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Objective . Microdosimetry offers a fast tool for radiation quality (RQ) verification to be implemented in treatment planning systems in proton therapy based on variable LET or RBE to move forward from the use of a fixed RBE of 1.1. It is known that the RBE of protons can increase up to 50% higher than that value in the last few millimetres of their range. Microdosimetry can be performed both experimentally and by means of Monte Carlo (MC) simulations. This paper has the aim of comparing the two approaches. Approach . Experimental measurements have been performed using a miniaturized Tissue equivalent proportional counter developed at the Legnaro National Laboratories of the Italian National Institute for Nuclear Physics with the aim of being used as RQ monitors for high intensity beams. MC simulations have been performed using the microdosimetric extension of TOPAS which provides optimized parameters and scorers for this application. Main results . Simulations were compared with experimental microdosimetric spectra in terms of shape of the spectra and their average values. Moreover, the latter have been investigated as possible estimators of LET obtained with the same MC code. The shape of the spectra is in general consistent with the experimental distributions and the average values of the distributions in both cases can predict the RQ increase with depth. Significance . This study aims at the comparison of microdosimetric spectra obtained from both experimental measurements and the microdosimetric extension of TOPAS in the same radiation field., (© 2023 Institute of Physics and Engineering in Medicine.)

Purpose: To measure the environmental doses from stray neutrons in the vicinity of a solid slab phantom as a function of beam energy, field size and modulation width, using the proton pencil beam scanning (PBS) technique., Method: Measurements were carried out using two extended range WENDI-II rem-counters and three tissue equivalent proportional counters. Detectors were suitably placed at different distances around the RW3 slab phantom. Beam irradiation parameters were varied to cover the clinical ranges of proton beam energies (100-220MeV), field sizes ((2×2)-(20×20)cm 2 ) and modulation widths (0-15cm)., Results: For pristine proton peak irradiations, large variations of neutron H ∗ (10)/D were observed with changes in beam energy and field size, while these were less dependent on modulation widths. H ∗ (10)/D for pristine proton pencil beams varied between 0.04μSvGy -1 at beam energy 100MeV and a (2×2)cm 2 field at 2.25m distance and 90° angle with respect to the beam axis, and 72.3μSvGy -1 at beam energy 200MeV and a (20×20) cm 2 field at 1m distance along the beam axis., Conclusions: The obtained results will be useful in benchmarking Monte Carlo calculations of proton radiotherapy in PBS mode and in estimating the exposure to stray radiation of the patient. Such estimates may be facilitated by the obtained best-fitted simple analytical formulae relating the stray neutron doses at points of interest with beam irradiation parameters., (Copyright © 2017 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.)

The authors attempt to establish the relative biological effectiveness (RBE) calculation for designing therapeutic proton beams on the basis of microdosimetry. The tissue-equivalent proportional counter (TEPC) was used to measure microdosimetric lineal energy spectra for proton beams at various depths in a water phantom. An RBE-weighted absorbed dose is defined as an absorbed dose multiplied by an RBE for cell death of human salivary gland (HSG) tumor cells in this study. The RBE values were calculated by a modified microdosimetric kinetic model using the biological parameters for HSG tumor cells. The calculated RBE distributions showed a gradual increase to about 1cm short of a beam range and a steep increase around the beam range for both the mono-energetic and spread-out Bragg peak (SOBP) proton beams. The calculated RBE values were partially compared with a biological experiment in which the HSG tumor cells were irradiated by the SOBP beam except around the distal end. The RBE-weighted absorbed dose distribution for the SOBP beam was derived from the measured spectra for the mono-energetic beam by a mixing calculation, and it was confirmed that it agreed well with that directly derived from the microdosimetric spectra measured in the SOBP beam. The absorbed dose distributions to planarize the RBE-weighted absorbed dose were calculated in consideration of the RBE dependence on the prescribed absorbed dose and cellular radio-sensitivity. The results show that the microdosimetric measurement for the mono-energetic proton beam is also useful for designing RBE-weighted absorbed dose distributions for range-modulated proton beams.

