Joel Poder
Centre of Medical Radiation Physics, University of Wollongong
Medical Physicist

Dean Cutajar
Research Fellow
Centre of Medical Radiation Physics, University of Wollongong

Susanna Guatelli
Senior Lecturer
Centre of Medical Radiation Physics, University of Wollongong

Marco Petasecca
Senior Lecturer
Centre of Medical Radiation Physics, University of Wollongong

Anatoly Rosenfeld
Director
Centre of Medical Radiation Physics, University of Wollongong

Background and Purpose: Due to uncertainties in catheter reconstruction during planning of high dose rate (HDR) brachytherapy treatments, as well as the potential for catheter shifts in the time between imaging and treatment, there exists a need for real-time source position verification to ensure safe and accurate treatment delivery. This study aims to assess the accuracy of source position verification using a 2D diode array [1] embedded below the patient in a carbon fibre couch. The effect of tissue inhomogeneities on the source triangulation accuracy is examined. Methods: Monte Carlo simulations of 12 source positions from a HDR prostate brachytherapy treatment were performed using the Geant4 platform [2]. An Ir-192 Flexisource (Isodose Control, Veenendaal, The Netherlands) was simulated inside a voxelised patient geometry, and the response of each detector in the couch embedded 11x11 diode array was evaluated. The detector response was then used to determine the distance of all detectors in the array to each of the 12 source positions. Finally, the source position was triangulated using an iterative procedure where the source position is first estimated, and then repeatedly refined based upon the agreement of the predicted geometric distance from the source to the detectors against those measured by the detectors in the array. Results: The accuracy of source position verification was found to be affected by the tissue inhomogeneities inherent in the patient geometry. As such, an inhomogeneity correction may be required, based on density information obtained from the patient CT scan performed prior to treatment. Furthermore, due to the relatively large distance between the source position and the diode array, it was found that source localisation accuracy can be improved with an increased number of detectors used in the triangulation algorithm. Conclusions: The effect of tissue inhomogeneities in the patient geometry on source localisation accuracy during a HDR prostate brachytherapy treatment was examined through Monte Carlo calculations. The localisation accuracy was found to be affected by the inhomogeneities; however this may be corrected for using density information obtained from CT. References: [1] Espinoza, A., B. Beeksma, M. Petasecca, et al., The feasibility study and characterization of a two-dimensional diode array in “magic phantom” for high dose rate brachytherapy quality assurance. Medical Physics, 2013. 40(11): p. 111702. [2] Archambault, L., L. Beaulieu, J.F. Carrier, et al. Overview of Geant4 applications in medical physics. in Nuclear Science Symposium Conference Record, 2003 IEEE. 2003.


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