miniTEPC

MICRODOSIMETRY OF HADRON THERAPY BEAMS USING MINI TISSUE-EQUIVALENT PROPORTIONAL COUNTERS

Along with the absorbed dose, the radiation quality is fundamental to characterise the radiation treatments in terms of their effects on cells. One of the two methods to assess the radiation quality is based on computational tools and verify the Linear Energy Transfer (LET) spectra, analytically or using Monte Carlo simulations, the Linear Energy Transfer (LET) spectra. The other is based on experimental measurements performed with microdosimeters to obtain the lineal energy spectra.

In carbon ion therapy, the radiation quality significantly changes significantly inside the treated volume, and such variations are kept into account to optimize the clinical treatment plan. There is a growing consensus about the idea that radiation quality variations should be accounted also for proton beams.

Tissue-Equivalent gas Proportional Counters (TEPCs) are the reference microdosimeters for such purpose. They are gas-filled spherical or cylindrical chambers, in which each ionization event traversing the sensitive volume gives rise to a signal pulse proportional to the ionization density of each charged particle that interacted with the detector. However, they typically have limited capability in measuring at counting rates higher than 106 s-1 due to the spectral distortions arising from the signal pile up.

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dett-minitepc

A WALLED MINI-TEPC WAS DEVELOPED BY INFN LEGNARO LABORATORIES (ITALY)

to cope with the beam intensities that are only moderately decreased compared to therapeutic conditions, and that provides distortion-free microdosimetric spectra. The cylindrical cavity is 0.9 mm in diameter and 0.9 mm in height corresponding to a sensitive volume of 0.6 mm3. It is inserted in a titanium sleeve of 2.7 mm and 20 cm length. The external size of this mini-TEPC of 2.7 mm is compatible with an in-vivo use of this microdosimeter.

Additionally, the detector small size reduces the depth-dose curve displacement effect and fluence perturbation. Hence, they allow performing microdosimetric measurements

with good spatial resolution also inside the relatively small Bragg peak region and on the distal edge, where the radiation quality grows very rapidly and the ion yield has a fast decrease.

The capability of mini-TEPC to perform microdosimetric measurements in therapeutic carbon ion beams was tested at CNAO facility (Pavia, Italy) [1]. Using a mono-energetic beam of 189.5 MeV/u directed toward a water phantom, the measurements were performed moving the mini TEPC at different depths both, at the Bragg peak proximal edge and distal edge.

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Microdosimetric spectra for 12C beams at seven different depths in the water phantom. The area under the curve for each spectrum in a given energy range is proportional to the relative contribution of the events in this range to the total absorbed dose [1]

DE.TEC.TOR. has started the industrialization of such technology, and, in collaboration with EBG MedAustron, it will contribute to the application of its routine use in clinical facilities.

[1] Conte, V. et al., (2017). Microdosimetric measurements at the CNAO active-scanning carbon-ion beam, EPJ Web of Conferences 153, 01012.

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