The microdosimetry of low-energy photons in radiotherapy

Low energy photons are more and more in use in clinical practice, for treatment in radiotherapy as well as for imaging purposes. Their relative biological effectiveness is however still debated. In this paper, some microdosimetric parameters have been calculated for different sources: (125)I, (103)Pd, (131)Cs, an electronic brachytherapy source and various clinical mammography X-ray qualities. These parameters have been used to deduce the quality factors as defined in ICRU 40.     

Analysis of cell-survival fractions for heavy-ion irradiations based on microdosimetric kinetic model implemented in the particle and heavy ion transport code system

It is considered that the linear energy transfer (LET) may not be the ideal index for expressing the relative biological effectiveness (RBE) of cell killing for heavy-ion irradiation, as the ion-species dependencies have clearly been observed in the relation between LET and RBE derived from cell-survival fraction data. The previously measured survival fractions of four cell lines irradiated by various ion species, employing the saturation-corrected dose-mean lineal energy, y*, instead of LET as the index of the RBE were therefore re-analysed. In the analysis, the initial slopes of the survival fractions, the so-called α-parameter in the linear-quadratic model, were plotted as a function of y*, which was calculated by the microdosimetric kinetic (MK) model implemented in the Particle and Heavy Ion Transport code System. It was found from the analysis that the ion-species dependencies observed in the relations between α and LET disappeared from those between α and y*, and their relations can be well reproduced by a simple equation derived from the MK model. These results clearly indicate the suitability of y* to be used in the estimation of the RBE of cell killing for heavy-ion irradiations, which is of great importance in the treatment planning of charged-particle therapy.     

125I密封籽源水中吸收剂量分布的测量

用热释光剂量计对125I密封籽源在水中的剂量分布进行具有量值溯源性的测量,结果表明,125II密封籽源在水中的剂量分布随着离源距离的增加而快速衰减,同时受照角度对剂量分布也有一定的影响。模拟实验有助于改进放射性籽源植入治疗的剂量准确性和安全性。对测量误差和实验方法也进行了有益的讨论。     

A fibre optic scintillator dosemeter for absorbed dose measurements of low-energy X-ray-emitting brachytherapy sources

A newly developed dosemeter using a 0.5 mm diameter x 0.5 mm thick cylindrical plastic scintillator coupled to the end of a fibre optic cable is capable of measuring the absorbed dose rate in water around low-activity, low-energy X-ray emitters typically used in prostate brachytherapy. Recent tests of this dosemeter showed that it is possible to measure the dose rate as a function of distance in water from 2 to 30 mm of a (103)Pd source of air-kerma strength 3.4 U (1 U = 1 microGy m(2) h(-1)), or 97 MBq (2.6 mCi) apparent activity, with good signal-to-noise ratio. The signal-to-noise ratio is only dependent on the integration time and background subtraction. The detector volume is enclosed in optically opaque, nearly water-equivalent materials so that there is no polar response other than that due to the shape of the scintillator volume chosen, in this case cylindrical. The absorbed dose rate very close to commercial brachytherapy sources can be mapped in an automated water phantom, providing a 3-D dose distribution with sub-millimeter spatial resolution. The sensitive volume of the detector is 0.5 mm from the end of the optically opaque waterproof housing, enabling measurements at very close distances to sources. The sensitive detector electronics allow the measurement of very low dose rates, as exist at centimeter distances from these sources. The detector is also applicable to mapping dose distributions from more complex source geometries such as eye applicators for treating macular degeneration.     

Microdosimetry of neutron field for boron neutron capture therapy at Kyoto university reactor

Microdosimetric single event spectrum in a human body simulated by an acrylic phantom has been measured for the clinical BNCT field at the Kyoto University Reactor (KUR). The recoil particles resulting from the initial reaction and subsequent interactions, namely protons, electrons, alpha particles and carbon nuclei are identified in the microdosimetric spectrum. The relative contributions to the neutron dose from proton, alpha particles and carbon are estimated to be about 0.9, 0.07 and 0.3, respectively, four depths between 5 and 41 mm. We estimate that the dose averaged lineal energy, yD decreased with depth from 64 to 46 keV microm(-1). Relative biological effectiveness (RBE) of this neutron field using a response function for the microdosimetric spectrum was estimated to decrease from 3.6 to 2.9 with increasing depth.     

