Development of a TLD-100 based set up for in vivo dosimetry in Intraoperative Electron Beam Radiation Therapy (IOERT): an experimental and clinical evaluation.

Purpose: This study presents an in-house developed set-up enabling the placement of multiple TLDs over the target region, for in vivo dosimetry in intraoperative electron beam radiotherapy (IOERT).

Methods and materials: TLD-100 (LiF:MgTi) rods were calibrated at 10 Gy and their response was determined for the nominal electron energies of the Mobetron LINAC within the dose range from 4 Gy to 20 Gy. Irradiation of various set-ups was performed using the ionisation chambers (IC) ROOS parallel plate 34001 (PTW, Freiburg) and 3D Semiflex 31021 (PTW, Freiburg), a microdiamond detector 60019 (PTW, Freiburg) and EBT3 films (Ashland™) to investigate beam perturbations that may result due to the structure of the set up. EGSnrc Monte Carlo (MC) simulations evaluated the response of the TLDs in clinical beams of the available electron energies, the influence of the catheter to the TLD dose scoring and the depth dependence of the TLD dose assessment. TLD measurements in-phantoms and in patient in vivo were realised and compared to the expected doses estimated using data of water phantom measurements and 3D MC electron dose calculations of a dedicated IOERT treatment planning system (Radiance TPS- GmV, Tres Cantos, Madrid).

Results: MC and measurements verified that no energy correction is needed for the used electron beams. Correction factors for the dose non-linear response were evaluated. High resolution dose measurements showed local hot spots beneath the flap. However, there are no significant perturbations of the electron beam or on the dose delivery to the targeted volume. MC simulations demonstrated no signal attenuation due to the catheter and 1 % effect of the depth of TLD measurement relative to the depth of calibration was noted. TLD measurements in phantom set-ups agreed with expected doses with less than 2.6 % in phantoms and by 1 % in patient in vivo.

Conclusions: Our results demonstrate the suitability of using the implemented TLD-based workflow for in vivo dosimetry purposes in the operation room (OR) environment.