Probe-Type Experimental Dosimetry in Terms of Absorbed Dose to Water in Photon-Brachytherapy A Proposal for a Radiation-Quality Index
U Quast1*, TW Kaulich2, GA Zakaria3, A Ahnesjö4, JT Alvarez-Romero5, D Medich6, F Mourtada7, A Pradhan8, M Rivard9, (1) ex- Essen University Hospital, Essen, Germany,(2) Tuebingen University Hospital, Tuebingen, Germany,(3) Cologne University Teaching Hospital, Gummersbach, Germany,(4) Nucletron Scandinavia AB, Uppsala, Sweden,(5) ININ, SSDL, Salazar, (6) University of Massachusetts Lowell, Lowell, MA, (7) Christiana Care Hospital, Newark, DE, (8) Ex- Bhabhba Atomic Research Centre, Mumbai, India,(9) Tufts Medical Center, Boston, MASU-E-T-313 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: 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.
Methods: 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 x 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.
Results: 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.
Conclusions: The main benefit of this new QBT-concept is that a type of BT-dosimetry-detector needs to be calibrated only for one reference radiation-quality, e.g., for Q0=192Ir. To measure the dose for different source-types, DW can be determined using calculated radiation-quality conversion factors kQ,QoBT, to be included in the AAPM-database and to be provided by the manufacturer for each detector-type. Typical BT-dosimetry-detectors are plastic scintillation detectors, radiochromic film, thermoluminescence detectors, optically stimulated detectors, and small volume ionization chambers. Recently, different DW(1cm)-primary standards have been developed in several European NMIs, enabling to calibrate BT-radiation-sources and BT-dosimetry-detectors and allowing to verify MC-calculated dose-rate constant values. The proposed definition of QBT has to be discussed internationally to find broad consensus.