Showing papers on "Dosimetry published in 1989"

Collapsed cone convolution of radiant energy for photon dose calculation in heterogeneous media

Abstract: A method for photon beam dose calculations is described. The primary photon beam is raytraced through the patient, and the distribution of total radiant energy released into the patient is calculated. Polyenergetic energy deposition kernels are calculated from the spectrum of the beam, using a database of monoenergetic kernels. It is shown that the polyenergetic kernels can be analytically described with high precision by (A exp( -ar) + B exp( -br)/r2, where A, a, B, and b depend on the angle with respect to the impinging photons and the accelerating potential, and r is the radial distance. Numerical values of A, a, B, and b are derived and used to convolve energy deposition kernels with the total energy released per unit mass (TERMA) to yield dose distributions. The convolution is facilitated by the introduction of the collapsed cone approximation. In this approximation, all energy released into coaxial cones of equal solid angle, from volume elements on the cone axis, is rectilinearly transported, attenuated, and deposited in elements on the axis. Scaling of the kernels is implicitly done during the convolution procedure to fully account for inhomogeneities present in the irradiated volume. The number of computational operations needed to compute the dose with the method is proportional to the number of calculation points. The method is tested for five accelerating potentials; 4, 6, 10, 15, and 24 MV, and applied to two geometries; one is a stack of slabs of tissue media, and the other is a mediastinum-like phantom of cork and water. In these geometries, the EGS4 Monte Carlo system has been used to generate reference dose distributions with which the dose computed with the collapsed cone convolution method is compared. Generally, the agreement between the methods is excellent. Deviations are observed in situations of lateral charged particle disequilibrium in low density media, however, but the result is superior compared to that of the generalized Batho method.

Dosimetry for Radiation Processing

Abstract: During the past few years significant advances have taken place in the different areas of dosimetry for radiation processing, mainly stimulated by the increased interest in radiation for food preservation, plastic processing and sterilization of medical products. Reference services both by international organizations (IAEA) and national laboratories have helped to improve the reliability of dose measurements. Several dosimeter systems like calorimetry, perspex, and radiochromic dye films are being improved and new systems have emerged, e.g. spectrophotometry of dichromate solution for reference and sterilization dosimetry, optichromic dosimeters in the shape of small tubes for food processing, and ESR spectroscopy of alanine for reference dosimetry. In this paper the special features of radiation processing dosimetry are discussed, several commonly used dosimeters are reviewed, and factors leading to traceable and reliable dosimetry are discussed.

An in vivo evaluation

Abstract: On the basis of dose readings in 102 patients treated with total body irradiation (TBI), a ‘tissue air ratio (TAR) curve’ has been produced. It could be useful to precalculate treatment time in TBI, for dose prescription to a specific point, provided the same source (60Co) and treatment setting (lateral irradiation; 3 m source-axis distance; reference point at thighs bifurcation, near the perineum) is used. The TAR curve produced, and the formula relating tissue depth to TAR value, are presented, and compared to preexisting data for ‘magna fields’ treatments. This curve is exponential, and in semilog representation becomes straight, as every classic TAR curve; it is lower than others, reflecting non full-scatter situation in patient irradiation. Total body irradiation (TBI) and hemibody irradiation (HBI) are increasingly used to treat advanced diseases. In addition, TBI plays an important role in treatment regimens involving bone marrow transplantation. The implementation of TBI and HBI presents a dosimetry challenge. In conventional radiotherapy treatment target dose is calculated on well established depth dose distribution data and only occasionally is the actual dose delivered confirmed ‘in vivo’; but common dosimetry concepts can in no way be applied to field sizes and treatment distances involved in TBI and HBI. Moreover, since patients undergoing TBI occupy part of the radiation field instead of presenting a uniform flat surface to the beam, an entirely new approach to treatment is required. The delivery of a uniform radiation dose to all parts of the patient’s body is difficult (1, 2) and there is no consensus on the methods of depth dose determination for TBI or HBI. It is therefore customary to measure the dose actually delivered to each patient at one or various reference points; on this basis depth dose is sometimes calculated

Ferrous sulphate gels for determination of absorbed dose distributions using MRI technique: basic studies

