Experimental evaluation of a MOSFET dosimeter for proton dose measurements

Abstract

The metal oxide semiconductor field-effect transistor (MOSFET) dosimeter has been widely studied for use as a dosimeter for patient dose verification. The major advantage of this detector is its size, which acts as a point dosimeter, and also its ease of use. The commercially available TN502RD MOSFET dosimeter manufactured by Thomson and Nielsen has never been used for proton dosimetry. Therefore we used the MOSFET dosimeter for the first time in proton dose measurements. In this study, the MOSFET dosimeter was irradiated with 190 MeV therapeutic proton beams. We experimentally evaluated dose reproducibility, linearity, fading effect, beam intensity dependence and angular dependence for the proton beam. Furthermore, the Bragg curve and spread-out Bragg peak were also measured and the linear-energy transfer (LET) dependence of the MOSFET response was investigated. Many characteristics of the MOSFET response for proton beams were the same as those for photon beams reported in previous papers. However, the angular MOSFET responses at 45, 90, 135, 225, 270 and 315 degrees for proton beams were over-responses of about 15%, and moreover the MOSFET response depended strongly on the LET of the proton beam. This study showed that the angular dependence and LET dependence of the MOSFET response must be considered very carefully for quantitative proton dose evaluations.

Investigation of an implantable dosimeter for single‐point water equivalent path length verification in proton therapy

“…Equation ͑3͒ can provide a qualitative explanation to the observed LET dependence in MOSFET detectors in measuring the dose of a Bragg peak. 13 It shows that for a given electric field with a fixed magnitude and orientation, i.e., a fixed value for the parameters , the fraction of charge pairs escaping recombination would decrease as the number of charge pairs created by the particle N 0 increases. At any point along the particle track, N 0 should be proportional to the LET at the point.…”

“…12 In a recent study of a MOSFET dosimeter for proton therapy, the dosimeter was found to undervalue the dose substantially around the Bragg peak where LET increases dramatically. 13 As will be shown, although the method for range verification under study here uses only the dose ratio rather than the dose itself, the LET effect will not cancel out. A correction method had to be developed for accurately calculating the response of MOS-FET detectors to proton beams.…”

“…12 It was demonstrated in a recent study that a MOSFET detector produced increasingly lower reading than an ion chamber toward the distal aspect of the Bragg peak depth-dose distribution. 13 It is necessary to model the effect of this LET dependence on the dose ratios to avoid having to measure the ratios at multiple depths for every possible SOBP and every individual choice of how to split those SOBPs into complimentary field pairs. The observed LET dependence may be attributed to electron-hole recombination among the charge pairs created by the particle beam.…”

“…Consequently, the net number of charge pairs detected by the device per unit dose actually decreases, resulting in the underdetection of the dose near the Bragg peak as reported. 13 The measurement reported in Ref. 13 was conducted with an external bias voltage over the MOSFET dosimeter during the irradiation, as was the practice for most of the applications of such dosimeters.…”

“…13 The measurement reported in Ref. 13 was conducted with an external bias voltage over the MOSFET dosimeter during the irradiation, as was the practice for most of the applications of such dosimeters. The DVS dosimeters, on the other hand, have no bias available, being implanted in the patient.…”