Experimental simulation of A-bomb gamma ray spectra for radiobiology studies

Improved radiation protection of humans requires a better understanding of the mechanisms of radiation action and accurate estimates of radiation risk for both internal and external radiations. The Japanese atomic bomb survivors represent one of the most important sources of human data on the late carcinogenic effects of ionising radiations. The present study was undertaken to investigate whether it would be possible to use hospital radiotherapy/radiobiology equipment to mimic the spectra encountered in Hiroshima and Nagasaki. The estimated total gamma ray fluence spectra (including both prompt and delayed photons) at both Hiroshima and Nagasaki, for distances of 500, 1000, 1500 and 2000 m have been evaluated using DS86 data and previously unpublished information for delayed gamma radiations which constitute the major contribution to survivor doses. Monte Carlo (EGS4) simulations were performed to transport these photons through the body in order to investigate the variation in electron spectra for various body organs. The electron spectra obtained for these fluences at, for example, the colon, have been matched with combinations of electron spectra produced by linear accelerators to within 5% SD. These will, for the first time, enable a direct link to be made between radiobiological studies (for example, on mammography spectra) and the epidemiological data from Japan, which currently underpin radiation risk estimates.     

Leukaemia following childhood radiation exposure in the Japanese atomic bomb survivors and in medically exposed groups

Incidence and mortality risks of radiation-associated leukaemia are surveyed in the Japanese atomic bomb (A-bomb) survivors exposed in early childhood and in utero. Leukaemia incidence and mortality risks are also surveyed in 16 other studies of persons who received appreciable doses of ionizing radiation in the course of treatment in childhood and for whom there is adequate dosimetry and cancer incidence or mortality follow-up. Relative risks tend to be lower in the medical series than in the Japanese A-bomb survivors. The relative risks in the medical studies tend to diminish with increasing average therapy dose. After taking account of cell sterilisation and dose fractionation, the apparent differences between the relative risks for leukaemia in the Japanese A-bomb survivors and in the medical series largely disappear. This suggests that cell sterilisation largely accounts for the discrepancy between the relative risks in the Japanese data and the medical studies. Excess absolute risk has also been assessed in four studies, and there is found to be more variability in this measure than in excess relative risk. In particular, there is a substantial difference between the absolute risk in the Japanese atomic bomb survivor data and those in three other (European) populations. In summary, the relative risks of leukaemia in studies of persons exposed to appreciable doses of ionizing radiation in the course of treatment for a variety of malignant and non-malignant conditions in childhood are generally less than those in the Japanese A-bomb survivor data. The effects of cell sterilisation can largely explain the discrepancy between the Japanese and the medical series.     

Point defects and the blue emission in fired quartz at high doses: a comparative luminescence and EPR study

The fundamental assumption implicit in the use of the atomic bomb survivor data to derive risk estimates for occupational and medical exposures is that the gamma rays of Hiroshima and Nagasaki are considered as equal efficiencies to other low LET radiations up to an LET of 10 keV.micron-1. For breast cancer induction, neoplastic cell transformation, mutation, reciprocal translocations and dicentrics in human lymphocytes, a strong and very similar dependence of the RBE values on photon energy or on LET is observed. Experimental data on mutation induction and neoplastic cell transformation in human cells show that 29 kVp X rays are by a factor of 4 and 3.4, respectively, more effective compared with 200 kVp X rays. These data are in excellent agreement with the data in the literature.     

Dosimetric analysis of thyroid doses from total cranial irradiation

The purpose was to estimate the radiation dose to the thyroid from total cranial irradiation with parallel opposed lateral fields of 1.25 MeV gamma rays of (60)Co, to represent the current treatment modality, and with 6 MV photon beams derived from a linear accelerator. The mean thyroid doses were measured using thermoluminescence dosemeters in the Rando phantom with two cranial irradiation set-ups: (1) the lateral beams were angled by rotating the collimator to avoid irradiation of the lens and healthy tissues and (2) the lateral beams were not angled, and shielding blocks were used to protect the lens and healthy tissues. For a cranial dose of 18 Gy, thyroid doses for child patients for (60)Co with the open and blocked fields were 0.27 and 0.32 Gy, respectively. With 6 MV, the thyroid dose was less than the doses for (60)Co open and blocked cranial fields by 17 and 10%, respectively. Unblocked cranial fields and a high-energy photon beam should be preferred for cranial irradiation of children.     

