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Volume 13, Number 3, 2008

Performance characteristics and commissioning of MOSFET as an in-vivo dosimeter for high energy photon external beam radiation therapy



AIM: In vivo dosimetry is an essential tool of quality assurance programmes in radiotherapy. In fact, the assessment of the fi nal uncertainty between the prescribed dose and the dose actually delivered to the patient is an effective way of checking the entire dosimetric procedure. Metal oxide semiconductor field effect transistors (MOSFETs) have recently been proposed for use in radiation therapy. The purpose of this work is to study the performance characteristics and to carry out the commissioning of MOSFET as an in-vivo dosimeter for high-energy photon external beam radiation therapy.
MATERIAL AND METHODS: Characterization and commissioning of low sensitivity TN502RD and high sensitivity TN1002RD MOSFETs for entrance and exit dosimetry respectively for application in in-vivo dosimetry in radiotherapy was carried out. The MOSFETs were characterized in terms of reproducibility, short-term constancy, long-term constancy, linearity, angular dependence, energy dependence, source to skin distance (SSD) dependence and field size dependence.
RESULTS: The reproducibility of standard sensitivity MOSFET is about 1.4% (1 SD) and 1.98% (1 SD) for high sensitivity detectors. The linearity of both MOSFETs was excellent (R2 = 0.996). The response of MOSFETs varies linearly for square fi elds from 3 x 3 cm2 to 30 x 30 cm2. For beam incidence ranging from ±45° the MOSFET response varies within ±3%. Commissioning of both MOSFETs was carried out in terms of entrance dose calibration factor, exit dose calibration factor, SSD correction factor, field size correction factor, wedge correction factor and shielding tray correction factor. The average calibration factor for low and high sensitivity MOSFET detectors is 0.9065 cGy/mV and 0.3412 cGy/mV respectively. The average SSD correction factors are quite small and vary between 0.968 and 1.027 for both types of detectors for the range of clinical SSDs from 80 cm to 120 cm. The fi eld size correction factor varies from 1.00 to 1.02 for both types of detectors. The wedge and the shielding tray correction
factors for both the detectors also show quite small variation. MOSFET characteristics are suitable for in vivo dosimetry of entrance and exit dose measurement relevant to 6 MV treatment.
CONCLUSION: It can be concluded that MOSFET dosimetry’s low energy dependence, high sensitivity and immediate readout make it a good replacement for TLD in radiation therapy dosimetry.

Signature: Rep Pract Oncol Radiother, 2008; 13(3) : 114-125


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