SU-E-T-313: Probe-Type Experimental Dosimetry in Terms of Absorbed Dose to Water in Photon-Brachytherapy a Proposal for a Radiation-Quality Index

Medical Physics
Ulrich QuastMark J Rivard

Abstract

In photon-brachytherapy (BT), all data for clinical dosimetry (e.g., the dose-rate constant) are not measured in water, but calculated, based on MC-simulation. To enable the measurement of absorbed dose to water, DW, in the vicinity of a source, the complex energy-dependence and other influence quantities must be considered. The detectors response, R=M/D, is understood as product of a detector-material dependent 'absorbed dose response', Ren, and Rin, the 'intrinsic response'. Ren is described by the Burlin-theory and because of dissimilarities between the detector-material and water, will have energy dependent correction factors which convert Ren into the clinically relevant DW,Qo=MQo × ND,W,Qo. To characterize BT- source-types, we propose a new 'radiation-quality index' QBT=Dprim(2cm)/Dprim(1cm), the ratio of the primary-dose to water at r=2cm to that at the reference distance r=1cm, similar to external beam dosimetry. Although QBT cannot be measured directly, it can be derived from primary and scatter separated dose-data, published as consensus data e.g., in the Carlton AAPM-TG-43-database. Mean QBT-values are: for nine HDR and four PDR 192Ir-sources: 0.2258±0.5%; one 169Yb- source: 0.2142; and one 125I-source: 0.1544. The m...Continue Reading

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