Measurement of microdosimetric spectra of high-LET particle beams using the ENCORE detector

Heavy ion radiotherapy offers distinct advantages over conventional treatments due to its superior dose distribution and enhanced biological effectiveness. However, the microdosimetric characteristics of high linear energy transfer (LET) ions, particularly near the Bragg peak, remain poorly characterized. Most experimental microdosimetry has been conducted at lower LETs, leaving a critical gap in high-LET microdosimetric data.

This study aims to extend microdosimetry to several different high-LET ions (700-2500 keV/µm) using the novel ENCORE detector.

Microdosimetry was performed at the Florida State University John D. Fox Accelerator Laboratory using the ENCORE detector, a multi-sampling ionization chamber filled with low-pressure tissue-equivalent gas capable of resolving energy deposition from approximately 200 keV to 3 MeV. ¹²C, ¹⁶O, and ²⁸Si beams were accelerated to 17-55 MeV and delivered to the segmented detector. Energy deposition patterns near the Bragg peak were recorded, and microdosimetric spectra were reconstructed. Monte Carlo was used to model the experimental setup. From each measured and simulated spectra, dose-mean lineal energy ( ${\overline{y}}_D$ ) was calculated and compared.

The 16 strips of the ENCORE corresponded to a tissue-equivalent thickness of 17.6 µm, enabling resolution of energy deposition immediately proximal and distal to the Bragg peak. In six measurement configurations, the ion beam stopped within this range, allowing precise mapping of the ${\overline{y}}_D$ profile around the peak; in the remaining configurations, the beam fully traversed the detector. Across all beams, difference between measured and simulated ${\overline{y}}_D$ averaged 12%, based on one measurement per strip. Peak measured ${\overline{y}}_D$ reached 920 ± 220, 1240 ± 206, and 2460 ± 620 keV/µm for ¹²C, ¹⁶O, and ²⁸Si beams, respectively.

This study demonstrated that ENCORE can resolve microdosimetric spectra with high spatial and LET resolution across a range of ion energies. Measured ${\overline{y}}_D$ values reasonably agreed with Monte Carlo simulations, validating the potential of ENCORE for position-resolved microdosimetry and model benchmarking in particle therapy.