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The interest in hadron therapy is growing fast thanks to the latest technological advances in accelerators and delivery technologies, to the development of more and more efficient and comprehensive treatment planning tools, and due to its increasing clinical adoption proving its efficacy. A precise and reliable beam quality assessment and an accurate and effective inclusion of the biological effectiveness of different radiation qualities are fundamental to exploit at best its advantages with respect to conventional radiotherapy. Currently, in clinical practice, the quality assurance (QA) is carried out by means of conventional dosimetry, while the biological effectiveness of the radiation is taken into account considering the Relative Biological Effectiveness (RBE). The RBE is considered a constant value for protons and it is estimated as a function of the absorbed dose in case of carbon ions. In this framework, microdosimetry could bring a significant improvement to both QA and RBE estimation. By measuring the energy deposited by the radiation into cellular or sub-cellular volumes, microdosimetry could provide a unique characterisation of the beam quality on one hand, and a direct link to radiobiology on the other. Different detectors have been developed for microdosimetry, from the more conventional tissue equivalent proportional counter (TEPC), silicon-based and diamond-based solid-state detectors, to ΔE-E telescope detectors, gas electrons multiplier (GEM), hybrid microdosimeters and a micro-bolometer based on Superconducting QUantum Interference Device (SQUID) technology. However, because of their different advantages and drawbacks, a standard device and an accredited experimental methodology have not been unequivocally identified yet. The establishment of accepted microdosimetry standard protocols and code of practice is needed before the technique could be employed in clinical practice. Hoping to help creating a solid ground on which future research, development and collaborations could be planned and inspired, a comprehensive state of the art of the detector technologies developed for microdosimetry is presented in this review, discussing their use in clinical hadron therapy conditions and considering their advantages and drawbacks.

Measurements in the stray radiation field from a proton therapy pencil beam at energies 70 and 146 MeV were performed using microdosimetric tissueequivalent proportional counters (TEPCs). The detector volumes were filled with a propane-based tissue-equivalent gas at low pressure simulating a mean chord length of 2 mmin tissue. Investigations were performed with and without a beam range shifter, and with different air gaps between the range shifter and a solid water phantom. The absorbed dose, the dose-mean lineal energy, and the dose equivalent were determined for different detector positions using the variance-covariance method. The influence from beam energy, detector- and range-shifter positions on absorbed dose, LET, and dose equivalent were investigated. Monte Carlo simulations of the fluence, detector response, and absorbed dose contribution from different particles were performed with MCNP 6.2. The simulated dose response for protons, neutrons, and photons were compared with, and showed good agreement with, previously published experimental data. The simulations also showed that the TEPC absorbed dose agrees well with the ambient absorbed dose for neutron energies above 20 MeV. The results illustrate that changes in both dose and LET variations in the stray radiation field can be identified from TEPC measurements using the variance-covariance method. The results are in line with the changes seen in the simulated relative dose contributions from different particles associated with different proton energies and range-shifter settings. It is shown that the proton contribution scattered directly from the range shifter dominates in some situations, and although the LET of the radiation is decreased, the ambient dose equivalent is increased up to a factor of 3. [ABSTRACT FROM AUTHOR]

Measurements in the stray radiation field from a proton therapy pencil beam at energies 70 and 146 MeV were performed using microdosimetric tissue-equivalent proportional counters (TEPCs). The detector volumes were filled with a propane-based tissue-equivalent gas at low pressure simulating a mean chord length of 2 μm in tissue. Investigations were performed with and without a beam range shifter, and with different air gaps between the range shifter and a solid water phantom. The absorbed dose, the dose-mean lineal energy, and the dose equivalent were determined for different detector positions using the variance–covariance method. The influence from beam energy, detector- and range-shifter positions on absorbed dose, LET, and dose equivalent were investigated. Monte Carlo simulations of the fluence, detector response, and absorbed dose contribution from different particles were performed with MCNP 6.2. The simulated dose response for protons, neutrons, and photons were compared with, and showed good agreement with, previously published experimental data. The simulations also showed that the TEPC absorbed dose agrees well with the ambient absorbed dose for neutron energies above 20 MeV. The results illustrate that changes in both dose and LET variations in the stray radiation field can be identified from TEPC measurements using the variance–covariance method. The results are in line with the changes seen in the simulated relative dose contributions from different particles associated with different proton energies and range-shifter settings. It is shown that the proton contribution scattered directly from the range shifter dominates in some situations, and although the LET of the radiation is decreased, the ambient dose equivalent is increased up to a factor of 3.