Intraoperative electron beam intercomparison of 6 sites using mailed thermoluminescence dosimetry: Absolute dose and energy.

PURPOSE: In 2018, the Netherlands Commission on Radiation Dosimetry subcommittee on IORT initiated a limited intercomparison of electron IORT (IOERT) in Belgium and The Netherlands. The participating institutions have enough variability in age, type of equipment, and in dose calibration protocols. METHODS: In this study, three types of IOERT-dedicated mobile accelerators were represented: Mobetron 2000, LIAC HWL and LIAC. Mobetron produces electron beams with energies of 6, 9 and 12 MeV, while LIAC HWL and LIAC can deliver 6, 8, 10 and 12 MeV electron beams. For all energies, the reference beam (10 cm diameter, 0° incidence) and 5 cm diameter beams were measured, as these smaller beams are used more frequently in clinic. The mailed TLD service from the Radiation Dosimetry Services (RDS, Houston, USA) has been used. Following RDS' standard procedures, each beam was irradiated to 300 cGy at dmax with TLDs around dmax and around depth of 50 % dose (R50). Absolute dose at 100 % and beam energy, expressed as R50, could be verified in this way. RESULTS: All absolute doses and energies under reference conditions were well within RDS-stated uncertainties: dose deviations were <5 % and deviations in R50 were <5 mm. For the small 5 cm beams, all results were also within acceptance levels except one absolute dose value. Deviations were not significantly dependent on manufacturer, energy, diameter and calibration protocol. CONCLUSIONS: All absolute dose values, except one of a non-reference beam, and all energy values were well within the measurement accuracy of RDS TLDs.

Narrowing the difference in dose delivery for IOERT and IOBT for locally advanced and locally recurrent rectal cancer.

Purpose: Intra-operative radiotherapy (IORT) has been used as a tool to provide a high-dose radiation boost to a limited volume of patients with fixed tumors with a likelihood of microscopically involved resection margins, in order to improve local control. Two main techniques to deliver IORT include high-dose-rate (HDR) brachytherapy, termed 'intra-operative brachytherapy' (IOBT), and electrons, termed 'intra-operative electron radiotherapy' (IOERT), both having very different dose distributions. A recent paper described an improved local recurrence-free survival favoring IOBT over IOERT for patients with locally advanced or recurrent rectal cancer and microscopically irradical resections. Although several factors may have contributed to this result, an important difference between the two techniques was the higher surface dose delivered by IOBT. This article described an adaptation of IOERT technique to achieve a comparable surface dose as dose delivered by IOBT. Material and methods: Two steps were taken to increase the surface dose for IOERT: 1. Introducing a bolus to achieve a maximum dose on the surface, and 2. Re-normalizing to allow for the same prescribed dose at reference depth. Conclusions: We describe and propose an adaptation of IOERT technique to increase surface dose, decreasing the differences between these two techniques, with the aim of further improving local control. In addition, an alternative method of dose prescription is suggested, to consider improved comparison with other techniques in the future.