Review of solid-state detectors for out-of-field dosimetry and microdosimetry in particle therapy

Fragments and neutrons produced by inelastic reactions in particle therapy can make a significant contribution to the Relative Biological Effectiveness (RBE) and as a consequence the biologically weighted-dose in the distal part of the Spread-out Bragg Peak (SOBP); this also make changes to the radiation quality and dose equivalent in surrounding organs near the target tumour. These mixed radiation fields of secondary fragments and neutrons are difficult to characterise which is why this paper reviews some solid state radiation detectors that can measure the radiation field produced out of primary field in particle therapy, and also assess the radiation quality and dose equivalent to normal organs located near the target volume. These detectors include active types such as the silicon on insulator (SOI) microdosimeter, the monolithic Δ E-E telescope, the pixel detectors TimePix3, and passive detectors such as CR-39 plastic charged particle track detector, and the fluorescent nuclear track detector (FNTD). This paper also reports on the microdosimetric measurements obtained with silicon-based microdosimeters, both laterally and downstream of the target volume, for proton and carbon ion beams. It also details the measurements taken by the TimePix3 detector, including a customised, miniaturized version with enhanced resolving power, for characterizing secondary radiation in proton therapy through single-particle tracking. The CR-39 and FNTD passive linear energy transfer (LET) detectors are also reviewed and the advantages and disadvantages of each type of detector are discussed. The SOI microdosimeter and Δ E-E telescope are suitable for characterising the typical mixed radiation field for out-of-field in particle therapy. An SOI microdosimeter can provide dose equivalent values to evaluate risk to normal organs, while the Δ E-E telescope can operate as a microdosimeter and distinguish the type of particles in the secondary radiation field. The TimePix3 chip can provide mixed field decomposition, particle flux, dose rate, and LET spectra while the CR-39 plastic detector could measure the LET spectra, absorbed dose, and dose equivalent from secondary particles of Carbon ion beam via nuclear targeted fragmentation reactions. • Solid state radiation detectors for characterization of radiation out of field in proton and heavy ion therapy. • Measurements of dose equivalent using an SOI microdosimeter outside treatment volume for ocular treatment with protons. • Characterise the secondary radiation field in proton therapy using miniature pixel detector with TimePix3 chip. • LET spectra, absorbed dose and dose equivalent measurements using CR-39 and FNTD detectors.