Dosimetric considerations on TEPC fluka-simulation and measurements

The response of a tissue equivalent proportional counter (TEPC) has been simulated with the Monte Carlo transport code FLUKA. The absorbed dose distribution of lineal energy y has been determined for several monoenergetic photon and neutron sources. The agreement between the calculated results and the measurements carried out with different well-known sources is well demonstrated. Work is in progress in order to evaluate the response of the instrument in the cosmic ray environment.     

Nanodosimetric measurements and calculations in a neutron therapy beam

A comparison of calculated and measured values of the dose mean lineal energy (y(D)) for the former neutron therapy beam at Louvain-la-Neuve is reported. The measurements were made with wall-less tissue-equivalent proportional counters using the variance-covariance method and simulating spheres with diameters between 10 nm and 15 microm. The calculated y(D)-values were obtained from simulated energy distributions of neutrons and charged particles inside an A-150 phantom and from published y(D)-values for mono-energetic ions. The energy distributions of charged particles up to oxygen were determined with the SHIELD-HIT code using an MCNPX simulated neutron spectrum as an input. The mono-energetic ion y(D)-values in the range 3-100 nm were taken from track-structure simulations in water vapour done with PITS/KURBUC. The large influence on the dose mean lineal energy from the light ion (A > 4) absorbed dose fraction, may explain an observed difference between experiment and calculation. The latter being larger than earlier reported result. Below 50 nm, the experimental values increase while the calculated decrease.     

Angular and radial dependence of the energy response factor for LIF-TLD micro-rods in 125L permanent implant source

EGSnrc Monte Carlo simulations were used to calculate the angular and radial dependence of the energy response factor for LiF-thermoluminescence dosemeters (TLDs) irradiated with a commercially available (125)I permanent brachytherapy source. The LiF-TLDs were modelled as cylindrical micro-rods of length 6 mm and with diameters of 1 mm and 5 mm. The results show that for a LiF-TLD micro-rod of 1 mm diameter, the energy response relative to (60)Co gamma rays is 1.406 +/- 0.3% for a polar angle of 90 degrees and radial distance of 1.0 cm. When the diameter of the micro-rod is increased from 1 to 5 mm, the energy response decreases to 1.32 +/- 0.3% at the same point. The variation with position of the energy response factor is not >5% in a 6 cm x 6 cm x 6 cm calculation grid for the 5 mm diameter micro-rod. The results show that there is a change in the photon spectrum with angle and radial distance, which causes the variation of the energy response.     

Microdosimetric spectra of the THOR neutron beam for boron neutron capture therapy

A primary objective of the BNCT project in Taiwan, involving THOR (Tsing Hua Open Pool Reactor), was to examine the potential treatment of hepatoma. To characterise the epithermal neutron beam in THOR, the microdosimetry distributions in lineal energy were determined using paired tissue-equivalent proportional counters with and without boron microfoils. Microdosimetry results were obtained in free-air and at various depths in a PMMA phantom near the exit of the beam port. A biological weighting function, dependent on lineal energy, was used to estimate the relative biological effectiveness of the beam. An effective RBE of 2.7 was found at several depths in the phantom.     

Spectra of scattered photons in large absorbers and their importance for the values of radiation weighting factor wR

In its review of the present values of radiation weighting factor w(R) and of possible revisions of this factor, the German Radiation Protection Commission has recommended to maintain the approach of ICRP 60 to base the selection of the w(R) value for a given radiation (e.g. fission neutrons) on observed values of the relative biological effectiveness (RBE) of this radiation 'regardless of whether the reference radiation is X rays or gamma rays'. The physical background of the German recommendation is the buildup of a strong field of energy-degraded Compton scattered photons in the human body if exposed to an external field of high-energy photons, so that the total radiation field inside the body is a mixture comprising low and high photon energies. Therefore, it is appropriate that the selection of the w(R) value of the given radiation is guided by RBE values averaged over X rays and gamma rays as the reference radiations. In support of this rationale, the present paper provides a sample of Monte Carlo calculated scattered photon spectra in large absorbers exposed to high-energy photons. Depth-dependent fractional dose contributions of the scattered photons are tabulated for incident energies from 1 to 10 MeV, and estimates of the influence of their degraded energies on the biological effectiveness of the incoming radiation are presented. Accordingly, we point out that it is appropriate to use, for the purposes of 'risk projection', RBE values averaged over X and gamma reference radiations.     

