Microdosimetric characteristics of proton beams from 50 keV to 200 MeV

Proton beams are of growing interest for radiation therapy due to their special physical and radiobiological properties. Microdosimetric characteristics of proton beams have strong influence on the relative biological effectiveness for each biological system. This study focused on the microdosimetric characteristics of monoenergetic protons from 50 keV to 200 MeV. Monte Carlo techniques were used to simulate track segments of protons in water. Dose mean lineal energies were derived to characterise proton beams with changing kinetic energy and changing radiation qualities at various depths and within spread-out Bragg peaks of clinic interests.     

Microdosimetric comparison of scanned and conventional proton beams used in radiation therapy

Multiple groups have hypothesised that the use of scanning beams in proton therapy will reduce the neutron component of secondary radiation in comparison with conventional methods with a corresponding reduction in risks of radiation-induced cancers. Loma Linda University Medical Center (LLUMC) has had FDA marketing clearance for scanning beams since 1988 and an experimental scanning beam has been available at the LLUMC proton facility since 2001. The facility has a dedicated research room with a scanning beam and fast switching that allows its use during patient treatments. Dosimetric measurements and microdosimetric distributions for a scanned beam are presented and compared with beams produced with the conventional methods presently used in proton therapy.     

Microdosimetric investigation at the therapeutic proton beam facility of CATANA

Proton beams (62 Mev) are used by the Laboratori Nazionali del Sud of the Italian Institute of Nuclear Physics to treat eye melanoma tumours at the therapeutic facility called CATANA. A cylindrical slim tissue-equivalent proportional counter (TEPC) of 2.7 mm external diameter has been used to compare the radiation quality of two spread-out Bragg peaks (SOBP) at the CATANA proton beam.     

Longitudinally polarized proton beams at TRIUMF

Beam line 4B at TRIUMF has been modified to allow production of polarized proton beams of all polarization components (normal, sideways and longitudinal). This was achieved by the installation of two 2.4 T m superconducting solenoids in the cyclotron vault section of the beam line. It was also required to be able to rotate one vault quadrupole. It was found that for longitudinal polarization in beam line 4B the twister could no longer be operated as a unit section. The standard achromatic and dispersed modes of operation of beam lines 4A and 4B are still available.     

Microdosimetric characteristics of carbon track-segments

Carbon ions were introduced to radiation therapy due to their special physical and radiobiological properties. In heavy ion therapy, specification of radiation quality is an important issue and rapid calculations are often required for treatment planning. Because radiation qualities are closely related to microdosimetric spectra and parameters, this study is intended to provide microdosimetric parameters of carbon track-segments in the energy range from 50 MeV to 5 GeV. Monte-Carlo techniques are used to simulate track-segments of carbon ions in water. Microdosimetric quantities (the dose mean lineal energies) are calculated for the two components of the track-segment: the track-core formed by energy deposition of carbon ions and the track-penumbra (an extended region around the track-core produced by energy deposition of secondary electrons).     

Dosimetry of clinical neutron and proton beams: an overview of recommendations

Neutron therapy beams are obtained by accelerating protons or deuterons on Beryllium. These neutron therapy beams present comparable dosimetric characteristics as those for photon beams obtained with linear accelerators; for instance, the penetration of a p(65)+Be neutron beam is comparable with the penetration of an 8 MV photon beam. In order to be competitive with conventional photon beam therapy, the dosimetric characteristics of the neutron beam should therefore not deviate too much from the photon beam characteristics. This paper presents a brief summary of the neutron beams used in radiotherapy. The dosimetry of the clinical neutron beams is described. Finally, recent and future developments in the field of physics for neutron therapy is mentioned. In the last two decades, a considerable number of centres have established radiotherapy treatment facilities using proton beams with energies between 50 and 250 MeV. Clinical applications require a relatively uniform dose to be delivered to the volume to be treated, and for this purpose the proton beam has to be spread out, both laterally and in depth. The technique is called 'beam modulation' and creates a region of high dose uniformity referred to as the 'spread-out Bragg peak'. Meanwhile, reference dosimetry in these beams had to catch up with photon and electron beams for which a much longer tradition of dosimetry exists. Proton beam dosimetry can be performed using different types of dosemeters, such as calorimeters, Faraday cups, track detectors and ionisation chambers. National standard dosimetry laboratories will, however, not provide a standard for the dosimetry of proton beams. To achieve uniformity on an international level, the use of an ionisation chamber should be considered. This paper reviews and summarises the basic principles and recommendations for the absorbed dose determination in a proton beam, utilising ionisation chambers calibrated in terms of absorbed dose to water. These recommendations are based on the recent IAEA TRS398 Code of Practice: 'Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water'.     