'Lifestyle' and cancer rates in former East and West Germany: the possible contribution of diagnostic radiation exposures

Breast and prostatic cancer as well as leukaemia in childhood have remarkably increased over some decades in the Federal Republic of Germany as well as in several other highly developed industrial nations. Such increase was much less or not observable in East Germany between 1960 and 1989 where diagnostic exposures were applied to a lesser extent. Low-level radiation can cause these diseases and the difference of cancer rates gives rise to renewed evaluation of current risk estimates. Risk factors for radiation-induced childhood leukaemia and breast cancer are derived from the literature, considering a higher relative biological effectiveness of diagnostic X rays in comparison to the A-bomb gamma rays in Hiroshima and Nagasaki. The prostate is not considered as radiation sensitive by the ICRP. But following a variety of low-level findings in the last two decades it was shown by Myles et al. in the UK that prostatic cancer is inducible by diagnostic X-ray procedures. From their study in men below the age of 60, a doubling dose of about 20 mSv can be estimated. Medical exposures of the considered tissues are taken from published data for East and West Germany. The difference in breast cancer mortality can be explained by diagnostic exposures. The contribution of these to prostatic cancer and childhood leukaemia must be regarded as relevant in current incidences. Reduction of diagnostic exposures would be an important measure for preventing several prominent cancer diseases.     

Current risk estimates based on the A-bomb survivors data - a discussion in terms of the ICRP recommendations on the neutron weighting factor

Currently, most analyses of the A-bomb survivors' solid tumour and leukaemia data are based on a constant neutron relative biological effectiveness (RBE) value of 10 that is applied to all survivors, independent of their distance to the hypocentre at the time of bombing. The results of these analyses are then used as a major basis for current risk estimates suggested by the International Commission on Radiological Protection (ICRP) for use in international safety guidelines. It is shown here that (i) a constant value of 10 is not consistent with weighting factors recommended by the ICRP for neutrons and (ii) it does not account for the hardening of the neutron spectra in Hiroshima and Nagasaki, which takes place with increasing distance from the hypocentres. The purpose of this paper is to present new RBE values for the neutrons, calculated as a function of distance from the hypocentres for both cities that are consistent with the ICRP60 neutron weighting factor. If based on neutron spectra from the DS86 dosimetry system, these calculations suggest values of about 31 at 1000 m and 23 at 2000 m ground range in Hiroshima, while the corresponding values for Nagasaki are 24 and 22. If the neutron weighting factor that is consistent with ICRP92 is used, the corresponding values are about 23 and 21 for Hiroshima and 21 and 20 for Nagasaki, respectively. It is concluded that the current risk estimates will be subject to some changes in view of the changed RBE values. This conclusion does not change significantly if the new doses from the Dosimetry System DS02 are used.     

Studies on the response of the TLD badge for high-energy photons

Absorbed tissue dose measurements are carried out for high-energy photon beams using CaSO4:Dy thermo-luminescence dosemeter (TLD) badge and the results are also verified using ionisation chamber used in radiation therapy. The photon beams generated using linear accelerator at 6 and 18 MV photon beam energies have been used and the absorbed doses are measured at the surface as well as at various depths. It has been found that the depth at which maximum dose is delivered increases with the increase in photon energy and the depth of maximum absorbed dose in tissue occurs beyond 10 mm. It has also been found that the evaluation of the absorbed dose (or Hp(10) as well) using thermoluminescence readout of disc D1 clearly shows that the current TLD badge provides a reasonable estimate of the effective dose for photon fields from 6 to 18 MV linacs for anterior-posterior incidence. The paper also provides information regarding the misinterpretation of radiation pattern in multi-element/filter TLD badge.     

The development of fetal dosimetry and its application to A-bomb survivors exposed in utero

The cohort of the atomic bomb survivors of Hiroshima and Nagasaki comprises the major basis for investigations of health effects induced by ionising radiation in humans. To study the health effects associated with radiation exposure before birth, fetal dosimetry is needed if significant differences exist between the fetal absorbed dose and the mother's uterine dose. Combining total neutron and gamma ray free-in-air fluences at 1 m above ground with fluence-to-absorbed dose conversion coefficients, fetal doses were calculated for various exposure orientations at the ground distance of 1500 m from the hypocentres in Hiroshima and Nagasaki. The results showed that the mother's uterine dose can serve as a good surrogate for the dose of the embryo and fetus in the first trimester. However, significant differences exist between doses of the fetus of different ages. If the mother's uterine dose were used as a surrogate, doses to the fetus in the last two trimesters could be overestimated by more than 20 % for exposure orientations facing towards and away from the hypocentre while significantly underestimated for lateral positions relative to the hypocentre. In newer fetal models, the brain is modelled for all fetal ages. Brain doses to the 3-month fetus are generally higher than those to an embryo and fetus of other ages. In most cases, brain absorbed doses differ significantly from the doses to the entire fetal body. In order to accurately assess radiation effects to the fetal brain, it is necessary to determine brain doses separately.     

