Comparison of percentage depth dose measurements obtained with three detector technologies and Monaco TPS calculations in a 1 × 1 cm² small photon field

Abstract The increasing clinical use of stereotactic radiosurgery (SRS), stereotactic body radiotherapy (SBRT), and intensity-modulated radiotherapy (IMRT) has increased the demand for accurate small-field dosimetry. In sub-centimetre photon fields, measurement 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 performed in a PTW BeamScan water phantom using three detectors with different sensitive volumes: the PTW Semiflex 3D ionization chamber (0.07 cm³), PTW PinPoint 3D ionization chamber (0.016 cm³), and PTW microDiamond detector (0.004 mm³). Measurements were acquired at a source-to-surface distance of 100 cm and repeated three times to assess reproducibility. Corresponding dose calculations were generated using the Monaco treatment planning system Monte Carlo algorithm with a 1 mm dose grid and 0.5% statistical uncertainty. The largest discrepancies between measured and calculated PDDs occurred in the surface and build-up regions, with differences reaching +106% at the phantom surface. Within the build-up region, the ionization chambers underestimated dose by up to 20%, whereas beyond the depth of maximum dose they increasingly over-responded, reaching approximately +7% at 250 mm for the Semiflex detector. In contrast, the microDiamond detector agreed with treatment planning system calculations within ±2% 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. These findings demonstrate that detector construction and sensitive volume significantly influence PDD accuracy in very small photon fields, with the microDiamond detector providing the closest agreement with Monte Carlo calculations. .