ESTRO 2021 Abstract Book

S127

ESTRO 2021

Purpose or Objective Proton therapy achieves very high dose conformity around the target, allowing a better protection of the organs at risk (decreasing radiation side effects) [1]. The determination of the relative biological effectiveness (RBE) of protons depends, among other factors, on the lineal energy transfer (LET). Currently, LET calculations are performed using Monte Carlo simulations, which might be affected by several uncertainties. Therefore, devices that help to reduce these uncertainties are essential. Likewise, there is a rising interest in the medical ‒ physics community in placing the enhanced LET of the beam within the tumour or removing it from the most sensitive normal structures around [2]. Nevertheless, there is no instrument to quantify the LET maps with high spatial resolution in clinical scenarios. In this work, we present the first 2D microdosimetry maps in a proton therapy facility in clinical conditions by means of an array of silicon 3D ‒ cylindrical The array of 11×11 independent microdetectors is based on a new diode architecture with 3D ‒ cylindrical electrodes (25 μm diameter, 20 μm depth, and 200 μm pitch) and an inner volume that matches the dimensions of the cellular nucleus. It covers 2.2 mm× 2.2 mm of total radiation sensitive surface. A pristine peak of 100 MeV protons was used in the Orsay Proton Therapy Centre (CPO, France) with clinical fluence rates (~ 10 8 cm -2 ·s -1 ). Measurements were performed at different depths along the Bragg peak with a phantom of solid ‒ water equivalent material. Results The microdosimetry spectra were obtained at different positions of the Bragg curve (Figure 1). Likewise, we present the first 2D maps of the lineal energy at a clinical facility (Figure 2) with the highest resolution so far (200 µm pitch covering a total area of 2.2 mm× 2.2 mm). microdetectors [3 ‒ 5]. Materials and Methods

Conclusion Microdosimetry measurements were performed at clinical fluence rates without saturation effects. The use of this device would allow medical physicists to guide beam arrangements to apply the LET ‒ painting technique and further RBE optimizations [2]. This work consolidates the capability of the new 3D ‒ cylindrical architecture as microdosimeters as well as commissioning under clinical conditions. [1] Schardt D. and Elsässer T., Reviews of Modern Physics, Vo. 82, 2010. [2] Bassler N, et al. Acta Oncol. 2010 Oct;49(7):1170-6. [3] Guardiola C., et al., Applied Physics Letters 107, 023505 (2015). [4] Prieto-Pena J. et al., IEEE Transactions on Nuclear Science, Vol. 66, No. 7, July 2019 [5] Bachiller-Perea D. et al., IEEE Transactions on Instrumentation and Measurement, vol. 70, pp. 1-11, 2021, Art no. 6005211, doi: 10.1109/TIM.2021.3054631.

OC-0199 Toward traceable dosimetry for electronic brachytherapy devices with skin applicators B. Reniers 1 , D. Colson 1 , M. Wieme 2 , S. Arits 3 , R. Federighi 2 , P. Aviles Lucas 4 , K. Elfried 4 , G.Y. Fernando 5 , J.