Differential absorbed dose distributions in lineal energy for neutrons and gamma rays at the mono-energetic neutron calibration facility

Absorbed dose distributions in lineal energy for neutrons and gamma rays of mono-energetic neutron sources from 140 keV to 15 MeV were measured in the Fast Neutron Laboratory at Tohoku University. By using both a tissue-equivalent plastic walled counter and a graphite-walled low-pressure proportional counter, absorbed dose distributions in lineal energy for neutrons were obtained separately from those for gamma rays. This method needs no knowledge of energy spectra and dose distributions for gamma rays. The gamma-ray contribution in this neutron calibration field >1 MeV neutron was <3%, while for <550 keV it was >40%. The measured neutron absolute absorbed doses per unit neutron fluence agreed with the LA150 evaluated kerma factors. By using this method, absorbed dose distributions in lineal energy for neutrons and gamma rays in an unknown neutron field can be obtained separately.     

Determination of dosimetric characteristics of a new design 125I brachytherapy source with the Monte Carlo code MCNPX

The dosimetric parameters of a new ¹²⁵I brachytherapy seed model were determined using Monte Carlo (MC) simulations according to the updated protocol of the American Association of Physicists in Medicine (TG-43U1 report) and compared with those of other brachytherapy ¹²⁵I seed models. The dose rate constant Λ, radial dose function g _L(r), and anisotropy function F(r, θ), were calculated in liquid water, and the dose rate constant of 0.915 ± 2% cGy h^?1 U^?1 in liquid water was obtained. Acceptable agreement between the calculated dosimetric parameters of this new source and other commercial source models (Amersham 6702 and Imagyn model IS-12 501) indicates that this seed has acceptable dose distributions for clinical use. However, the anisotropy function of this model is considerably better than that of Imagyn model IS-12 501 ¹²⁵I source, especially on the points along and close to the longitudinal axis of the source. This model provides more isotropic angular dose distribution in tissue than does Imagyn model IS-12501 ¹²⁵I source.     

Empirical model estimation of relative biological effectiveness for proton beam therapy

A simple model for proton relative biological effectiveness (RBE) is proposed. It describes the RBE as a function of proton depth, the dose and the linear energy transfer (LET) when proton passes through tissue-like materials. Radiobiological parameters were first obtained by fitting the published experimental cell survival data. The dose-averaged LET values were calculated for 250-MeV proton beam in a water phantom by using GEANT4 Monte Carlo simulation code and were then used as input values to calculate the values of RBE as function of depths. The model was also applied to proton spread-out Bragg peak, where the increasing RBE with depth causes an extended RBE-weighted dose in the distal fall-off region. This model was found to be able to reproduce the measured RBE values as a function of LET, depth and dose for a specific cell line.     

Dosimetric characteristic of a new 125I brachytherapy source

A new brachytherapy (125)I source has been investigated at Iranian Agricultural, Medical and Industrial Research School. Dosimetric characteristics [dose-rate constant Λ, radial dose function g(l)(r) and anisotropy function F(r,)] of IRA-(125)I were theoretically determined in terms of the updated AAPM task group 43 (TG-43U1) recommendations. Versions 5 and 4C of the Monte Carlo radiation transport code were used to calculate the dosimetry parameters around the source. The Monte Carlo calculated dose-rate constant of the (125)I source in water was found to be 92×10(-4) Gy h(-1) U(-1) with an approximate uncertainty of ±3 %. Brachytherapy seed model, 6711-(125)I, carrying (125)I radionuclides, was modelled and benchmarked against previously published values. Finally, the calculated results were compared with the published results of those of other source manufacturers.     