Abstract: Two gels have been found to be suitable to load with ferrous sulphate solution. In these soft tissue equivalent phantoms, the absorbed dose distribution can be measured after irradiation in clinically used MR imaging equipment. The present studies were carried out using a 0.25 T NMR analyser without imaging properties. A ferrous sulphate solution, 0.05 M with respect to sulphuric acid, can be gelled with 4% gelatin to give a dosemeter which has a response which is linearly correlated (r = 0.998) with the absorbed dose in the interval 0-40 Gy. Ferrous sulphate solution can also be gelled with 1% agarose, but this gel has to be purged with oxygen to obtain a linear relationship (r = 0.997) in the same absorbed dose interval. The ferrous sulphate loaded gels have a sensitivity which is a factor of 2.2 or 4.0 times higher for gelatin and agarose, respectively, than the ordinary dosemeter solution. Because the standard deviation of background measurements is higher for the gels than for the dosemeter solution, the minimum detectable absorbed dose is about the same, or 1.0 Gy, for the two gels and the dosemeter solution. The sensitivity of the ferrous sulphate loaded gels shows no dependence on dose rate if the mean dose rate and the absorbed dose per pulse are within the limits normally used by accelerators for radiotherapy.

Backscatter dose perturbation at high atomic number interfaces in megavoltage photon beams.

Abstract: Most computer algorithms used clinically for photonbeamtreatment planning are unable to predict the effect of electron backscattering on dose distribution from high atomic number materials. It has been observed that there is a significant dose enhancement at such an interface. We define the dose enhancement in terms of backscatter dose factor (BSDF), which depends on the energy of the photonbeam, thickness and width of the inhomogeneity, distance from the interface, and the atomic number of the inhomogeneity. For all energies studied, the dose fall‐off is initially very rapid and disappears beyond a few millimeters upstream from the interface. Empirically derived equations are presented for dose calculation at the interfaces of various media, including bone and soft tissue, for photon energies in the range of Co‐60 gamma rays to 24 MV x rays.

Low dose radiation : biological bases of risk assessment

Abstract: Part 1 Review of current status of Japanese A-bomb study: the new radiation dosimetry for the A-bombs in Hiroshima and Nagasaki the new dose system and radiation induced cancer risks among A-bomb survivors radiation-related damage to the developing human brain the genetic effects of the atomic bombs - problems in extrapolating from somatic cell findings to risk for children cancer risk estimation from the A-bomb survivors - extrapolation to low doses, use of relative risk models, and other uncertainties. Part 2 Review of epidemiological studies: epidemiological studies of children irradiated in utero epidemiologic studies on uranium miners and other groups exposed to radon carcinogenesis following medical uses of ionising radiation cancer risks from internally deposited radium and thorotrast evidence from mammalian studies on genetic effects of low-level irradiation. Part 3 Risk projection models: models for projecting radiation risks in the BEIR V report the role of dose inhomogeneity in biological models of dose-response relative and absolute risk models for cancer mortality in ankylosing spondylitis patients epistemiological limits of risk assessments at low radiation doses. Part 4 Animal studies: the influence of physical factors on carcinogenesis in experimental animals age, sex and other factors in radiation carcinogenesis is a dose/response relationship a valid concept for the induction of leukaemia by bone-seeking alpha-emitting radionuclides irradiation of lymph nodes after deposition of radioactive particles in the lung. Part 5 Sensitive groups in the population: human ill health, abnormal radiation induced cytotoxicity and aberrant DNA metabolism radiosensitive individuals in the population determination of the proportion of persons in the population-at-large who ehhibit abnormal sensitivity to ionizing radiation. Part 6 Radiation effects in the lung: requirements for local dosimetry and risk evaluation in inhomogeneously irradiated organs lung tumour induction in mice after X-rays and neutrons. Part 7 Epidemiology and effects on the foetus. Part 8 Molecular biology and cell transformation. Part 9 Radiation quality. Part 10 Reverse dose rate effect.

Dose parameters of 125I and 192Ir seed sources

Abstract: As mandated by an NCI brachytherapy contract, we measured dosimetric parameters for 1 9 2Ir seeds and two models of 1 2 5I seeds. Measurements were with LiF powder in a water‐equivalent phantom. Data were corrected for background, sample mass, and finite detector volume. Selected parameters were also investigated through Monte Carlo calculations. Results are presented in terms of a dose parametrization that is described in detail, and are compared to published data. Our results agreed well with published data for relative quantities such as radial and angular dose dependence. Our measured value for the 1 9 2Ir dose factor was 4.55 cGy(H2O) cm2 mCi− 1 h− 1, also in good agreement with commonly used values. However, the measured dose factors for 1 2 5I seed models 6702 and 6711 were 1.18 and 1.06 cGy(H2O) cm2 mCi− 1 h− 1, values well below those in general use.

