Year

2010

Degree Name

Doctor of Philosophy

Department

Engineering physics

Abstract

[Background and Purpose] Nasopharyngeal carcinoma (NPC) is a serious health problem in southern China. The continuing struggle to control tumor gives constant impetus to efforts to refine the various available treatment modalities. It has been widely recognized that only a well developed and disciplined quality assurance (QA) program can achieve a high degree of accuracy and reliability in radiation treatment of cancer patients. The main purpose of the present study is to introduce a new metal oxide semiconductor field effect transistor (MOSFET) dosimetry system as a QA tool for intensity modulated radiation therapy (IMRT) and high dose rate (HDR) brachytherapy.

[Materials and Methods] A comprehensive set of experiments has been performed to characterize the performance of the new miniature MOSFET detector, including the reproducibility, linearity, energy response, angular dependence, etc. The feasibility and efficiency of this MOSFET detector in absolute dose rate measurements for the HDR192Ir source and in accurate skin dose assessments for the IMRT treatment are explored, respectively. The new MOSFET detectors are then used in vivo to monitor the treatment delivery for both HDR intracavitary brachytherapy and serial tomotherapy IMRT. The preliminary results of in vivo measurements are presented.

[Results] The MOSFET detector, with a 5 V gate bias, shows a good dose linearity (R2=1), small angular effect (<2%) and flat energy response in high energy photon and electron fields. It can provide dose accuracy within measurement uncertainty for applied doses above 20 cGy and within ±1 mV for small doses less than 10 cGy.

The MOSFET dosimetry system, after proper calibration and correction, is applied to verify the dosimetric accuracy of HDR brachytherapy treatment plan. Results show the phantom verification method using MOSFET detectors is reliable and also very sensitive to errors such as source position inaccuracies, data transfer errors and source strength discrepancies.

The miniature MOSFET detector has a minimal but highly reproducible intrinsic buildup of 7 mg/cm2 corresponding to the requirement of personal surface dose equivalent Hp (0.07). Phantom measurements demonstrate that the MOSFET detector agrees well with the Attix chamber and the EBT Gafchromic® film in terms of surface and buildup region dose measurements, even for oblique incident beams. In our anthropomorphic phantom investigation, an overestimation of up to 8.5% in surface dose calculations has been found for a commercial treatment planning system (TPS). Similar trend has also been observed through in vivo patient dosimetry during IMRT treatments.

From 2007 till now, a total of 70 in vivo measurements in 11 NPC patients receiving HDR intracavitary brachytherapy have been performed at Sun Yat-sen University Cancer Center (SYSUCC), China using the new MOSFET dosimetry system. Results indicate good agreement between theory and experiment. Less than ±7% deviation from the planned dose is measured in all patients.

Another in vivo dosimetry practice with miniature MOSFET detectors has also been conducted at SYSUCC for serial tomotherapy IMRT. The mean difference between 48 MOSFET measured doses and their calculated values for 8 NPC patients is 3.33%, ranging from -2.2% to 7.89%. More than 90% of the total measurements are less than 5% deviation from the planned doses.

[Conclusions] The miniature MOSFET detector, due to its small physical size, ease of use and real time readout, provides a useful QA tool not only for routine in vivo dosimetry, but also for more advanced techniques such as IMRT and HDR brachytherapy. The dosimetry methods we presented here are universal and can also be applied to other cancer treatments receiving radiotherapy.

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