HDR brachytherapy in vivo source position verification using a 2D diode array: A Monte Carlo study

Abstract

PurposeThis study aims to assess the accuracy of source position verification during high‐dose rate (HDR) prostate brachytherapy using a novel, in‐house developed two‐dimensional (2D) diode array (the Magic Plate), embedded exactly below the patient within a carbon fiber couch. The effect of tissue inhomogeneities on source localization accuracy is examined.MethodMonte Carlo (MC) simulations of 12 source positions from a HDR prostate brachytherapy treatment were performed using the Geant4 toolkit. An Ir‐192 Flexisource (Isodose Control, Veenendaal, the Netherlands) was simulated inside a voxelized patient geometry, and the dose deposited in each detector of the Magic Plate evaluated. The dose deposited in each detector was then used to localize the source position using a proprietary reconstruction algorithm.ResultsThe accuracy of source position verification using the Magic Plate embedded in the patient couch was found to be affected by the tissue inhomogeneities within the patient, with an average difference of 2.1 ± 0.8 mm (k = 1) between the Magic Plate predicted and known source positions. Recalculation of the simulations with all voxels assigned a density of water improved this verification accuracy to within 1 mm.ConclusionSource position verification using the Magic Plate during a HDR prostate brachytherapy treatment was examined using MC simulations. In a homogenous geometry (water), the Magic Plate was able to localize the source to within 1 mm, however, the verification accuracy was negatively affected by inhomogeneities; this can be corrected for by using density information obtained from CT, making the proposed tool attractive for use as a real‐time in vivo quality assurance (QA) device in HDR brachytherapy for prostate cancer.

A Monte Carlo study on the feasibility of real-time in vivo source tracking during ultrasound based HDR prostate brachytherapy treatments

“…The presence of the TRUS probe was found to significantly affect the accuracy of source tracking in all three dimensions. Source tracking accuracy without the TRUS probe using the MP900 was found to be similar, but improved by approximately 0.5 mm, to that found previously using the MP121 device [17]. The major discrepancy remains in the z direction (patient anterior-posterior direction).…”

“…The composition by weight of the stainless-steel shell is modelled as follows: Fe 67.92%, Cr 19%, Ni 10%, Mn 2%, Si 1% and C 0.08% and the physical density is 7.999 g/cm 3 . A more detailed description and validation of the Flexisource model used in the simulations has been presented in a previous study [17].…”

“…The total energy deposited in the sensitive region of each diode was then divided by the mass of the sensitive volume to calculate the dose deposited. The source positions were then determined using the average diode dose as input into an source tracking algorithm [17,18,31] relative to the patient geometry is known, fixed distance (the MP900 is rigidly embedded into the carbon fibre couch below the patient) within the simulation geometry. The simulations were repeated with the TRUS probe replaced by tissue equivalent material in the patient geometry to examine the effect of the TRUS probe on the source tracking accuracy.…”

“…The distance from the source to each of the diodes is used in an algorithm based on a nonlinear least squares fit method to localize the source in three dimensions. Further details of the algorithm are described in a previous study [17]. Studies performed with a previous version of the device, the 'Magic Plate 121' (MP121), in homogeneous phantom media (i.e., the presence of the TRUS probe was not modelled) have reported source localization accuracy of less than 1 mm [18][19][20] and timing resolution of 0.1 seconds [18].…”

“…Studies performed with a previous version of the device, the 'Magic Plate 121' (MP121), in homogeneous phantom media (i.e., the presence of the TRUS probe was not modelled) have reported source localization accuracy of less than 1 mm [18][19][20] and timing resolution of 0.1 seconds [18]. A previous Monte Carlo study of the MP121, showed that source tracking accuracy with the device embedded in a carbon fibre couch below a patient in the lithotomy position was achievable to within 2 mm for CT based HDR pBT [17].…”