Session Item

High dose rate (HDR) multi fraction treatments are currently used in gynaecological brachytherapy treatments. To ensure correct dosage for patient safety and treatment efficacy, it is important to monitor dose accurately. Use of in vivo dosimetry (IVD) for brachytherapy involves complexities such as measuring doses from different dwell positions in the applicators, measuring dose accurately in steep dose gradients and difficulties that arise from positioning a detector into the patient such that it is close enough to the treatment site to measure the dose to target. Therefore, IVD is not routinely performed in brachytherapy. Real time IVD measurements allow the treatment to be interrupted if an issue is detected. Previous real time IVD in HDR prostate brachytherapy demonstrated good agreement with predicted dose within measurement uncertainties ^[1]. This research focuses on the feasibility of real time IVD for HDR gynaecological brachytherapy. 

Stage one followed the commissioning of the metal-oxide semiconductor field effect transistor (MOSFET) devices using an in-house phantom placed in a water tank to immobilise the MOSFET device within a treatment catheter at a fixed distance from the HDR source. Stage two consisted of calibration measurements to determine the dose (in Gy) each MOSFET reading (in mV) corresponds to when irradiated. Stage three consisted of measuring dose in the Venezia™ applicator by placing the MOSFET device into an interstitial needle within the applicator. The applicator was partially submerged in a water tank for the purpose of replicating a patient. 

The mean dose from applicator measurements resulted in a -1.8% difference (with STDEV of 1.5) from the Oncentra® treatment planning system (TPS) dose. The total plan uncertainty (k=2), including MOSFET angular dependence uncertainties acquired from commissioning measurements, calibration uncertainties and TPS dose uncertainties, was calculated to be ±11.2%. This compares to prostate IVD measurements which resulted in -6.4% mean dose (range +5.1% to -15.2%) compared to TPS dose and total plan uncertainty (k=2) between 11-17%^[1].

Use of the MOSFET device for IVD in HDR gynaecological brachytherapy applicators in a water tank showed good agreement with predicted dose (extracted from the TPS) within measurement uncertainties. These results provide confidence in the accuracy of dose delivery for brachytherapy. The next stage of this project will implement real-time IVD measurements in patients during cervix brachytherapy treatments, based on MOSFET readings taken automatically every 20 seconds.