P. Olko

CALCULATION OF RELATIVE BIOLOGICAL EFFECTIVENESS FOR PROTON BEAMS USING BIOLOGICAL WEIGHTING FUNCTIONS

PURPOSE The microdosimetric weighting function approach is used widely for beam comparison studies. The suitability of this model to predict the relative biological effectiveness (RBE) of therapeutic proton beams was studied. The RBE(alpha) (i.e., linear approximation) dependence on the type of biological end point, initial proton energy, energy spread of the input proton beam, and depth of beam penetration was investigated. METHODS AND MATERIALS Proton transport calculations for a proton energy range from 70 to 250 MeV were performed to obtain proton energy spectra at a given depth. The corresponding microdosimetric distributions of lineal energy were calculated. To these distributions the biological response function approach was applied to calculate RBE(alpha) the biological effectiveness based on a linear dose-response relationship. The early intestinal tolerance assessed by crypt regeneration in mice and the inactivation of V79 cells were taken as biological end points. RESULTS The RBE(alpha) values approach about 1 in the plateau region and gradually increase with the proton penetration depth. In the center of the Bragg peak, at the maximum dose delivery, the values of RBE(alpha) range from 1.1 (250-MeV beam, early intestinal tolerance in mice) to 1.9 (70-MeV beam, Chinese hamster V79 cells in G1/S phase). Distal to the Bragg peak, where only a small fraction of dose is delivered, the RBE(alpha) was found to be even higher. For modulated proton beams we found an increasing RBE(alpha) with depth in the spread-out Bragg peak (SOBP). Values up to 1.37 at the distal end of the SOBP plateau (155-MeV beam, SOBP between 5.3 and 13.2 cm) were obtained. CONCLUSION More experimental work on the determination of microdosimetric weighting functions is needed. The results of the presented calculations indicate that for therapy planning it may be necessary to account for a depth dependence on proton RBE, especially for lower energy.

The microdosimetric one-hit detector model for calculating the response of solid state detectors

The microdosimetric one-hit detector model has been developed and applied to calculate the dose response, energy response and relative efficiency of thermoluminescent LiF:Mg,Cu,P and CaF2:Tm detectors, and of the free-radical alanine dosimeter, after their exposure to radiation of different quality. The one-hit detector is described by two model parameters: the target diameter, d and the saturation parameter, α. Combining these parameters with microdosimetric distributions in nanometer-size targets calculated using Monte Carlo track structure codes TRION and MOCA-14, it was possible to describe and predict a great variety of experimental data for photon, X-ray, beta-electron, proton, alpha-particles and heavy ion irradiation. Within the framework of this biophysical model of radiation action, some mechanistic insight into the physics of radiation action in solid state detectors can be obtained.

Development of a Method for Passive Measurement of Radiation Doses at Ultra-High Dose Range

The thermoluminescent (TL) detectors are an old-established method of passive dose measurement. In the last several years MCP-N (LiF:Mg,Cu,P) detectors have been widely used in modern TL dosimetry due to their very high sensitivity (at microgray level) and a simple signal-dose relation. Their dose response does not show any supralinearity up to saturation at about 1 kGy. Only recently we have discovered their quite unexpected properties at high and ultra-high doses, which enable us to use them for measurements of doses from micrograys up to a megagray. The presented method is based on these properties. This method is also suitable for measurements of doses in mixed radiation fields.

Measurement of stray radiation within a scanning proton therapy facility: EURADOS WG9 intercomparison exercise of active dosimetry systems

