Experience of using MOSFET detectors for dose verification measurements in an end-to-end 192Ir brachytherapy quality assurance system

Publication date: Available online 27 October 2017
Source:Brachytherapy
Author(s): Maria Persson, Josef Nilsson, Åsa Carlsson Tedgren
PurposeEstablishment of an end-to-end system for the brachytherapy (BT) dosimetric chain could be valuable in clinical quality assurance. Here, the development of such a system using MOSFET (metal oxide semiconductor field effect transistor) detectors and experience gained during 2 years of use are reported with focus on the performance of the MOSFET detectors.Methods and MaterialsA bolus phantom was constructed with two implants, mimicking prostate and head & neck treatments, using steel needles and plastic catheters to guide the ¹⁹²Ir source and house the MOSFET detectors. The phantom was taken through the BT treatment chain from image acquisition to dose evaluation. During the 2-year evaluation-period, delivered doses were verified a total of 56 times using MOSFET detectors which had been calibrated in an external ⁶⁰Co beam. An initial experimental investigation on beam quality differences between ¹⁹²Ir and ⁶⁰Co is reported.ResultsThe standard deviation in repeated MOSFET measurements was below 3% in the six measurement points with dose levels above 2 Gy. MOSFET measurements overestimated treatment planning system doses by 2–7%. Distance-dependent experimental beam quality correction factors derived in a phantom of similar size as that used for end-to-end tests applied on a time-resolved measurement improved the agreement.ConclusionsMOSFET detectors provide values stable over time and function well for use as detectors for end-to-end quality assurance purposes in ¹⁹²Ir BT. Beam quality correction factors should address not only distance from source but also phantom dimensions.



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