Angular Dependence of the MOSFET Dosimeter and its Impact on In vivo Surface Dose Measurement in Breast Cancer Treatment

Abstract

The focus of this study is the angular dependence of two types of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters (MOSFET20 and OneDose/ OneDosePlus) when used for surface dose measurements. External beam radiationat different gantry angles were delivered to a cubic solid water phantom with a MOSFET placed on the top surface at CAX. The long axis of the MOSFET was oriented along the gantry axis of rotation, with the dosimeter (bubble side) facing the radiation source. MOSFET-measured surface doses were compared against calibrated radiochromic film readings. It was found that both types of MOSFET dosimeters exhibited larger than previously reported angular dependence when measuring surface dose in beams at large oblique angles. For the MOSFET20 dosimeter the measured surface dose deviation against film readings was as high as 17% when the incident angle was 72 degrees to the norm of the phantom surface. It is concluded that some MOSFET dosimeters may have a strong angular dependence when placed on the surface of water-equivalent material, even though they may have an isotropic angular response when surrounded by uniform medium. Extra on-surface calibration maybe necessary before using MOSFET dosimeters for skin dose measurement in tangential fields.

Surface and Buildup Region Dose Measurements with Markus Parallel-Plate Ionization Chamber, GafChromic EBT3 Film, and MOSFET Detector for High-Energy Photon Beams

“…Scalchi et al [27] demonstrated surface angular dependence on a special unencapsulated MOSFET dosimeter; and Qi et al [20] measured the angular dependence at surface with Attix ionization chamber and MOSkin detector, finding that the oblique beam incidence increases the surface dose. Qin et al [28] found the maximum deviation at an oblique angle of 72 ∘ . The epoxy side of the MOSFET dosimeter showed less angular dependence at 30 ∘ and 60 ∘ while the result of 80 ∘ was still high.…”

Contact lens‐type ocular in vivo dosimeter for radiotherapy

“…Small-sized detectors, such as optically stimulated luminescent dosimeters and metal oxide semiconductor fieldeffect transistors (MOSFETs), can be used to measure the surface dose during treatment. [23][24][25][26] The available in vivo dosimeters for eye lenses can be applied to measure the dose on the skin of the eyelids. However, this surface dose does not represent the eye lens dose.…”

“…As mentioned earlier, a few eye lens dosimeters have been introduced in the field of radiation protection and several in vivo dosimeters have been introduced in the field of radiotherapy. Small‐sized detectors, such as optically stimulated luminescent dosimeters and metal oxide semiconductor field‐effect transistors (MOSFETs), can be used to measure the surface dose during treatment . The available in vivo dosimeters for eye lenses can be applied to measure the dose on the skin of the eyelids.…”

Validation of OSLD and a treatment planning system for surface dose determination in IMRT treatments

“…Practical detectors include TLD, surface diodes, and MOSFETS and each has potential accuracy limitations that are caused by factors such as a complicated readout process, temperature dependence, and angular dependence. 12,13 OSLD is particularly attractive for surface dose measurement because the measuring element is thin, easy to handle and position on the skin, and has a track record for accurate dosimetric results in other contexts. Yukihara et al 14 evaluated Al 2 O 3 :C OSLD (Luxel, Landauers, Inc., Glenwood, IL) and reported that the OSLD measurements agree with ion chamber measurements within ±1% in low gradient regions and in high gradient regions the distance-to-agreement (DTA) is between 0.5 and 1.0 mm.…”