In gynecological radiotherapy with high dose rate (HDR)192Ir brachytherapy, the treatment complexity has increased due to improved optimization techniques and dose constraints. As a consequence, it has become more important to verify the dose delivery to the target and also to the organs at risk (e.g., the bladder). In vivodosimetry, where dosimeters are placed in or on the patient, is one way of verifying the dose but until recently this was hampered by motion of the radiation detectors with respect to the source. The authors present a novel dosimetry method using a position sensitive radiation detector.Methods:
The prototype RADPOS system (Best Medical Canada) consists of a metal oxide field effect transistor (MOSFET)dosimeter coupled to a position-sensor, which deduces its 3D position in a magnetic field. To assess the feasibility ofin vivodosimetry based on the RADPOS system, different characteristics of the detector need to be investigated. Using a PMMA phantom, the positioning accuracy of the RADPOS system was quantified by comparing position readouts with the known position of the detector along the x and y-axes. RADPOS dose measurements were performed at various distances from a Nucletron192Ir source in a PMMA phantom to evaluate the energy dependence of the MOSFET. A sensitivity analysis was performed by calculating the dose after varying (1) the position of the RADPOS detector to simulate organ motion and (2) the position of the first dwell position to simulate errors in delivery. The authors also performed an uncertainty analysis to determine the action level (AL) that should be used during in vivodosimetry.Results:
Positioning accuracy is found to be within 1 mm in the 1-10 cm range from the origin along the x-axis (away from the transmitter), meeting the requirements forin vivodosimetry. Similar results are obtained for the other axes. The ALs are chosen to take into account the total uncertainty on the measurements. As a consequence for in vivodosimetry, it is determined that the RADPOS sensor, if placed, for example, in the bladder Foley balloon, would detect a 2 mm motion of the bladder, at a 5% chance of a false positive, with an AL limit of 9% of the dose delivered. The authors found that source position errors, caused by, e.g., a wrong first dwell position, are more difficult to detect; indeed, with our single RADPOS detector, positioned in the bladder, dwell position errors below 5 mm and resulting in a dose error within 10%, could be detected in the tandem but not in the colpostats. A possible solution to improve error detection is to use multiple MOSFETs to obtain multiple dose values.Conclusions:
In this study, the authors proposed a dosimetry procedure, based on the novel RADPOS system, to accurately determine the position of the radiation dosimeter with respect to the applicator. The authors found that it is possible to monitor the delivered dose in a point and compare it to the predetermined dose. This allows in principle the detection of problems such as bladder motion/filling or source mispositioning. Further clinical investigation is warranted.
The authors would like to acknowledge the work of Dr. Esther Bloemen-van Gurp and Justyna Wittych during the early phase of the testing of the positioning accuracy. The authors would like also to thank Best Medical Canada for allowing us to test their prototype of the RADPOS system. Dr Reniers is supported by a Marie Curie Reintegration Grant No. (PIRG05-GA-2009-247878) and G. Landry is supported by a PGSD2 scholarship from the Natural Sciences and Engineering Research Council of Canada and by the O’Brien Foundation (New-Brunswick, Canada). This work is also supported by an Maastro-Atrium grant (2009-12) on Assessment of dose-effect relationship between radiation dose and complications in prostate brachytherapy.
II. MATERIALS AND METHODS
II.A. RADPOS system
II.B. Accuracy of the positioning sensors
II.C. Energy response of the MOSFET
II.D. Use of the RADPOS system for in vivo dosimetry
II.D.1. Catheter mapping
II.D.2. Effect of presence of metal
II.D.3. Measurements for a GYN plan
II.D.4. Sensitivity study
III.A. Positioning accuracy
III.B. Energy response of the MOSFET
III.C. In vivo dosimetry
III.C.1. Effect of presence of metal
III.C.3. Measurements for a GYN plan
III.C.4. Sensitivity study
- Position sensitive detectors
- Magnetic field sensors
Data & Media loading...
Article metrics loading...
Full text loading...