PURPOSE To characterize stray radiation around the target volume in scanning proton therapy and study the performance of active neutron monitors. METHODS Working Group 9 of the European Radiation Dosimetry Group (EURADOS WG9-Radiation protection in medicine) carried out a large measurement campaign at the Trento Centro di Protonterapia (Trento, Italy) in order to determine the neutron spectra near the patient using two extended-range Bonner sphere spectrometry (BSS) systems. In addition, the work focused on acknowledging the performance of different commercial active dosimetry systems when measuring neutron ambient dose equivalents, H(∗)(10), at several positions inside (8 positions) and outside (3 positions) the treatment room. Detectors included three TEPCs--tissue equivalent proportional counters (Hawk type from Far West Technology, Inc.) and six rem-counters (WENDI-II, LB 6411, RadEye™ NL, a regular and an extended-range NM2B). Meanwhile, the photon component of stray radiation was deduced from the low-lineal energy transfer part of TEPC spectra or measured using a Thermo Scientific™ FH-40G survey meter. Experiments involved a water tank phantom (60 × 30 × 30 cm(3)) representing the patient that was uniformly irradiated using a 3 mm spot diameter proton pencil beam with 10 cm modulation width, 19.95 cm distal beam range, and 10 × 10 cm(2) field size. RESULTS Neutron spectrometry around the target volume showed two main components at the thermal and fast energy ranges. The study also revealed the large dependence of the energy distribution of neutrons, and consequently of out-of-field doses, on the primary beam direction (directional emission of intranuclear cascade neutrons) and energy (spectral composition of secondary neutrons). In addition, neutron mapping within the facility was conducted and showed the highest H(∗)(10) value of ∼ 51 μSv Gy(-1); this was measured at 1.15 m along the beam axis. H(∗)(10) values significantly decreased with distance and angular position with respect to beam axis falling below 2 nSv Gy(-1) at the entrance of the maze, at the door outside the room and below detection limit in the gantry control room, and at an adjacent room (<0.1 nSv Gy(-1)). Finally, the agreement on H(∗)(10) values between all detectors showed a direct dependence on neutron spectra at the measurement position. While conventional rem-counters (LB 6411, RadEye™ NL, NM2-458) underestimated the H(∗)(10) by up to a factor of 4, Hawk TEPCs and the WENDI-II range-extended detector were found to have good performance (within 20%) even at the highest neutron fluence and energy range. Meanwhile, secondary photon dose equivalents were found to be up to five times lower than neutrons; remaining nonetheless of concern to the patient. CONCLUSIONS Extended-range BSS, TEPCs, and the WENDI-II enable accurate measurements of stray neutrons while other rem-counters are not appropriate considering the high-energy range of neutrons involved in proton therapy.

Development of a method of passive measurement of radiation doses at ultra-high dose range

Thermoluminescent (TL) detectors are an established method for passive dose measurement. Over the last several years MCP-N (LiF:Mg,Cu,P) detectors have been widely used in modern TL dosimetry due to their very high sensitivity (at the microgray level) and their simple signal-dose relation. Their dose response does not show any supralinearity up to saturation at about 1 kGy. Only recently we have discovered their quite unexpected properties at high and ultra-high doses, which enable us to use them for measurements of doses from micrograys up to a megagray. Significant changes of the glow-curve shape occur for doses higher than a few kGy and what is most important, a new peak appears in their glow-curve at exposures above 50 kGy, the position of which shifts towards higher temperatures with increasing dose. The presented method is based on these properties. This method is also a promising way of measurements of doses in mixed radiation fields.

Ultra-thin LiF:Mg,Cu,P detectors for beta dosimetry

Abstract Standard thermoluminescent (TL) detectors, owing to their relatively large thickness, may seriously underestimate personal skin doses which are defined at the depth of 7 mg cm −2 . New TL ultra-thin, LiF:Mg,Cu,P-based detectors have been developed at the Institute of Nuclear Physics to fulfill simultaneously the requirements of flat energy response for beta rays and the ability to measure low beta ray doses. In our detectors a thin layer of MCP phosphor is bonded with a thick base of undoped LiF. We assess the effective thickness of this detector to be 8.5 mg cm −2 . Tests of these detectors exposed with and without covering foil to 147 Pm, 204 Tl and 90 Sr/ 90 Y calibrated beta fields indicate that our detectors feature an essentially flat energy response and good angular characteristics. The sensitivity of our detectors permits doses in the microsievert range to be measured reliably.

EPR studies of free radicals decay and survival in gamma irradiated aminoglycoside antibiotics: sisomicin, tobramycin and paromomycin.

Radiation sterilization technology is more actively used now that any time because of its many advantages. Gamma radiation has high penetrating power, relatively low chemical reactivity and causes small temperature rise. But on the other hand radiosterilization can lead to radiolytic products appearing, in example free radicals. Free radicals in radiative sterilized sisomicin, tobramycin and paromomycin were studied by electron paramagnetic resonance (EPR) spectroscopy. Dose of gamma irradiation of 25kGy was used. Concentrations and properties of free radicals in irradiated antibiotics were studied. EPR spectra were recorded for samples stored in air and argon. For gamma irradiated antibiotics strong EPR lines were recorded. One- and two-exponential functions were fitted to experimental points during testing and researching of time influence of the antibiotics storage to studied parameters of EPR lines. Our study of free radicals in radiosterilized antibiotics indicates the need for characterization of medicinal substances prior to sterilization process using EPR values. We propose the concentration of free radicals and other spectroscopic parameters as useful factors to select the optimal type of sterilization for the individual drug. The important parameters are i.a. the τ time constants and K constants of exponential functions. Time constants τ give us information about the speed of free radicals concentration decrease in radiated medicinal substances. The constant K(0) shows the free radicals concentration in irradiated medicament after long time of storage.

A comprehensive spectrometry study of a stray neutron radiation field in scanning proton therapy.