The applicability of radiophotoluminescence dosemeter (RPLD) for measuring medical radiation (MR) doses

The applicability of radiophotoluminescence dosimetry was determined by assessing various radiophotoluminescence dosemeter (RPLD) properties for measuring medical radiation doses from radiation sources of a continuous spectrum. The RPLD was found to be accurate for measuring doses in diagnostics (50-125 keV) and radiation therapy (6, 10 and 18 MV photons, 6 and 15 MeV electrons). The RPLD shows excellent dose linearity (R(2) > 0.99), reproducibility and batch uniformity, and minimal fading and accurate accumulated dose measurement. The dosemeter material is independent of photon energy in the diagnostic range; however, the dosemeter requires additional calibration in the mammography energy range and also for accurate dose measurement with photon or electron energies in radiation therapy. RPLD measurements with a tin filter show considerable angular dependence at angles exceeding 50° between the photon beam and the normal to the long axis of the dosemeter. The RPLD measurement accuracy at high doses can be improved with optimised pre-heating schemes.     

The variation in biological effectiveness of X-rays and gamma rays with energy

The ICRP has attributed the same relative risk for all low-LET (linear energy transfer) radiations, including X and gamma radiations of all energies. However, very low energy X-rays are expected to be more biologically effective, per unit absorbed dose, than high energy X-rays or gamma rays due to the production of lower energy secondary electrons, with a correspondingly higher LET. This increase in relative biological effectiveness (RBE) is also seen experimentally for a range of biological end-points, however, a wide range of RBE values have been reported. The assessment of risks is particularly important due to the use of low energy X-rays for mammography screening. A review of the published data on the variation in biological effectiveness with energy is presented here.     

ISFSI site boundary radiation dose rate analyses

Across the globe nuclear utilities are in the process of designing and analysing Independent Spent Fuel Storage Installations (ISFSI) for the purpose of above ground spent-fuel storage primarily to mitigate the filling of spent-fuel pools. Using a conjoining of discrete ordinates transport theory (DORT) and Monte Carlo (MCNP) techniques, an ISFSI was analysed to determine neutron and photon dose rates for a generic overpack, and ISFSI pad configuration and design at distances ranging from 1 to -1700 m from the ISFSI array. The calculated dose rates are used to address the requirements of 10CFR72.104, which provides limits to be enforced for the protection of the public by the NRC in regard to ISFSI facilities. For this overpack, dose rates decrease by three orders of magnitude through the first 200 m moving away from the ISFSI. In addition, the contributions from different source terms changes over distance. It can be observed that although side photons provide the majority of dose rate in this calculation, scattered photons and side neutrons take on more importance as the distance from the ISFSI is increased.     

New National Air-Kerma-Strength Standards for 125I and 103Pd Brachytherapy Seeds

The new U.S. measurement standard for the air-kerma strength from low-energy photon-emitting brachytherapy seed sources is formally described in detail. This instrument-based standard was implemented on 1 January 1999, with its salient features and the implications of differences with the previous standard given only through a series of informal communications. The Wide-Angle Free-Air Chamber (WAFAC) is specially designed to realize air kerma from a single-seed source emitting photons with energies up to about 40 keV, and is now used to measure the wide variety of seeds used in prostate-cancer therapy that has appeared in the last few years. For the two ¹²⁵I seed models that have been subject to both the old and new standards, the new standard reduces the air-kerma strength by 10.3 %. This change is mainly due to the removal of the influence on the measurement of the Ti K x rays produced in the source encapsulation, a component with no clinical significance.     

Differential dosimetry in a neutron-proton mixed field with low-pressure proportional counters

For a mixed radiation field of neutrons and protons, radiation events were discriminated between photons, neutrons, and protons using a thin plastic scintillator. Distributions of lineal energy were measured with low-pressure proportional counters (LPPCs). To estimate the distribution of lineal energy for ICRU muscle, measurements were carried out using A-150-walled, graphite-walled, ZrO2-walled, and Zr-walled counters. Data were corrected for different atomic compositions between the A-150 plastic and ICRU muscle.