A brachytherapy photon radiation quality index QBT for probe-type dosimetry

Abstract

Introduction: In photon brachytherapy (BT), experimental dosimetry is needed to verify treatment plans if planning algorithms neglect varying attenuation, absorption or scattering conditions. The detector's response is energy dependent, including the detector material to water dose ratio and the intrinsic mechanisms. The local mean photon energy E-(r) must be known or another equivalent energy quality parameter used. We propose the brachytherapy photon radiation quality index QBT(E-), to characterize the photon radiation quality in view of measurements of distributions of the absorbed dose to water, Dw, around BT sources. Materials and methods: While the external photon beam radiotherapy (EBRT) radiation quality index QEBRT(E-)=TPR1020(E-) is not applicable to BT, the authors have applied a novel energy dependent parameter, called brachytherapy photon radiation quality index, defined as QBT(E-)=Dprim(r=2cm,θ0=90°)/Dprim(r0=1cm,θ0=90°), utilizing precise primary absorbed dose data, Dprim, from source reference databases, without additional MC-calculations. Results and discussion: For BT photon sources used clinically, QBT(E-) enables to determine the effective mean linear attenuation coefficient μ-(E) and thus the effective energy of the primary photons Eprimeff(r0,θ0) at the TG-43 reference position Pref(r0=1cm, θ0=90°), being close to the mean total photon energy E-tot(r0,θ0). If one has calibrated detectors, published E-tot(r) and the BT radiation quality correction factor kQ,Q0BT(E-,r,θ) for different BT radiation qualities Q and Q0, the detector's response can be determined and Dw(r,θ) measured in the vicinity of BT photon sources. Conclusions: This novel brachytherapy photon radiation quality index QBT characterizes sufficiently accurate and precise the primary photon's penetration probability and scattering potential.