Limitations of Conventional Internal Dosimetry at the Cellular Level

Abstract: A theoretic examination of the validity at the cellular level of assumptions used in classic internal dosimetry has been undertaken. An alternate dosimetric model accounting for the consequences of selective uptake of a radiolabeled compound by specific cells in a multicellular cluster of hexagonal geometry has been developed. At the cellular level, derived dose estimates for electrons have been compared to dose estimates obtained employing the assumptions of conventional internal dosimetry. The study has been performed for all electron energies and then applied specifically to electrons emitted by 99mTc, 201Tl, 111In, and 123I. The dosimetric consequences of altering (a) the intracellular-to-extracellular radionuclide concentration, (b) the labeled cell density, and (c) the cell size have been examined for the labeled and nonlabeled cells in a cell cluster, and the conditions in which conventional dosimetry underestimates or overestimates the dose to individual cells have been indicated. It is shown that when selective intracellular uptake of a radiolabeled compound occurs in specific cells within a cell cluster, conventional dosimetry underestimates the radiation dose delivered to the labeled cells by twofold to more than 25-fold if the emitted electrons have ranges of a few micrometers or less, i.e., energies smaller than approximately 10 keV. Under the same conditions, conventional dosimetry overestimates slightly (20% to 50%) the electron radiation dose to the nonlabeled cells of the cell cluster. It is shown that inclusion of photons in the calculation of the total dose to individual cells does not alter significantly the conclusions of the present investigation.

Spectral measurement during thermoluminescence—an essential requirement

Abstract: Emission spectra recorded during thermoluminescence provide a fuller understanding of defect properties. Also, as detailed changes in spectra occur with dose, dose rate and thermal treatments, knowledge of emission spectra will make even simple radiation dosimetry, as well as applications such as archaeological and geological dating, more reliable.

Mutant frequency of radiotherapy technicians appears to be associated with recent dose of ionizing radiation.

Abstract: The frequency of hypoxanthine phosphoribosyl transferase (HPRT) mutants among peripheral T-lymphocytes of radiotherapy technicians primarily exposed to 60Co was measured by the T-cell cloning method. Mutant frequencies of these technicians in 1984 and 1986 were significantly higher than those of physiotherapy technicians who worked in a neighboring service, and correlated significantly with thermoluminescence dosimeter readings recorded during the 6 mo preceding mutant frequency determination. Correlations decreased when related to dose recorded over longer time intervals. HPRT mutant frequency determination in peripheral lymphocytes is a good measure of recently received biologically effective radiation dose in an occupationally exposed population.

A direct comparison of water calorimetry and Fricke dosimetry

Abstract: Considerable effort has been devoted to measuring the absorbed dose to water using water calorimetry. Most of these efforts have been hampered by a lack of adequate knowledge of the heat defect of water. The authors argue that there is now sufficient information to establish with considerable confidence the heat defect of high-purity water containing various dissolved gases. For the present work the authors used water saturated with a 50/50 mixture of H2 and O2 gases, for which the heat defect is calculated to be -2.1%. As a test of this assignment, they have compared the absorbed dose to water as measured using water calorimetry with that obtained from Fricke dosimetry. The authors find that for 20 MV X-rays, the dose to water determined by water calorimetry is 1.006+or-0.004 times the dose determined by Fricke dosimetry. Within 0.6(+or-0.4)%, this result supports the calculated heat defect of -2.1% for water saturated with a 50/50 mixture of H2 and O2 gases.

Simple theory, measurements, and Rules of thumb for dosimetry during photodynamic therapy

Abstract: Dosimetry of laser fluence rates within a tissue are required for proper planning of photodynamic therapy at a specific site on a given individual. A simple one-dimensional theory of light penetration into tissue is presented. A device for measurement of the total reflectance and the lateral diffusion of light provides a simple means for specifying the tissue optical parameters that govern laser dosimetry. Rules of thumb for complicating factors such as narrow laser beams and optical fiber delivery are discussed.

