Abstract
Source tracking is becoming a more widely used approach in HDR brachytherapy treatment verification. While it provides a sensitive method to detect deviations from the treatment plan during delivery, it does not show the clinical significance of any detected changes. By incorporating a tool that calculates volumetric doses and DVH indices from measurements, source tracking systems can be expanded to assess dosimetric significance of any deviations from the plan. The source tracking dose calculation tool, MaxiCalc, was developed in MATLAB. Validation was performed by comparing doses and DVH indices calculated in MaxiCalc to those calculated by the clinical TPS, for several test plans and 10 clinical plans. Clinical implementation was demonstrated by calculating volumetric doses from a clinical source tracking event. MaxiCalc showed excellent agreement with the clinical TPS for point and volumetric doses (mean difference<0.01% and 0.1% respectively). MaxiCalc calculates dosimetrically equivalent plans to the TPS with agreement<0.3% for all DVH indices except PTV V200%. Small differences seen for the clinical source tracking event were consistent with the known tracking uncertainties enabling them to be quantified for clinical decision making. Calculations are fast, enabling real-time use. MaxiCalc is an independent tool that calculates doses and DVH indices from dwells measured with any clinical HDR brachytherapy source tracking system. This extends the capabilities of source tracking systems from determining discrepancies in positions or times during delivery to assessing the dosimetric impact of any detected deviations, allowing for more comprehensive treatment verification and evaluation.
Highlights
Since our earliest work in the development of a HDR brachytherapy verification system based on a flat panel detector, [1] it has been clear that source tracking provides a wealth of additional information that is not necessarily readily available from in vivo dosimetry (IVD) alone
We have shown that source tracking for treatment verification is a viable solution in the clinical environment, and our system has been in routine use for every fraction across every patient since 2015
Point dose module The absolute local differences in dose calculated for the 27 validation points between MaxiCalc and Oncentra Brachy (OCB) is shown in Table 1 for both the literature look up tables (LUTs) and the interpolated and extrapolated LUTs used in OCB
Summary
Since our earliest work in the development of a HDR brachytherapy verification system based on a flat panel detector, [1] it has been clear that source tracking provides a wealth of additional information that is not necessarily readily available from in vivo dosimetry (IVD) alone. [3] The richness of information from source tracking extends the capabilities of a treatment verification system from the primary aims of error detection and patient safety, to monitoring smaller changes – for example due to implant movements between planning and treatment delivery. While such changes are known to occur, there are few practical strategies for assessing their significance or potential impact on patient outcomes. Several groups with well-developed IVD systems have recognised the value of source tracking, either by measuring the individual 3D source positions or by measuring implant movements relative to the in vivo dosimeter. If the verification system is capable of capturing the time the source dwells at each position, it follows that the volumetric dose that was delivered is able to be reconstructed using the TG-43U1 formalism. [17] If the anatomical relationship is known relative to the measured dwell (http://creativecommons.org/licenses/by/4.0/)
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