Dose‐rate considerations for the INTRABEAM electronic brachytherapy system: Report from the American Association of Physicists in Medicine task group no. 292

Abstract

The purpose of this report is to provide detailed guidance on the dosimetry of the INTRABEAMâ (Carl Zeiss Medical AG, Jena, Germany) electronic brachytherapy (eBT) system as it stands at the present time. This report has been developed by the members of American Association of Physicists in Medicine (AAPM) Task Group 292 and endorsed by the AAPM. Members of AAPM Task Group 292 on Electronic-Brachytherapy Dosimetry have reviewed pertinent publications and user manuals regarding the INTRABEAM system dosimetry and manufacturer-supplied dose calculation protocols. Formal written correspondence with Zeiss has also provided further clarification. Dose-rate calculations for the INTRABEAM system are highly dependent on choice of dosimetry protocol. Even with careful protocol selection, large uncertainties remain due to the incomplete characterization of the ionization chambers used for verification with respect to their energy dependence as well as manufacturing variations. There are two distinct sets of dose-rate data provided by Zeiss for the INTRA-BEAM system. One dataset (Calibration V4.0) is representative of the physical dose surrounding the source and the other dataset (TARGIT) has been adjusted to be consistent with a clinical trial named TARGIT (TARGeted Intraoperative RadioTherapy). The adjusted TARGIT doses are quite dissimilar to the physical doses, with differences ranging from 14% to 30% at the surface of a spherical applicator, depending on its diameter, and up to a factor of two at closer distances with the smaller needle applicators. In addition, ion chamber selection and associated manufacturing tolerances contribute to significant additional uncertainties. With these substantial differences in dose rates and their associated uncertainties, it is important for users to be aware of how each value is calculated and whether it is appropriate to be used for the intended treatment. If users intend to deliver doses that are the same as they were in 1998 at the onset of the TARGIT trial, then the TARGIT dose-rate tables should be used. The Calibration V4.0 dose rates may be more appropriate to use for applications other than TARGIT trial treatments, since they more closely represent the physical doses being delivered. Users should also be aware of the substantial uncertainties associated with the provided dose rates, which are due to beam hardening, chamber geometry, and selection of the point-of-measurement for a given ionization chamber. This report serves to describe the details and implications of the manufacturer-

Kilovoltage therapy is well and truly alive and needed in a modern radiotherapy centre

“…In the field of intraoperative radiotherapy for breast treatment, several groups including my own tried to determine a robust framework for dosimetry, in the face of steep dose gradients and substantial energy dependence of dosimeters [29]. However, we lacked the tools to accurately measure the dose to very small volumes, and relied on manufacturer provided equipment for output determination, leading to partial volume errors of 14-30% that were only recently demonstrated through Monte Carlo simulations [30]. Should we have challenged our own assumptions more at the time?…”

Organ absorbed doses in the IORT treatment of breast cancer with the INTRABEAM device: a Monte-Carlo study

“…Firstly, the measurement method of the INTRA-BEAM treatment planning is highly dependent on the employed dosimetry protocol; the Calibration V4.0 protocol and/or the TARGIT protocol [12]. The Calibration V4.0 protocol represents physical dose surrounding the INTRABEAM source while the TARGIT protocol reflects dose that has been adjusted to follow dose used in TARGIT (TARGeted Intraoperative radioTherapy) clinical trial.…”

“…The Calibration V4.0 protocol represents physical dose surrounding the INTRABEAM source while the TARGIT protocol reflects dose that has been adjusted to follow dose used in TARGIT (TARGeted Intraoperative radioTherapy) clinical trial. Dose differences between the both protocols are up to 30% and depend on the size of the applicator [12].The measurement method of the INTRABEAM is also limited, time-consuming, and difficult to perform. Using ionization chambers, optically stimulated luminescence dosimeters, and radiochromic films only provide a limited amount of data and, at best provide dosimetric information of the distribution on a plane of interest [13].…”

Dose distributions of the INTRABEAM electronic brachytherapy system measured with traceable detectors

“…The need for standardization in dosimetry has also been pointed out by the AAPM Task Group TG-292. In their published guidance [4] for the dosimetry of the INTRABEAM it is stressed that substantial uncertainties in dose rate calculations arise from the choice of dosimetry protocol and from the incomplete characterization of the ionization chambers used for calibration. This work recommends a methodology to measure dose distribution around the INTRABEAM system using small, commercially available detectors.…”