Experimental validation of absorbed dose-to-medium calculation algorithms in heterogeneous media

Abstract

Objective.The aim of this work was to determine heterogeneous correction factorshQclin,Qreffclin,frefdetm,wto validate absorbed dose-to-mediumDm,Qclinm,fclincalculation algorithms from detector readings. The impact of detector orientation perpendicular and parallel to the beam central axis on the correction factors was also investigated.Approach.ThehQclin,Qreffclin,frefdetm,wfactors were calculated for four types of detectors (PTW PinPoint T31016, PTW microDiamond T60019, PTW microSilicon T60023 and EBT3 film) placed in different media (cortical bone, lung, adipose tissue, Teflon and RW3) for the 6 MV energy beam with a 10 × 10 cm²field size. These corrections were then applied to the detector measurements performed at different depths in heterogeneous phantoms.Main results.ThehQclin,Qreffclin,frefdetm,wfactors mainly depended on the media and slightly on the type of detector. Considering all detectors, the largest corrections were found in high-density media with values ranging from 0.911 to 0.934 in cortical bone. For comparison, the corrections in other media were closer to unity with values from 0.966 (lung and RW3) to 0.991 (adipose tissue). Except for the PinPoint T31016, detector orientation-dependence was observed especially in high-density media. A good agreement (≤1.5%) was found betweenDm,Qclinm,fclincalculations and the detector readings corrected with thehQclin,Qreffclin,frefdetm,wfactor for all studied heterogeneous phantoms.Significance.This paper could serve as an initial guideline for medical physicists involved in the validation of the advanced type-b dose calculation algorithms reportingDm,Qclinm,fclin. To our knowledge, this is the first study to assess the impact of the orientation of different detectors in heterogeneous media. The orientation dependence of the detector response observed in water may not reflect what is observed in heterogeneous media, especially in high-density media. The knowledge of thehQclin,Qreffclin,frefdetm,wfactors becomes mandatory for accurate interpretation of detector readings and comparisons withDm,Qclinm,fclincalculations.

Keywords: MV photon beams; detectors; dose-to-medium; formalism; heterogeneity.