ABSTRACT
BACKGROUND AND PURPOSE: The purpose of this work was to characterize metal oxide semiconductor field-effect transistors (MOSFETs) in a 6 MV conventional linac and investigate their use for quality assurance of radiotherapy treatments with a tomotherapy Hi-Art unit. MATERIALS AND METHODS: High sensitivity and standard sensitivity MOSFETs were first calibrated and then tested for reproducibility, field size dependence, and accuracy of measuring surface dose in a 6 MV beam as well as in a tomotherapy Hi-Art unit. In vivo measurements were performed on both a RANDO phantom and several head and neck cancer patients treated with tomotherapy and compared to TLD measurements and treatment plan doses to evaluate the performance of MOSFETs in a high gradient radiation field. RESULTS: The average calibration factor found was 0.345+/-2.5%cGy/mV for the high sensitivity MOSFETs tested and 0.901+/-2.4%cGy/mV for the standard sensitivity MOSFETs. MOSFET measured surface doses had an average agreement with ion chamber measurements of 1.55% for the high sensitivity MOSFET and 5.23% for the standard sensitivity MOSFET when averaged over all trials and field sizes tested. No significant dependence on field size was found for the standard sensitivity MOSFETs, however a maximum difference of 5.34% was found for the high sensitivity MOSFET calibration factors in the field sizes tested. Measurements made with MOSFETS on head and neck patients treated on a tomotherapy Hi-Art unit had an average agreement of (3.26+/-0.03)% with TLD measurements, however the average of the absolute difference between the MOSFET measurements and the treatment plan skin doses was (12.2+/-7.5)%. The MOSFET measured patient skin doses also had good reproducibility, with inter-fraction deviations ranging from 1.4% to 6.6%. Similar results were found from trials using a RANDO phantom. CONCLUSIONS: The MOSFETs performed well when used in the tomotherapy Hi-Art unit and did not increase the overall treatment set-up time when used for patient measurements. It was found that MOSFETs are suitable detectors for surface dose measurements in both conventional beam and tomotherapy treatments and they can provide valuable skin dose information in areas where the treatment planning system may not be accurate.
Subject(s)
Head and Neck Neoplasms/radiotherapy , Quality Assurance, Health Care , Radiometry/instrumentation , Tomography, Spiral Computed , Calibration , Equipment Failure Analysis , Humans , Radiotherapy Dosage , Semiconductors , Sensitivity and Specificity , Transistors, ElectronicABSTRACT
Using a 50 W x-ray tube as the fluorescing source, an x-ray fluorescence system was designed to measure arsenic in superficial layers of tissue-simulating phantoms. The detection limit for arsenic in the phantoms at a setting of 35 kV and with 200 microm of a Mo filter was 0.40 +/- 0.06 microg As g(-1). This measurement results in an effective dose of 0.6 microSv.
Subject(s)
Arsenic/analysis , Spectrometry, X-Ray Emission/methods , Arsenic/chemistry , Calibration , Electronics , Equipment Design , Humans , Models, Statistical , Monte Carlo Method , Phantoms, Imaging , Scattering, Radiation , Spectrometry, X-Ray Emission/instrumentation , X-RaysABSTRACT
An x-ray fluorescence measurement system has been developed with an 125I source to detect arsenic in superficial layers of phantoms and tissue. Based on in vivo measurements, in conjunction with Monte Carlo simulations, the detection limit for arsenic in skin ranges between 2.6+/-0.5 and 5.7+/-1.1 microg g(-1), depending on skin thickness and assuming that arsenic is uniformly distributed in the skin. The effect of skin arsenic distribution was also examined.
