Online in vivo dosimetry in high dose rate prostate brchytherapy with MOSkin detectors: In phantom feasibility study

Highlights

• A needles implant was set-up in phantom to simulate prostate brachytherapy treatments.

• In vivo dosimetry was performed in the urethral catheter with MOSkin dosimeters.

• Dual-MOSkin detectors resulted to be accurate dosimeters to perform this task.

Abstract

MOSkin detectors were studied to perform real-time in vivo dose measurements in high dose rate prostate brachytherapy. Measurements were performed inside an urethral catheter in a gel phantom simulating a real prostate implant. Measured and expected doses were compared and the discrepancy was found to be within 8.9% and 3.8% for single MOSkin and dual-MOSkin configurations, respectively. Results show that dual-MOSkin detectors can be profitably adopted in prostate brachytherapy treatments to perform real-time in vivo dosimetry inside the urethra.

Introduction

The recent developments of more sophisticated radiotherapy and brachytherapy (BT) techniques call for the improvement of instruments and methodologies employed for the quality control of the performed treatments. Due to the achievable high conformity of modern BT associated with steep dose gradients, a careful verification of the accuracy in the delivered dose distributions, as planned by the Treatment Planning System (TPS) through mathematical models, is gaining importance.

In vivo dosimetry is a reliable method to compare planned and delivered dose distributions, representing therefore a valid tool to systematically verify treatment accuracy and improve radiotherapy quality control (Lambert et al., 2007, Mijnheer, 2008). Particularly advantageous for in vivo dosimetry are detectors that allow online dose reading. These dosimeters provide in fact real-time measurements during treatment, avoiding therapy misadministration and allowing at the same time intraoperative dose re-planning for treatment error correction.

Current methods for in vivo dosimetry are mainly based on the application of thermoluminescence detectors (TLDs) (Toye et al., 2009) or semiconductor diodes (Waldhäusl et al., 2005). TLDs involve offline process providing the integral dose absorbed during patient treatment and require special procedures in order to achieve good precision of the results. On the other hand, diodes show rapid processing time, high sensitivity and immediate reuse, however, they show a high energy dependence and the delivered dose is therefore not promptly inferred from the diode reading. Moreover, the major disadvantages of diodes are their relative large sizes, which make them unable to be held in many catheters placed inside the patient to perform in vivo dosimetry.

New detectors such as fiber optic coupled scintillation dosimeters (Suchowerska et al., 2007, Therriault-Proulx et al., 2011) and metal oxide semiconductor field effect transistors (MOSFETs) (Zilio et al., 2006, Fagerstrom et al., 2008) have recently been introduced to perform in vivo dosimetry. In particular, MOSFETs show many advantages, such as good spatial resolution, high sensitivity, real-time read-out without deterioration of information, negligible radiation field perturbation owing to their small size and ease of use. In particular, great interest was dedicated to the application of MOSFETs to BT, because the typical large dose gradients achieved in BT necessitate a small detector with a reduced active volume for accurate dosimetry. In this work, a specific type of MOSFET dosimeter called “MOSkin” which was developed by the Center for Medical Radiation Physics (CMRP) of the University of Wollongong (Australia) (Qi et al., 2007, Kwan et al., 2008, Kwan et al., 2009) has been studied.

High dose rate (HDR) prostate BT allows the delivery of local and high conformal dose directly into the tumor, minimizing exposure of the surrounding healthy tissues. Due to the large dose delivered to the target in a single fraction and the dose constraints to be simultaneously satisfied for organs at risk, it is very important to have as small as possible discrepancy between planned and delivered dose. The development and application of reliable and accurate methods for monitoring the dose delivered to critical organs is therefore crucial.

Among these organs at risk, the urethra is most likely susceptible to acute and/or late toxicity resulting from the treatment (i.e. urethritis, stenosis), as it is inside the target volume (Fig. 1a). However, its localization for treatment planning purposes is particularly difficult due to images artefacts generated by the presence of source catheters, especially if transrectal ultrasound imaging is performed. Moreover, source catheters are themselves difficult to be accurately localized on the same images and therefore calculated dose distributions are susceptible to inaccuracies (Fig. 1b). The real-time dosimetry in the urethra is therefore very important and will be supplementary to reinforce existing QA programs.

Studies aimed at characterizing the dosimetric properties of MOSkin dosimeters have already demonstrated that they are promising instruments for performing in vivo dosimetry during HDR BT treatments (Qi et al., 2007, Qi et al., 2012, Kwan et al., 2008). Measurements finalized to detect the accuracy of the dosimeters and the change in sensitivity as a function of depth and angle of incidence of the radiation, have already shown good agreement between MOSkin response and dose calculated by the TPS (Hardcastle et al., 2010). Aim of this work was to study and develop the applicability of the MOSkin dosimeters for urethral dose measurement in prostate HDR BT.