The purpose of this study is to compare performance of several dosimetric methods in heterogeneous phantoms irradiated by 6 and 18 MV beams. Monte Carlo (MC) calculations were used, along with two versions of Acuros XB, anisotropic analytical algorithm (AAA), EBT2 film, and MOSkin dosimeters. Percent depth doses (PDD) were calculated and measured in three heterogeneous phantoms. The first two phantoms were a 30×30×30 cm3 solid‐water slab that had an air‐gap of 20×2.5×2.35 cm3. The third phantom consisted of 30×30×5 cm3 solid water slabs, two 30×30×5 cm3 slabs of lung, and one 30×30×1 cm3 solid water slab. Acuros XB, AAA, and MC calculations were within 1% in the regions with particle equilibrium. At media interfaces and buildup regions, differences between Acuros XB and MC were in the range of +4.4% to −12.8%. MOSkin and EBT2 measurements agreed to MC calculations within ∼2.5%, except for the first centimeter of buildup where differences of 4.5% were observed. AAA did not predict the backscatter dose from the high‐density heterogeneity. For the third, multilayer lung phantom, 6 MV beam PDDs calculated by all TPS algorithms were within 2% of MC. 18 MV PDDs calculated by two versions of Acuros XB and AAA differed from MC by up to 2.8%, 3.2%, and 6.8%, respectively. MOSkin and EBT2 each differed from MC by up to 2.9% and 2.5% for the 6 MV, and by −3.1% and ∼2% for the 18 MV beams. All dosimetric techniques, except AAA, agreed within 3% in the regions with particle equilibrium. Differences between the dosimetric techniques were larger for the 18 MV than the 6 MV beam. MOSkin and EBT2 measurements were in a better agreement with MC than Acuros XB calculations at the interfaces, and they were in a better agreement to each other than to MC. The latter is due to their thinner detection layers compared to MC voxel sizes.PACS numbers: 87.55.K‐, 87.55.kd, 87.55.km, 87.53.Bn, 87.55.k

Linac backscattered radiation (BSR) into the monitor chamber affects the chamber's signal and has to be accounted for in radiotherapy dose calculations. In Monte Carlo (MC) calculations, the BSR can be modelled explicitly and accounted for in absolute dose. However, explicit modelling of the BSR becomes impossible if treatment head geometry is not available. In this study, monitor backscatter factors (MBSFs), defined as the ratio of the charge collected in the monitor chamber for a reference field to that of a given field, have been evaluated experimentally and incorporated into MC modelling of linacs with either known or unknown treatment head geometry. A telescopic technique similar to that by Kubo (1989 Med. Phys. 16 295-98) was used. However, instead of lead slits, a 1.8 mm diameter collimator and a small (2 mm diameter) detector positioned at extended source to detector distance were used. This setup provided a field of view to the source of less than 3.1 mm and allowed for MBSF measurements of open fields from 1 × 1 to 40 × 40 cm(2). For the fields with both X and Y dimensions exceeding 15 cm, a diode detector was used. A pinpoint ionization chamber was used for smaller fields. MBSFs were also explicitly modelled in MC calculations using BEAMnrc and DOSXYZnrc codes for 6 and 18 MV beams of a Varian 21EX linac. A method for deriving the D(ch)(forward) values that are used in MC absolute dose calculations was demonstrated. These values were derived from measured MBSFs for two 21EX and four TrueBeam energies. MBSFs were measured for 6 and 18 MV beams from Varian 21EX, and for 6 MV, 10 MV-FFF, 10 MV, and 15 MV beams from Varian TrueBeam linacs. For the open field sizes modelled in this study for the 21EX, the measured MBSFs agreed with MC calculated values within combined statistical (0.4%) and experimental (0.2%) uncertainties. Variation of MBSFs across field sizes was about a factor of two smaller for the TrueBeam compared to 21EX Varian linacs. Measured MBSFs and the derived [Formula: see text] factors allow for the incorporation of the BSR effect into accurate radiotherapy dose calculations without explicit backscatter modelling.

The purpose of this work was to obtain [Formula: see text] factors for microDiamond and EFD-3G detectors in very small (less than 5 mm) circular fields. We also investigated the impact of possible variations in microDiamond detector design schematics on the calculated [Formula: see text] factors. Output factors (OF's) of 6 MV beams from TrueBeam linac collimated with 1.27-40 mm diameter cones were measured with EBT3 films, microDiamond and EFD-3G detectors as well as calculated (in water) using Monte Carlo (MC) methods. Based on EBT3 measurements and MC calculations [Formula: see text] factors were derived for these detectors. MC calculations were performed for microDiamond detector in parallel and perpendicular orientations relative to the beam axis. Furthermore, [Formula: see text] factors were calculated for two microDiamond detector models, differing by the presence or absence of metallic pins. The measured OFs agreed within 2.4% for fields  ⩾10 mm. For the cones of 1.27, 2.46, and 3.77 mm maximum differences were 17.9%, 1.8% and 9.0%, respectively. MC calculated output factors in water agreed with those obtained using EBT3 film within 2.2% for all fields. MC calculated [Formula: see text] factors for microDiamond detector in fields  ⩾10 mm ranged within 0.975-1.020 for perpendicular and parallel orientations. MicroDiamond detector [Formula: see text] factors calculated for the 1.27, 2.46 and 3.77 mm fields were 1.974, 1.139 and 0.982 with detector in parallel orientation, and these factors were 1.150, 0.925 and 0.914 in perpendicular orientation. Including metallic pins in the microDiamond model had little effect on calculated [Formula: see text] factors. EBT3 and MC obtained [Formula: see text] factors agreed within 3.7% for fields of  ⩾3.77 mm and within 5.9% for smaller cones. Including metallic pins in the detector model had no effect on calculated [Formula: see text] factors. Our results show that microDiamond and EFD-3G detectors can be used in very small (1.27-3.77 mm) fields once [Formula: see text] corrections determined in this work are applied. Expected uncertainty of such measurements will be in the range of 8%-2.5%.