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]

• Miniature Tissue Equivalent Proportional Counters can work without gas-flow. • The new sealed mini-TEPC gives reproducible results over a period of 4 months. • The new mini-TEPC can be used as a precise LET-monitor in proton therapy. • The new mini-TEPC measurements result in a reasonable RBE-assessment. • The mini-TEPC can be used for a comparative study of different radiotherapy centres. A new mini-TEPC with cylindrical sensitive volume of 0.9 mm in diameter and height, and with external diameter of 2.7 mm, has been developed to work without gas flow. With such a mini counter we have measured the physical quality of the 62 MeV therapeutic proton beam of CATANA (Catania, Italy). Measurements were performed at six precise positions along the Spread-Out Bragg Peak (SOBP): 1.4, 19.4, 24.6, 29.0, 29.7 and 30.8 mm, corresponding to positions of clinical relevance (entrance, proximal, central, and distal-edge of the SOBP) or of high lineal energy transfer (LET) increment (distal-dose drop off). Without refilling the microdosimeter with new gas, the measurements were repeated at the same positions 4 months later, in order to study the stability of the response in sealed-mode operation. From the microdosimetric spectra the frequency-mean lineal energy y - F and the dose-mean lineal energy y - D were derived and compared with average LET values calculated by means of Geant4 simulations. The comparison points out, in particular, a good agreement between microdosimetric y - D and the total dose-average L E T ¯ d , which is the average LET of the mixed radiation field, including the contribution by nuclear reactions. [ABSTRACT FROM AUTHOR]

Neutron spectrometry measurements with an extended-range Bonner Sphere Spectrometer (BSS), as well as neutron H*(10) measurements using an extended-range rem meter WENDI-2, a conventional rem meter LB 6411 and a tissue-equivalent proportional counter, were performed inside and around the Fixed-Beam Treatment Room at the proton therapy facility of Essen, in Germany. The WENDI-2 stood out as the easiest detector for making accurate neutron H*(10) measurements, since its direct measurements were equivalent to the H*(10) rates obtained with the BSS. The measurements were also compared to simulation results obtained with MCNPX 2.7.0 using two different selections of physics models for the hadron interactions above 150 MeV: the Bertini & Dresner models and the CEM03 model. For neutron H*(10) rates outside the treatment room, factors of 1.6–1.8 were obtained between the results of the two simulations, the Bertini & Dresner models yielding the largest values in all positions. The comparison of the simulation results with the WENDI-2 and BSS measurements for positions inside the treatment room showed that the Bertini & Dresner models reproduce the global neutron production in the water phantom relatively well, whereas the CEM03 model underestimates it by a factor of ∼1.3. At the most-forward angle, however, the Bertini model (unlike the CEM03 model) seemed to overestimate the production of neutrons with energies above 100 MeV. Outside the shielding, the simulated H*(10) overestimated the WENDI-2 measurements by factors of 2–3 with the Bertini & Dresner models, and 1.1–1.7 with the CEM03 model. Both simulations were thus conservative with respect to the neutron fluxes transmitted through the concrete walls. This conservative behaviour is probably caused by a combination of several uncertainties, including for instance uncertainties on the proton and neutron interaction cross-sections and uncertainties on the concrete composition and density. [ABSTRACT FROM AUTHOR]

