MOSFET sensitivity dependence on integrated dose from high‐energy photon beams

Abstract

The ability of a commercially available dual bias, dual MOSFET dosimetry system to measure therapeutic doses reproducibly throughout its vendor-defined dose-based lifetime has been evaluated by characterizing its sensitivity variation to integrated/cumulative doses from,high-energy (6 and 15 MV) photon radiotherapy beams. The variation of sensitivity as a function of total integrated dose was studied for three different dose-per-fraction levels; namely, 50, 200, and 1200 cGy/fraction. In standard sensitivity mode (i.e., measurements involving dose-per-fraction levels > or =100 cGy), the response of the MOSFET system to identical irradiations increased with integrated dose for both energies investigated. Dose measurement reproducibility for the low (i.e., 50 cGy) dose fractions was within 2.1% (if the system was calibrated before each in-phantom measurement) and 3.1% [if the system was calibrated prior to first use, with no intermediate calibration(s)]. Similarly, dose measurement reproducibility was between 2.2% and 6.6% for the conventional (i.e., 200 cGy) dose fractions and between 1.8% and 7.9% for escalated (i.e., 1200 cGy) dose fractions. The results of this study suggest that, due to the progressively increasing sensitivity resulting from the dual-MOSFET design, frequent calibrations are required to achieve measurement accuracy of < or =3% (within one standard deviation).

Evaluation of the 4D RADPOS dosimetry system for dose and position quality assurance of CyberKnife

“…For film, the percent corrections for imaging dose were 0.3% (static) and 0.8% to 1.1% (dynamic). Dosimetric uncertainties for RADPOS and film measurements are composed of beam delivery and dosimetry uncertainties combined (amounting to approximately 1%), Solid Water vs water differences (0.7%) and detector measurement reproducibility (2% for single film measurement, 1% for single RADPOS measurement). Taking these sources of uncertainty into account, the total film uncertainty (single measurement) was approximately 2.3%, while total RADPOS uncertainty (single measurement) was approximately 1.6%.…”

Energy dependence corrections to MOSFET dosimetric sensitivity

“…This is known to be caused by alterations in the effective electric field applied to the MOSFET during irradiations which causes an accumulation of holes at the Si-SiO2 interface 28 . Depending on the performance characteristics of the MOSFET device, eg, dual versus singular bias voltage) this effect can be larger or smaller for an individual type of device 29 . Figure 3 shows the decreasing sensitivity of the CSDS MOSFET (UOWCentre for Medical Radiation Physics) dosimeters as a function of applied dose for 3 energies, 100, kVp, 250 kVp and 6MVp.…”

“…Another aspect of MOSFET dosimetry is the sensitivity dependence to integrated dose history of a MOSFET device. It is known in general that the sensitivity of the device can change with dose history [28][29] . We also plan to investigate this effect using the newly designed CSDS MOSFET devices at superficial, orthovoltage and megavoltage x-ray energies for comparison.…”

Peripheral dose measurement in high-energy photon radiotherapy with the implementation of MOSFET

“…In general, it is of extreme importance to calculate the PD down to the level of 0.1% of the central axis maximum dose (dmax) [8] and its determination has been the subject of extensive investigation [2,[9][10][11][12][13][14][15][16][17][18][19][20] . Metal oxide semiconductor field effect transistor (MOSFET) is used as a clinical dosimeter for radiotherapy beams, and mobileMOSFET seems to be the appropriate dose verification system [21][22][23][24][25][26][27][28][29][30] , since due to its small size it can be positioned very easily on the patient's skin, and can evaluate the delivered dose both at the target and at organs at risk [21] . This paper aims to assess the PD in high-energy photon beam radiotherapy as a function of the distance from the edge of the field, the depth, the field size and the energy of the photon beam, while the overall accuracy has been investigated by comparing the derived experimental results to corresponding ones obtained with an ionization chamber.…”