Response of CaSO4:Dy based TL dosemeter system to high-energy photon beams: theoretical simulation

Response of thermoluminescence (TL) discs under different filter regions of a CaSO(4):Dy based TL dosemeter system was simulated to high-energy photon beams in the energy range of 1.25 MeV to 24 MV ( approximately 9 MeV). This was done using FLUKA Monte Carlo code and also experimentally verified for some energy points. Response of disc D1 under metal filter combination was found to increase with photon energy, whereas that for the discs under polystyrene filter and open window regions of the dosemeter decreases continuously. The changes in the response of the discs under polystyrene filter and open window were attributed to the lack of build-up material. The increase in the response of disc D1 was due to the contribution from secondary electrons produced through Compton and pair production processes mainly arising out from the metal filter combination. The knowledge of the change in the response of individual discs and the ratio of discs' responses under different filter regions of the dosemeter system could be used for the measurement of energy of bremsstrahlung radiation that exists in and around high-energy electron accelerator and could be used for accurate evaluation of personal dose equivalent in high-energy photon field.     

Estimation of bremsstrahlung photon energy in the environment of high-energy electron accelerator using CaSO4:Dy based TL dosemeter

High-energy bremsstrahlung X rays constitute the major radiation hazard to working personnel around the high-energy electron accelerators. Thermoluminescent (TL) dosemeter system based on CaSO4:Dy Teflon disc used in the routine individual monitoring was used to estimate the bremsstruhlung photon energy at different locations of the experimental hall of 450-MeV synchrotron accelerator. The response of TL discs under different filter regions of the dosemeter system undergo change with photon energy due to the lack of build up and interaction of photon in the metal filters. This change in the response of the discs used to estimate the energy of the bremsstrauhlung photon in conjunction with suitable calibration curve generated using known photon energy from medical linear accelerator. The photon energies estimated were in the range 1-4 MeV, depending on the locations.     

水量热法加速器光子水吸收剂量绝对测量与国际比对

为实现加速器光子水吸收剂量绝对测量,研制了水量热计系统,在此基础上取得国际互认并建立加速器光子水吸收剂量基准,进一步提高了我国放疗剂量量值传递能力。通过水浴与半导体制冷系统二级控温,将量热计水模体的温度漂移控制在0.5μ℃/s。利用惠斯通交流电桥测量辐射所致的热敏探针阻值变化,逐次校准热敏探针和交流电桥,实现了医用加速器光子水吸收剂量的绝对测量,合成标准不确定度为0.30%。参加了国际计量局加速器光子水吸收剂量关键比对,复现的6MV和10MV光子水吸收剂量值与比对参考值之比分别为0.9917和0.9949,在不确定度允许的范围内一致。     

Monte Carlo simulation of a medical linear accelerator for radiotherapy use

A Monte Carlo code MCNPX (Monte Carlo N-particle) was used to model a 25 MV photon beam from a PRIMUS (KD2-Siemens) medical linear electron accelerator at the Centre Antoine Lacassagne in Nice. The entire geometry including the accelerator head and the water phantom was simulated to calculate the dose profile and the relative depth-dose distribution. The measurements were done using an ionisation chamber in water for different square field ranges. The first results show that the mean electron beam energy is not 19 MeV as mentioned by Siemens. The adjustment between the Monte Carlo calculated and measured data is obtained when the mean electron beam energy is approximately 15 MeV. These encouraging results will permit to check calculation data given by the treatment planning system, especially for small fields in high gradient heterogeneous zones, typical for intensity modulated radiation therapy technique.     

加速器辐射质测量方法的比较分析

针对加速器高能光子辐射质的不同测量方法,在剂量比(D20/D10)通过公式计算法和组织模体比(TPR20,10)直接实验法的基础上,提出了定剂量率和定剂量的测量方法.在中国计量科学研究院加速器实验室分别对6 MV、10 MV、25 MV光子的辐射质进行测量.实验结果表明:在测量加速器辐射质时,采用定剂量法比定剂量率法不确定度明显变小;采用定剂量法时,6 MV、10 MV、25 MV采用公式法计算的值比实际测量值分别偏小0.27％、0.31％、0.35％,采用剂量率法进行修正后和此结果一致.     

Organ dose conversions from ESR measurements using tooth enamel of atomic bomb survivors

Dose conversions were studied for dosimetry of atomic bomb survivors based upon electron spin resonance (ESR) measurements of tooth enamel. Previously analysed data had clarified that the tooth enamel dose could be much larger than other organ doses from a low-energy photon exposure. The radiation doses to other organs or whole-body doses, however, are assumed to be near the tooth enamel dose for photon energies which are dominant in the leakage spectrum of the Hiroshima atomic bomb assumed in DS02. In addition, the thyroid can be a candidate for a surrogate organ in cases where the tooth enamel dose is not available in organ dosimetry. This paper also suggests the application of new Japanese voxel phantoms to derive tooth enamel doses by numerical analyses.     