A new miniaturized Tissue Equivalent Proportional Counter (mini‒TEPC) with cylindrical sensitive volume of 1.0 mm in diameter and height, and with external diameter of 1.53 cm, has been developed to work without gas flow. With this mini counter we have measured the radiation quality of the 148 MeV energy-modulated proton beam at the radiobiological research line of the Trento Proton Therapy Centre. The irradiation field had a diameter of about 6 cm, a modulation width of 2.5 cm and a range of about 13.5 cm. At four specific depths, the response function of the new microdosimeter has also been compared to that of a commercial spherical microdosimeter, the LET-1/2" Spherical TEPC produced by Far West Technology. Afterwards, measurements were performed with the mini‒TEPC at nine precise positions across the Spread-Out Bragg Peak (SOBP). From the microdosimetric spectra the frequency and the dose‒mean lineal energy values, y ‾ F and y ‾ D , were derived for both detectors. Afterwards, microdosimetric assessment of Relative Biological Effectiveness ( R B E μ) has been performed by weighting the dose distribution of the lineal energy, d(y), with the Loncol's biological weighting function r(y). R B E μ has been used to calculate the R B E μ D at different dose levels D. In the distal fall-off region the R B E μ D (d) increases significantly, up to a factor 3. Finally, the R B E μ D -weighted dose was calculated and compared to that obtained with a fixed RBE = 1.1. The use of a variable RBE to weight the physical dose leads to a biological range extension evaluated at R80 of about 2.0 mm, with respect to the R80 obtained with a fixed RBE = 1.1. The use of a mini‒TEPC in proton therapy has been so far limited to 60 MeV proton beams, used for the treatment of ocular melanoma. This is the first time that a mini-TEPC was used with an higher energy proton beam of 148 MeV, and the spectra measured with the mini-TEPC were compared to those measured with the commercial FWT LET1/2 counter for the first time. These new results corroborate the usability of mini-TEPCs in proton therapy. • Mini-TEPC for radiation quality characterization. • Microdosimetry of a 148 MeV proton beam. • RBE assessment in proton therapy through microdosimetry. • RBE increases at the distal edge of proton beams. [ABSTRACT FROM AUTHOR]

Abstract: For a passive scattering nozzle at the Proton Medical Research Center (PMRC), the distribution of the neutron dose equivalent in the direction transverse to the proton beam line was measured for 200MeV protons using a tissue equivalent proportional counter REM500. The neutron dose equivalent per Gray of proton-absorbed dose at the isocenter was approximately 2.3mSv/Gy. The neutron dose outside of the treatment field was approximately 2mSv/Gy, at maximum and it should be reduced as low as possible according to the ALARA principle. The aperture size of the multi-leaf collimator (MLC), which is a pre-collimator for shielding unwanted protons and mounted at a position upstream from the patient bolus and collimator, was varied in order to investigate its reduction effect on the neutron dose equivalent outside of the treatment field. We confirmed that the neutron dose equivalent outside of the treatment field can be limited depending on the aperture size of the MLC. This could be considered in future treatment planning optimization and patient treatment. MCNPX calculations showed good agreement with the measurements within 10%. [Copyright &y& Elsevier]

To study the agreement between proton microdosimetric distributions measured with a silicon-based cylindrical microdosimeter and a previously published analytical microdosimetric model based on Geant4-DNA in-water Monte Carlo simulations for low energy proton beams. Distributions for lineal energy (y) are measured for four proton monoenergetic beams with nominal energies from 2.0 MeV to 4.5 MeV, with a tissue equivalent proportional counter (TEPC) and a silicon-based microdosimeter. The actual energy for protons traversing the silicon-based microdosimeter is simulated with SRIM. Monoenergetic beams with these energies are simulated with Geant4-DNA code by simulating a water cylinder site of dimensions equal to those of the microdosimeter. The microdosimeter response is calibrated by using the distribution peaks obtained from the TEPC. Analytical calculations for y ‾ F and y ‾ D using our methodology based on spherical sites are also performed choosing the equivalent sphere to be checked against experimental results. Distributions for y at silicon are converted into tissue equivalent and compared to the Geant4-DNA simulated, yielding maximum deviations of 1.03% for y ‾ F and 1.17% for y ‾ D. Our analytical method generates maximum deviations of 1.29% and 3.33%, respectively, with respect to experimental results. Simulations in Geant4-DNA with ideal cylindrical sites in liquid water produce similar results to the measurements in an actual silicon-based cylindrical microdosimeter properly calibrated. The found agreement suggests the possibility to experimentally verify the calculated clinical y ‾ D with our analytical method. • Validation of analytical microdosimetric models for protons of low energy. • Silicon-based microdosimeter is calibrated and employed for experimental data. • Distributions and averages of lineal energy are compared. • We found agreement among experiments, Geant4-DNA and analytical results. [ABSTRACT FROM AUTHOR]