Proton dose distribution measurements using a MOSFET detector with a simple dose-weighted correction method for LET effects

Abstract

We experimentally evaluated the proton beam dose reproducibility, sensitivity, angular dependence and depth-dose relationships for a new Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detector. The detector was fabricated with a thinner oxide layer and was operated at high-bias voltages. In order to accurately measure dose distributions, we developed a practical method for correcting the MOSFET response to proton beams. The detector was tested by examining lateral dose profiles formed by protons passing through an L-shaped bolus. The dose reproducibility, angular dependence and depth-dose response were evaluated using a 190 MeV proton beam. Depth-output curves produced using the MOSFET detectors were compared with results obtained using an ionization chamber (IC). Since accurate measurements of proton dose distribution require correction for LET effects, we developed a simple dose-weighted correction method. The correction factors were determined as a function of proton penetration depth, or residual range. The residual proton range at each measurement point was calculated using the pencil beam algorithm. Lateral measurements in a phantom were obtained for pristine and SOBP beams. The reproducibility of the MOSFET detector was within 2%, and the angular dependence was less than 9%. The detector exhibited a good response at the Bragg peak (0.74 relative to the IC detector). For dose distributions resulting from protons passing through an L-shaped bolus, the corrected MOSFET dose agreed well with the IC results. Absolute proton dosimetry can be performed using MOSFET detectors to a precision of about 3% (1 sigma). A thinner oxide layer thickness improved the LET in proton dosimetry. By employing correction methods for LET dependence, it is possible to measure absolute proton dose using MOSFET detectors.

Figure 1

Experimental arrangement for measurement of dose distribution (top view). The bolus was made of polyethylene.

Figure 2

MOSFET sensitivity for 200, 157, 150, 100 and 50 MeV proton beams.

Figure 3

Angular dependence of MOSFET detectors exposed to a 190 MeV proton beam. The correction value for the angular response of the MOSFET detector is also plotted.

Figure 4

Comparison of Bragg curves obtained using IC and MOSFET detectors at high‐bias setting for a 190 MeV proton beam. The correction factor for the response of the MOSFET detector was calculated as a function of proton penetration depth.

Figure 5

Comparison of SOBP obtained using IC and MOSFET detectors. The correction factor for the response of the MOSFET detector was calculated as a function of proton penetration depth.

Figure 6

Comparison of lateral‐dose distribution obtained using IC, uncorrected MOSFET (MOSFET) and corrected MOSFET detectors (MOSFET with Correction) at PE thicknesses of 0 (a), 100 (b), 105 (c), 110 (d) and 115 (e) mm for a 190 MeV mono‐energetic proton beam.

Figure 7

Comparison of lateral‐dose distribution measurements obtained using IC, uncorrected MOSFET (MOSFET) and corrected MOSFET detectors (MOSFET with Correction) at PE thicknesses of 0 (a), 50 (b) and 100 (c) mm for a SOBP proton beam.