Biological weighting of absorbed dose in radiation therapy

Absorbed dose is a quantity which is scientifically rigorously defined and used to quantify the exposure of biological objects, including humans, to ionising radiation. There is, however, no unique relationship between absorbed dose and induced biological effects. The effects induced by a given absorbed dose to a given biological object depend also on radiation quality and temporal distribution of the irradiation. In radiation therapy, empirical approaches are still used today to account for these dependencies in practice. In hadron therapy (neutrons, protons, ions), radiation quality is accounted for with a diversity of (almost hospital specific) methods. The necessity to account for temporal aspects is well known in external beam therapy and in high dose rate brachytherapy. The paper reviews the approaches for weighting the absorbed dose in radiation therapy, and focusses on the clinical aspects of these approaches, in particular the accuracy requirements.     

Measurement of absorbed dose by 7-GeV bremsstrahlung in a PMMA phantom

High-energy electron storage rings generate energetic bremsstrahlung photons through radiative interaction of the particle beam with the residual gas molecules and other components inside the storage ring. At synchrotron radiation facilities, where beamlines are channeled out of the storage ring, a continuous bremsstrahlung spectrum, with a maximum energy of the stored particle beam, will be present. At the advanced photon source (APS), where the stored beam energy is 7 GeV, bremsstrahlung generated in the straight sections of the insertion device beamlines, which are a total of 15.38 m in length, can be significant. The contribution from each bremsstrahlung interaction adds up to produce a narrow mono-directional bremsstrahlung beam that comes down through the insertion device beamlines. The resulting absorbed dose distributions by this radiation in a 300 mm×300 mm×300 mm tissue substitute cube phantom were measured with LiF:Mg,Ti (TLD-700) thermoluminescent dosemeters. The normalized absorbed dose, in a cross-sectional area of 100 mm² at a depth of 150 mm of the PMMA phantom, was measured as 3.3×10⁶ mGy h⁻¹W⁻¹ for 7-GeV bremsstrahlung spectrum.     

Review of reconstruction of radiation incident air kerma by measurement of absorbed dose in tooth enamel with EPR

Electron paramagnetic resonance dosimetry with tooth enamel has been proved to be a reliable method to determine retrospectively exposures from photon fields with minimal detectable doses of 100 mGy or lower, which is lower than achievable with cytogenetic dose reconstruction methods. For risk assessment or validating dosimetry systems for specific radiation incidents, the relevant dose from the incident has to be calculated from the total absorbed dose in enamel by subtracting additional dose contributions from the radionuclide content in teeth, natural external background radiation and medical exposures. For calculating organ doses or evaluating dosimetry systems the absorbed dose in enamel from a radiation incident has to be converted to air kerma using dose conversion factors depending on the photon energy spectrum and geometry of the exposure scenario. This paper outlines the approach to assess individual dose contributions to absorbed dose in enamel and calculate individual air kerma of a radiation incident from the absorbed dose in tooth enamel.     

Evaluation of absorbed dose rate and annual effective dose equivalent due to terrestrial gamma radiation in rocks in a part of Southwestern Nigeria

The average outdoor absorbed dose rate in air and the average annual effective dose equivalent due to terrestrial gamma radiation from 40K, 238U and 232Th in rocks in Ondo and Ekiti States, Southwestern Nigeria have been evaluated from measurements of the concentrations of these radionuclides in this environmental material. The concentration measurements were obtained using a very sensitive gamma spectroscopic system consisting of a 7.6 cm x 7.6 cm NaI(Tl) scintillation detector coupled to a computerised ACCUSPEC installation. The average absorbed dose rate and average annual effective dose equivalent was found to be 8.33 +/- 2.76 nGy x h(-1) and 8.7 +/- 2.9 microSv x y(-1) respectively.     

Optically stimulated luminescence of some thermoluminescent detectors as an indicator of absorbed radiation dose

Stimulation spectra of several TLD materials in the short-wave spectral region are measured using the optically stimulated afterglow (OSA) method for determination of absorbed dose. Optical stimulation spectra are studied in the region of wavelengths lower than those of emission spectra. The effective optical stimulation hands have been found for examined materials in the regions of wavelengths which overlap with fluorescence excitation bands. Application of short-wave OSA bands for determination of absorbed dose is analysed.     

Neutron absorbed dose determination by calculations of recoil energy

The aim of this work is to calculate the absorbed dose to matter due to neutrons in the 5-150 MeV energy range. Materials involved in the calculations are Al2O3, CaSO4 and CaS, which may be used as dosemeters and have already been studied for their luminescent properties. The absorbed dose is assumed to be mainly due to the energy deposited by the recoils. Elastic reactions are treated with the ECIS code while for the non-elastic ones, a Monte Carlo code has been developed and allowed to follow the nucleus decay and to determine its characteristics (nature and energy). Finally, the calculations show that the absorbed dose is mainly due to non-elastic process and that above 20 MeV this dose decreases slightly with the neutron energy.     

