S Hoefel¹*, M Fix², M Drescher³, F Zwicker⁴, (1)University of Konstanz ,Konstanz, ,DE, (2) Inselsptial - University Hosptial Bern, Bern, BE, CH, (3) University of Konstanz ,Konstanz, ,DE(4) German Cancer Research Center (DKFZ), Heidelberg, ,DE

PO-GePV-T-196 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: This phantom study investigates the applicability of Electron Paramagnetic Resonance (EPR) dosimetry for measuring superficial total doses delivered by helical tomotherapy treatments.

Methods: For this purpose, eight EPR dosimeters were attached to distinct spots next to the surface of an anthropomorphic head phantom: i) within a superficial simultaneously integrated boost (SIB) volume, ii) near to organs at risk including the parotids and the lenses, iii) at the thyroid lying out-of-field. Each dosimeter consisted of two pellets: one lithium formate monohydrate (LFM) pellet and one L-alanine (ALA) pellet. A helical tomotherapy plan representing a realistic head and neck treatment was applied daily over a period of about 6 weeks (32 fractions). Planned total doses to the dosimeters ranged from about 2 Gy (lenses, thyroid) to 64 Gy (SIB volume). EPR read out was performed by means of a previously proposed EPR dosimetry system after all fractions were applied. The results were compared to thermoluminescence dosimeter (TLD) measurements and to planned total doses derived from the treatment planning system (TPS).

Results: By taking uncertainties into account, EPR measurements were in agreement with TLD and the TPS. Exceptions occurred at the thyroid (out-of-field) and lenses (extreme sparing). Remarkably, each LFM and ALA pellet placed inside the SIB volume provided dose values that were within plus/minus 3% of the respective planned dose. LFM and ALA provided comparable results. However, LFM is preferable at lower doses due to lower measuring uncertainties.

Conclusion: This study suggests that EPR dosimetry is a valuable tool for measuring superficial total doses in-vivo being applicable in a wide dose range and in various irradiation conditions.

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