医用加速器光子水吸收剂量国际比对及进展

医用加速器光子水吸收剂量国际比对采用星式比对方式,主导实验室为国际计量局,其石墨量热计复现的水吸收剂量为比对参考值.各国计量实验室依据各自医用加速器辐射质参数,进行2～3个辐射质的水吸收剂量绝对测量,现已完成7个国家的比对工作,比对数据公布在关键比对数据库中.     

ESR dosimetry for atomic bomb survivors and radiologic technologists

An individual absorbed dose for atomic bomb (A-bomb) survivors and radiologic technologists has been estimated using a new personal dosimetry. This dosimetry is based on the electron spin resonance (ESR) spectroscopy of the CO₃³⁻ radicals, which are produced in their teeth by radiation. Measurements were carried out to study the characteristics of the dosimetry; the ESR signals of the CO₃³⁻ radicals were stable and increased linearly with the radiation dose. In the evaluation of the absorbed dose, the ESR signals were considered to be a function of photon energy. The absorbed doses in ten cases of A-bomb victims and eight cases of radiologic technologists were determined. For A-bomb survivors, the adsorbed doses, which were estimated using the ESR dosimetry, were consistent with the ones obtained using the calculations of the tissue dose in air of A-bomb, and also with the ones obtained using the chromosome measurements. For radiologic technologists, the absorbed doses, which were estimated using the ESR dosimetry, agreed with the ones calculated using the information on the occupational history and conditions. The advantages of this method are that the absorbed dose can be directly estimated by measuring the ESR signals obtained from the teeth of persons, who are exposed to radiation. Therefore, the ESR dosimetry is useful to estimate the accidental exposure and the long term cumulative dose.     

Comparison of the energy-response factor of LiF and Al2O3 in radiotherapy beams

A Monte Carlo study of the energy-response factor of aluminium oxide (Al2O3) and lithium fluoride (LiF) TLDs in kilovoltage and megavoltage photon beams relative to 60Co gamma rays has been performed using EGSnrc Monte Carlo simulations. The sensitive volume of the detector was simulated as a disc of diameter 2.85 mm and thickness 1 mm. The phantom material was water and the irradiation depth was 2.0 cm in kilovoltage photon beams and 5.0 cm for megavoltage photon beams. The results show that the energy-response of the Al2O3 and LiF-TLDs is constant within 3% for photon beam energies in the energy range of 60Co gamma rays to 25 MV X rays. However, both detectors show an enhanced response for kilovoltage photon beams, which in the case of 50 kV X rays is 3.2 times higher than that for 60Co gamma rays. The energy-response factor was 1.46 for LiF irradiated in 50 kV X rays. The Al2O3 detector has an energy-response that is 2.2 times higher than that of LiF in 50 kV X rays decreasing to 1.19 for 250 kV X rays. The results show that the addition of 0.1 or 1% of carbon by weight (as dopant) into the Al2O3 does not change the Monte Carlo determined energy-response factor by more than 1%.     

Water equivalent plastic scintillation detectors in radiation therapy

A review of the dosimetric characteristics and properties of plastic scintillation detectors for use in radiation therapy is presented. The detectors show many desirable qualities when exposed to megavoltage photon and electron beams, including water equivalence, energy independence, reproducibility, dose linearity, resistance to radiation damage and near temperature independence. These detectors do not require the usual conversion and/or correction factors used to convert the readings from common dosemeters to absorbed dose. Due to their small detecting volume, plastic scintillation detectors exhibit excellent spatial resolution. Detector performance, in certain specific cases, can be affected by radiation-induced light arising in the optical fibres that carry the scintillator signal to a photodetector. While this effect is negligible for photon beams, it may not be ignored for electron beams and needs to be accounted for.     

光电技术在合肥光源束测量系统中的应用

介绍了光电技术在合肥同步辐射加速器束流测量系统中的应用。介绍了合肥 2 0 0 Me V电子直线加速器利用光电二极管阵列的能谱测量系统、合肥 80 0 Me V电子储存环利用 CCD技术的束流截面测量系统以及利用单光子计数法测量束团纵向精细结构。本文给出了测量结果。     

Evaluation of the EDR-2 film for relative dosimetry of high-energy photon and electron beams

A sensitometric study of Kodak XV and EDR-2 radiographic films (Eastman Kodak Company, Rochester, NY) was performed using photons ranging from 75 kV to 18 MV and electrons ranging from 6 to 20 MeV. To investigate the applicability of the EDR-2 film for clinical radiation dosimetry, percentage depth-doses, profiles and distributions in open and dynamically wedged fields were measured using film and compared with data from a linear diode. Moreover, conventional quality assurance dose parameters were measured, including open-field dose profiles to determine flatness and symmetry of photon and electron beams. Finally, film was employed to validate dose distributions produced by complex computerised treatment planning techniques. Our conclusion is that the EDR-2 film is an effective tool for relative dosimetry of photon and electron beams.