The purpose of this study is to characterize the stray neutron radiation field in scanning proton therapy considering a pediatric anthropomorphic phantom and a clinically-relevant beam condition. Using two extended-range Bonner sphere spectrometry systems (ERBSS), Working Group 9 of the European Radiation Dosimetry Group measured neutron spectra at ten different positions around a pediatric anthropomorphic phantom irradiated for a brain tumor with a scanning proton beam. This study compares the different systems and unfolding codes as well as neutron spectra measured in similar conditions around a water tank phantom. The ten spectra measured with two ERBSS systems show a generally similar thermal component regardless of the position around the phantom while high energy neutrons (above 20 MeV) were only registered at positions near the beam axis (at 0°, 329° and 355°). Neutron spectra, fluence and ambient dose equivalent, H (*)(10), values of both systems were in good agreement (<15%) while the unfolding code proved to have a limited effect. The highest H (*)(10) value of 2.7 μSv Gy(-1) was measured at 329° to the beam axis and 1.63 m from the isocenter where high-energy neutrons (E  ⩾  20 MeV) contribute with about 53%. The neutron mapping within the gantry room showed that H (*)(10) values significantly decreased with distance and angular position with respect to the beam axis dropping to 0.52 μSv Gy(-1) at 90° and 3.35 m. Spectra at angles of 45° and 135° with respect to the beam axis measured here with an anthropomorphic phantom showed a similar peak structure at the thermal, fast and high energy range as in the previous water-tank experiments. Meanwhile, at 90°, small differences at the high-energy range were observed. Using ERBSS systems, neutron spectra mapping was performed to characterize the exposure of scanning proton therapy patients. The ten measured spectra provide precise information about the exposure of healthy organs to thermal, epithermal, evaporation and intra-nuclear cascade neutrons. This comprehensive spectrometry analysis can also help in understanding the tremendous literature data based rem-counters while also being of great value for general neutron shielding and radiation safety studies.

Dosimetry properties of Tm-doped single CaF2 crystals

Abstract The dosimetry properties of thermoluminescence CaF 2 : Tm detectors with thulium concentrations of 0.1%, 0.3%, 0.5%, 0.6% and 0.7%, developed from single crystals grown by the Stockbarger method at the Institute of Geochemistry, Irkutsk, Russia, were investigated. The relative TL efficiency, η, after 5.3 MeV Am-241 α-particle irradiation and the Cs-137 γ-ray dose response after doses up to 10 Gy , of interest for radiation therapy, were evaluated. Linearity and reproducibility of the dose response, the shape of the glow-curve (ratio of area under peaks 3 and 5 for α- and γ-radiation) were studied and referred to the properties of standard Harshaw TLD-300 detectors. The relative TL α-particle efficiency, η, for the high temperature peak 5 was found to grow with increasing Tm concentration, from 0.13 for 0.1% Tm, 0.17 for 0.3% Tm to 0.92 for 0.5% Tm where the corresponding values of η for peak 3 were 0.073, 0.059 and 0.211. Detectors with 0.6 wt % Tm appear to reproduce the properties of TLD-300 quite well, showing a similar shape of the glow curve and a similar ratio of peak areas under peaks 5 and 3. The highest ranges of linear dose-response for 137Cs γ-rays for peaks 3 and 5, were obtained for detectors with 0.5% Tm. These detectors, due to their high efficiency after high-LET irradiation and extended linearity range are most suitable for dosimetry of high-LET radiotherapy beams.

Dosimetric properties of sintered LiF:Mg,Cu,Na,Si TL detectors

Abstract Selected dosimetric properties of sintered LiF : Mg , Cu , Na , Si detectors were studied. Solid LiF : Mg , Cu , Na , Si pellets of diameters 4.5 mm and thickness 0.9 mm , dark blue in colour, were obtained by cold pressing and sintering the powder at 820°C. The dosimetric properties of the newly developed detectors were studied and compared with the properties of LiF : Mg , Cu , P pellets (MCP-N). The annealing conditions were the same as those used for MCP-N detectors. X-ray exposures were performed at the KAERI,Taejon,Korea, while other irradiation and readout were carried out at the INP in Krakow, Poland. The glow-curve structure of LiF : Mg , Cu , Na , Si pellets is found to be comparable to that of MCP-N ( LiF : Mg , Cu , P ) detectors but the absolute sensitivity is about 50% lower. The photon energy response after doses of X-rays of energy about 100 keV shows a decrease, similar to that in LiF : Mg , Cu , P . For lower energies the response is higher than that for LiF : Mg , Cu , P due to the presence of high- Z elements (Na,Cu, and Si). The relative TL efficiency after doses of alpha particles from an 214 Am source of the sintered LiF : Mg , Cu , Na , Si detectors is similar to that of MCP-N ( LiF : Mg , Cu , P ) .