Practical issues regarding angular and energy response in in vivo intraoperative electron radiotherapy dosimetry

Abstract

Aim

To estimate angular response deviation of MOSFETs in the realm of intraoperative electron radiotherapy (IOERT), review their energy dependence, and propose unambiguous names for detector rotations.

Background

MOSFETs have been used in IOERT. Movement of the detector, namely rotations, can spoil results.

Materials and methods

We propose yaw, pitch, and roll to name the three possible rotations in space, as these unequivocally name aircraft rotations. Reinforced mobile MOSFETs (model TN-502RDM-H) and an Elekta Precise linear accelerator were used. Two detectors were placed in air for the angular response study and the whole set of five detectors was calibrated as usual to evaluate energy dependence.

Results

The maximum readout was obtained with a roll of 90° and 4[[ce:hsp sp="0.25"/]]MeV. With regard to pitch movement, a substantial drop in readout was achieved at 90°. Significant overresponse was measured at 315° with 4[[ce:hsp sp="0.25"/]]MeV and at 45° with 15[[ce:hsp sp="0.25"/]]MeV. Energy response is not different for the following groups of energies: 4, 6, and 9[[ce:hsp sp="0.25"/]]MeV; and 12[[ce:hsp sp="0.25"/]]MeV, 15[[ce:hsp sp="0.25"/]]MeV, and 18[[ce:hsp sp="0.25"/]]MeV.

Conclusions

Our proposal to name MOSFET rotations solves the problem of defining sensor orientations. Angular response could explain lower than expected results when the tip of the detector is lifted due to inadvertent movements. MOSFETs energy response is independent of several energies and differs by a maximum of 3.4% when dependent. This can limit dosimetry errors and makes it possible to calibrate the detectors only once for each group of energies, which saves time and optimizes lifespan of MOSFETs.