Speaker
Description
Introduction:
Radiation therapy using protons and heavier ions, referred to as ion-beam therapy in the following, is a highly precise form of radiation therapy. A prerequisite for the best therapeutic outcome is patient-specific treatment planning. It includes the calculation and optimization of the physical dose distribution in the patient and as another key parameter the efficiency of biological damage. While physical dose is a clinical parameter routinely measured during treatment quality assurance (QA), radiation quality in the form of linear energy transfer (LET)—which is considered the primary factor governing the relationship between physical dose and the actual biological effect—is not yet sufficiently monitored in proton and helium ion therapy.
To date, RBE models often rely on LET spectra obtained through Monte Carlo (MC) simulations. Although electronic energy loss of ions is well modelled, nuclear interaction cross sections and inconsistencies of different MC-scoring options can cause uncertainties in LET (especially dose-averaged $LET_D$) of several ten percent [1]. Hence, LET measurements that can verify MC simulations are of great interest. However, measurement devices and methods are still rare and partly not suitable for QA purposes.
For this reason, we have been conducting a project in Heidelberg over the past three years aimed at developing a method for measuring LET distributions with the potential for future use in clinical quality assurance. Key findings of this work will be presented in this contribution.
Methods:
We have chosen silicon pixel detectors as primary technology, since they are active detectors and enable the detection and energy-deposition measurement of individual ions. The Timepix3 detector [2] combined with an AdvaPIX readout interface offers plug-and-play characteristics and therefore user-friendly handling.
Results:
After optimization of operating parameters and essential correction of a detector artifact termed “cheese clusters”, LET spectra of monoenergetic proton, helium, carbon, and oxygen-ion beams could be successfully measured in a partially depleted silicon sensor. Deviations between the measured and MC-simulated mean values of the spectra ($LET_t$) were all below 7%. Thus, the first milestone of accurate LET measurements up to $LET_t$ ≈ 100 keV/µm in silicon has been achieved that opens the possibility of measuring LET in proton and helium-ion beams at the end of their range [3].
Building on this, we were able to apply the method in mixed ion fields to test it under more realistic conditions for clinical proton and helium ion therapy. LET spectra, dose-and track-averaged LET values ($LET_D, LET_t$) and resulting RBEs were measured for a total of 20 positions along depth-dose curves of proton and helium ion beams at ~149 MeV/u. Deviations from detailed MC simulations were below 10% for both ion types in front of the Bragg peak (within 1σ uncertainty) and below 17% (within 3σ) in the region of the Bragg peak. This already represents a promising agreement [4].
A key insight that made this possible is that ions stopping inside the detector have a significant influence on $LET_D$ measurements and are by no means negligible in the Bragg peak region. Consequently, particle track measurements with two detector planes restricting the analysis to particles reaching both detector planes would lead to significant underestimations and are not suitable in complex mixed ion fields.
Conclusion:
Overall, the developed method based on a single silicon pixel detector is a promising and practical tool for measuring $LET_D$ during optimized quality assurance in the field of ion-beam therapy. An outlook towards next essential steps for future clinical implementation will be presented.
References
1. Granville, D.A. and G.O. Sawakuchi, Physics in Medicine & Biology, 2015. 60(14): p. N283.
2. Poikela, T., et al., Journal of Instrumentation, 2014. 9(05): p. C05013–C05013.
3. Félix-Bautista, R., et al., Physics in Medicine & Biology, 2019. 69(12): p. 125030.
4. Hamad, Y., et al., Medical Physics, 2025. 52(9): p. e18085.