Assessment of personnel absorbed dose at production of medical radioisotopes by a cyclotron

The medical radioisotope (201)Tl is produced by a cyclotron through the (203)Tl(p, 3n)(201)Pb reaction in the nuclear medicine research group of Agricultural, Medical and Industrial Research Schools in Iran. The produced (201)Pb decays to (201)Tl by electron capture. One of the most important problems that may occur is malfunction of a part of target or beam line, so that it needs the bombardment to be stopped and the problem fixed. In this work, induced radioactivity of the target, aluminium case of target, beam line and concrete walls of the thallium target room were calculated by Monte Carlo method. Then by using the results of the Monte Carlo simulation, the whole body absorbed dose to cyclotron personnel during repair and after stopping the bombardment, were assessed at different places of target room.     

Determination of absorbed dose in the vicinity of a neutron source

A computational method is presented that transfers the spectral neutron fluence from the measuring position to a reference point in a biological sample. The method is applied to the determination of the absorbed dose in adult blood irradiated with monoenergetic neutrons of 0.035 and 14.8 MeV.     

The estimation of absorbed doses received by a victim of a Chinese radiation accident

The aim of this work was to estimate absorbed doses received by a victim of the radiation accident with a 60Co source in Henan province, China. With a Monte Carlo stochastic simulation method, an estimation method for doses to the radiation accident victim was made. It utilised a mathematical model of adult man (MIRD) and a relative applied computer program was developed. By means of the simulated conditions of the accident, the absorbed doses to the victim's main organ and total body doses were estimated. The results estimated by our Monte Carlo method are close to those of experimental simulation measurement of the accident. With its convenience and rapidity, this method will be valuable for radiation dose reconstruction for victims in radiation accidents.     

Determination of the absorbed dose and the average LET of space radiation in dependence on shielding conditions

The HTR method, developed for determination of absorbed dose and average LET of mixed radiation fields in space, was applied during several space missions on space station MIR, space shuttles and satellites. The method utilises the changes of peak height ratios in the glow curves in dependence on the linear energy transfer LET. Due to the small size of the dosemeters the evaluation of the variation of absorbed dose and average LET in dependence on the position of the dosemeters inside the space station is possible. The dose and LET distribution was determined during the experiment ADLET where dosemeters were exposed in two positions with different shielding conditions and during two following experiments (MIR-95, MIR-96) using six positions inside the space station. The results were compared with the shielding conditions of the positions. Calculations of the absorbed dose were carried out for comparison. Results have shown that the average LET increases with increasing absorbing thickness while the absorbed dose decreases.     

Determination of dose-build-up thickness for absorbed dose measurement in high energy electron-photon radiation at electron storage rings

Radiation field during accidental electron beam loss near electron storage rings comprises of high-energy electrons and bremsstrahlung photons. Due to high-energy nature of the radiation, the absorbed dose is expected to build up with depth of the body of an exposed worker and hence absorbed dose measurements in 30 cm water/tissue equivalent phantom is essential. Carrying out the measurements with in such phantoms is not practically feasible for routine applications and hence the use of an equivalent solid material around the dosemeters would be convenient. Monte Carlo calculations have been carried out in order to determine the dose-build-up thickness required in copper and lead, corresponding to the maximum absorbed within 30 cm depth of water in such radiation fields. Equivalent build-up thickness calculated for a case was verified by measurement using an ion chamber detector. The results are found to be in agreement within +/-20%.     

Retrospective biological dosimetry of absorbed radiation

Frequencies of chromosomal translocations in human peripheral blood lymphocytes irradiated in vitro by 200 kV X rays have been estimated by the fluorescence in situ hybridisation (FISH) technique. Probes specific for whole chromosomes 1, 3 and 4 were labelled with, biotin 16 dUTP + digoxigenin 11 dUTP; biotin 16 dUTP and digoxigenin 11 dUTP respectively, to achieve three colour FISH to distinguish exchanges involving individual chromosomes. The translocation frequencies for individual chromosomes suggested a relationship between the response and the genomic content of the chromosomes studied. Dose-response curves derived from translocations involving all the three painted chromosomes (22.3% of the genome) showed a linear-quadratic response with alpha and beta coefficients of 0.027 +/- 0.009 Gy-1 and 0.033 +/- 0.004 Gy-2, corresponding coefficients for the response curves extrapolated to the entire genome are 0.072 +/- 0.026 Gy-1 and 0.075 +/- 0.011 Gy-2 respectively. Dose-response curves for the induction of dicentrics involving painted chromosomes also showed a similar response, but the dicentric frequencies were consistently lower than the translocation frequencies for all the doses studied. Dose fractionation resulted in a small but significant reduction in the yield of exchanges, thus suggesting repair of some radiation damage during the period between the two fractions. Retrospective biological dosimetry was attempted with FISH assay of five radiation workers with chronically accumulated doses of approximately 500 mSv received over a period of 2-3 decades. Among the 2421 metaphases scored in these subjects 28 translocations involving the painted chromosomes were detected. The dose estimates based on FISH assay ranged from 0.215 +/- 0.116 Gy to 0.635 +/- 0.292 mGy, thus suggesting the usefulness of this assay in retrospective biological dosimetry.