Three-dimensional radiochromic film dosimetry of proton clinical beams using a gafchromic EBT2 film array

In this work, three-dimensional (3D) film-based proton beam measurements were used for the first time to verify the patient-specific radiation dose distribution, beam range and compensator shape. Three passively scattered proton beams and one uniform scanning proton beam were directed onto an acrylic phantom with inserted Gafchromic EBT films. The average gamma index for a comparison of the dose distributions was less than one for 97.2 % of all pixels from the passively scattered proton beams and 98.1 % of all pixels for the uniform scanning proton beams, with a 3 % dose and a 3 mm distance-to-dose agreement tolerance limit. The results also showed that the average percentage of points within the acceptance criteria for proton beam ranges was 94.6 % for the passively scattered proton beams. Both the dose distribution and the proton beam range determined by the 3D EBT film measurement agreed well with the planning system values.     

Microdosimetric assessment of the radiation quality of a therapeutic proton beam: comparison between numerical simulation and experimental measurements

Using protons for the treatment of ocular melanoma (especially of posterior pole tumours), the radiation quality of the beam must be precisely assessed to preserve the vision and to minimise the damage to healthy tissue. The radiation quality of a therapeutic proton beam at the Centre Antoine Lacassagne in Nice (France) was measured using microdosimetric techniques, i.e. a miniaturised version of a tissue-equivalent proportional counter. Measurements were performed in a 1-μm site at different depths in a Lucite phantom. Experimental data showed a significant increase in the beam quality at the distal edge of the spread-out Bragg peak (SOBP). In this paper, the numerical simulation of the experimental setup is done with the FLUKA Monte Carlo radiation transport code. The calculated microdosimetric spectra are compared with the measured ones at different depths in tissue for a monoenergetic proton beam (E=62 MeV) and for a modulated SOBP. Numerically and experimentally predicted relative biological effectiveness values are in good agreement. The calculated frequency-averaged and dose-averaged lineal energy mean values are consistent with measured data.     

Preparation of laser-accelerated proton beams for radiobiological applications

This paper presents the concept of transport and filtering of laser-accelerated proton pulses used for the first cell irradiation experiments performed with the Dresden 150 TW laser DRACO. Based on a simple non-focusing magnetic dipole equipped with two apertures the concept makes use of an energy dependent angular asymmetry of the proton spectra. For micron thin target foils protons of interest with energies above 7 MeV are observed to be significantly offset from target normal where low energy emission is dominantly centered. As the effect can be controlled via the target rotation with respect to the incoming light, it can be used to optimize the transport efficiency for high energy protons while simultaneously suppressing background radiation.     

High current, high energy proton beams accelerated by a sub-nanosecond laser

The sub-nanosecond laser system available at PALS facility in Prague has been used in order to produce MeV proton beams with typical current density approaching 1 A/cm² at few tens of centimeters from the target surface. In spite of the relatively long pulse duration (0.3 ns) and low intensity (∼10¹⁶ W/cm²), far away from the forefront laser facilities used for advanced proton beam acceleration in the recent years (from tens of femtoseconds to few picoseconds), the obtained results are promising both in terms of maximum proton energy and fast proton current. Real-time diagnostics systems, mainly in time-of-flight (TOF) configuration, have been used in order to estimate maximum and peak energy of the plasma fast proton component, peak current density, total number of fast protons and conversion efficiency of laser energy into accelerated fast proton total energy. Optimization of the maximum attainable proton energy and current has been carried out by irradiating targets of different composition as well as varying the laser energy and the focal spot diameter. Experimental results, as well as possible applications in material science and nuclear physics, are discussed and compared with literature data.     

Two-dimensional dosimetry of radiotherapeutical proton beams using thermoluminescence foils

In modern radiation therapy such as intensity modulated radiation therapy or proton therapy, one is able to cover the target volume with improved dose conformation and to spare surrounding tissue with help of modern measurement techniques. Novel thermoluminescence dosimetry (TLD) foils, developed from the hot-pressed mixture of LiF:Mg,Cu,P (MCP TL) powder and ethylene-tetrafluoroethylene (ETFE) copolymer, have been applied for 2-D dosimetry of radiotherapeutical proton beams at INFN Catania and IFJ Krakow. A TLD reader with 70 mm heating plate and CCD camera was used to read the 2-D emission pattern of irradiated foils. The absorbed dose profiles were evaluated, taking into account correction factors specific for TLD such as dose and energy response. TLD foils were applied for measuring of dose distributions within an eye phantom and compared with predictions obtained from the MCNPX code and Eclipse Ocular Proton Planning (Varian Medical Systems) clinical radiotherapy planning system. We demonstrate the possibility of measuring 2-D dose distributions with point resolution of about 0.5 x 0.5 mm(2).     

