radiotherapy has heightened the need for accurate small-field dosimetry. In sub-centimetre photon fields, dosimetric uncertainties arise from steep dose gradients, loss of lateral charged particle equilibrium, source occlusion, and detector-related volume-averaging and perturbation effects. This study evaluated the influence of detector construction and sensitive volume on percentage depth dose (PDD) measurements in a 1 × 1 cm², 6 MV photon field delivered by an Elekta Versa HD linear accelerator. PDD measurements were acquired using three detectors with differing sensitive volumes: the PTW Semiflex 3D ionisation chamber (0.07 cm³), PTW PinPoint 3D ionisation chamber (0.016 cm³), and PTW microDiamond detector (0.004 mm³). Measurements were performed in a PTW BeamScan water phantom at a source-to-surface distance of 100 cm and repeated to assess reproducibility. Three repeated measurements were acquired for each detector. Corresponding dose calculations were generated using the Monaco treatment planning system (TPS) Monte Carlo algorithm with a 1 mm grid resolution and 0.5% statistical uncertainty. Substantial deviations between measured and calculated PDDs were observed in the surface and build-up regions, with differences reaching +106% at the phantom surface. These discrepancies reflect the combined effects of steep dose gradients and the absence of charged particle equilibrium. Within the build-up region, the ionisation chambers exhibited under-response of up to approximately -20% at shallow depths, while beyond the depth of maximum dose they demonstrated increasing over-response with depth, reaching approximately +7% at 250 mm for the Semiflex detector. This behaviour is consistent with energy dependence and small-field cavity-theory effects. In contrast, the microDiamond detector-maintained agreement with TPS calculations within approximately ±2% across the entire depth range beyond the build-up region. One-dimensional gamma analysis (global normalisation, 10% dose threshold) yielded pass rates of 98.8%, 72.1%, and 25.9% for the microDiamond, PinPoint, and Semiflex detectors, respectively, using the 1%/1 mm criterion. The findings demonstrate that detector construction and sensitive volume significantly influence PDD accuracy in very small photon fields. The microDiamond detector showed the closest agreement with Monte Carlo TPS calculations, highlighting the advantages of solid-state detectors for small-field dosimetry. Nevertheless, accurate dosimetry in the build-up region remains challenging irrespective of detector type.