质子束在液体介质中声学效应的实验研究

最近,在布鲁克海文国立实验室(BNL)和哈佛大学的实验证明,当高能质子束通过液体介质时能产生可探测到的声信号.观测到的声波与热膨胀模型预期的一致.结果表明,提出的另外两种发声机制,即:微气泡破裂及分子离解都没有任何有意义的贡献.我们研究了声波的频率和振幅分布、辐射图形,以及与温度、压力和介质的依赖关系.我们认为这个现象直接可用于束流的监测和高能重离子的探测.信号的阈值可能很低,或许可探测到(如下一代高能加速器产生的或来自高能宇宙线的)单个粒子所产生的粒子簇射.由于换能器成本低廉及声波在液体中传播较远,所以建造质量比目前使用的探测器大许多数量级的高能粒子探测器是有可能实现的.     

Scintillation characteristics of cosh-Gaussian beams

By using the generalized beam formulation, the scintillation index is derived and evaluated for cosh-Gaussian beams in a turbulent atmosphere. Comparisons are made to cos-Gaussian and Gaussian beam scintillations. The variations of scintillations against propagation length at different values of displacement and focusing parameters are examined. The dependence of scintillations on source size at different propagation lengths is also investigated. Two-dimensional scintillation index distributions covering the entire transverse receiver planes are given. From the graphic illustrations, it is found that in comparison to pure Gaussian beams cosh-Gaussian beams have lower on-axis scintillations at smaller source sizes and longer propagation distances. The focusing effect appears to impose more reduction on the cosh-Gaussian beam scintillations than those of the Gaussian beam. The distribution of the off-axis scintillation index values of the Gaussian beams appears to be uniform over the transverse receiver plane, whereas that of the cosh-Gaussian beam is arranged according to the position of the slanted axis.     

Microdosimetric study for interpretation of outcomes from boron neutron capture therapy clinical trials

Boron neutron capture therapy is a brachyradiotherapy utilizing the (10)B(n,alpha)(7)Li reaction that has been used to treat glioblastoma multiforme (GBM), melanoma and colon carcinoma liver metastases. GBM clinical trials resulted in modestly improved life expectancies compared with conventional therapies. Early results trials focused on malignant melanoma and colon carcinoma provide dramatically better results. Macrodosimetry cannot explain these apparent differences. The dichotomy can only be understood using microdosimetry techniques. A computer program has been created to provide an improved tissue model. This model permits the dose in each cell's cytoplasm, nucleus, and the interstitium to be calculated for ellipsoidal cells placed in either random or ordered locations. The nuclei can be centered or eccentric. The new model provides insight into the micro level for differences in the trials. The differences arise from the tissue's cellular geometry and the effects of neighboring cells. These results help to explain the observed clinical outcomes.     

A method of polarizing relativistic proton beams by laser radiation

The method of polarization of protons in the process of optical orientation of hydrogen atoms, obtained in neutralizing negative hydrogen ions, accelerated up to an energy of 200–600 MeV is discussed. The method uses the Doppler shift of a transition frequency for relativistic atoms ($\text{ν}\text{c}\text{? 0.5}$) into the region accessible for lasers. The implementation of the method will allow a beam of polarized protons with a pulse intensity of up to 50 mA to be obtained. This value exceeds the intensity achieved so far by three orders of magnitude. The kinetics of the polarization process is calculated. The possible scheme of a laser polarizer is discussed.     

Microdosimetric evaluation of the neutron field for BNCT at Kyoto University reactor by using the PHITS code

In this study, microdosimetric energy distributions of secondary charged particles from the (10)B(n,α)(7)Li reaction in boron-neutron capture therapy (BNCT) field were calculated using the Particle and Heavy Ion Transport code System (PHITS). The PHITS simulation was performed to reproduce the geometrical set-up of an experiment that measured the microdosimetric energy distributions at the Kyoto University Reactor where two types of tissue-equivalent proportional counters were used, one with A-150 wall alone and another with a 50-ppm-boron-loaded A-150 wall. It was found that the PHITS code is a useful tool for the simulation of the energy deposited in tissue in BNCT based on the comparisons with experimental results.     

Microdosimetric interpretation of the photon energy response of LiF:Mg,Ti detectors

Photon energy response of MTS-N (LiF:Mg,Ti) detectors (TLD Poland) and of MTS-N detectors sensitised with 200 Gy of 60Co gamma rays, followed by UV irradiation (sMTS-N), has been determined using X rays with narrow energy spectra, in the energy range from 20 to 300 keV. The over-response of LiF:Mg,Ti detectors for X rays (relative TL efficiency eta = 1.1) can be explained as an ionisation density effect. Low energy X rays produce short electron tracks, which locally deposit a high radiation dose and, consequently, lead to an enhanced (supralinear) response. This over-response has not been observed in sensitised MTS-N where supralinearity in the response after gamma ray doses above 1 Gy is not seen. Using the dose-response curves measured for MTS-N detectors after 137Cs gamma ray irradiation and local doses calculated using Monte Carlo generated electron tracks, it was possible to predict the relative TL effectiveness for different X ray energies. The calculation procedure can be applied to predict the photon energy response of LiF:Mg,Ti detectors in an arbitrary photon field.