A model of the peritoneal cavity for use in internal dosimetry.

Abstract: Several therapeutic and diagnostic techniques involve injection of radioactive material into the peritoneal cavity. Estimation of the radiation dose to the surface of the peritoneum or to surrounding organs is hampered by the lack of a suitable source region in the phantom commonly used for such calculations. We have modified the Fisher-Snyder phantom to include a region representing the peritoneal cavity which may be employed to estimate such radiation doses. A geometric model is described which is coordinated with the existing organ regions in the phantom. Specific absorbed fractions (derived by Monte Carlo techniques) for photon emissions originating within the cavity are listed. Photon S-values for several radionuclides which have been administered intraperitoneally are shown. Dose conversion factors for electrons irradiating the peritoneal cavity wall, from either a thin plane or volume source of activity within the cavity, are also given for several nuclides.

The physics and radiobiology of fast neutron beams

Abstract: Part 1 Production of neutron beams: basic properties of the neutron neutron sources comparison of generators for neutron therapy comparison of generators for nuclear methods of analysis (activation). Part 2 Neutron interactions and kerma: cross sections kerma neutron spectra in terms of soft-tissue kerma. Part 3 Dosimetry: methods of dosimetry principles of mixed-field dosimetry absolute instruments relative instruments - ionisation chambers separation of neutron and x-ray components miscellaneous dosimetric method neutron dosimetry intercomparisons. Part 4 Penetration in tissue: phantom tissue measurement of dose distributions examples of dose distributions back scatter gamma radiation target thickness and filtration transition zone imhomogeneities. Part 5 Sheilding, collimation and radiation protection: shielding collimation doses outside the beam induced radioactivity doses received by staff due to activation room shielding protection measurements. Part 6 Radiation quality: neutron spectra quality from secondary charged-particles. Part 7 Radiobiology: general concepts RBE and neutron energy chemical modifiers of radiation response RBE and position in the cell cycle repair of damage after photons and neutrons RBE and dose level RBE in relation to tissue and end-point tumours RBE and secondary charged-particle equilibrium the gamma-ray component. Part 8 Neutron therapy: effects of hypoxia radioresistant tumours growth rate other trials in neutron therapy selection of cases treatment schedules other forms of high-LET radiation. Part 9 The radiation hazard of neutrons: quality factor use of chromosome abberations to assess dose-equivalent RBE values for life-shortening and carcinogenesis doses and risks to patients investigated by nuclear methods of analysis (activation).

Comparison of contralateral breast doses from 1/2 beam block and isocentric treatment techniques for patients treated with primary breast irradiation with 60CO.

Abstract: The risk of carcinogenesis in breast tissue subject to low to moderate radiation doses may be of concern to the clinical radiotherapist. With earlier diagnosis, more women, and especially younger women, are electing breast preservation radiation therapy. During therapy, tissue outside the treatment field is exposed to leakage and scattered radiation. Such exposure could lead to significant doses of radiation resulting in carcinogenesis. Therefore, reduction of contralateral breast dose may be an important factor to consider when selecting a treatment technique. This study measured dose in the contralateral breast on 15 patients treated with 60Co gamma rays with the source to skin distance (SSD), 1/2 beam block, and the source to axis distance (SAD), no 1/2 beam block, techniques. Thermoluminescent dosimeters (TLD), with 0.5 cm of superflab used as build-up material, were placed on the contralateral breast to measure dose from the medial tangential beam and from the lateral tangential beam. Dose measurements were done on patients in the treatment position and do not represent phantom or formula calculation of dose to the opposite breast. Our results indicated that total opposite breast dose ranged from 325-650 cGy for SSD treatments as opposed to 200-450 cGy for SAD treatments, in patients receiving a total prescribed dose of 5,040 cGy in 28 fractions to the involved breast. This paper points out that a simple solution for reduction of opposite breast dose for patients treated with 60Co may be utilization of the modified SAD treatment technique.

The influence of the size of the grid used for dose calculation on the accuracy of dose estimation.

Abstract: The standard presentation of a dose distribution as an isodose map is based on interpolation between dose values calculated on a matrix of equally spaced points. We explored the question of how the spacing of the grid used for the dose matrix affects the error due to interpolating the dose at any point. We defined two types of errors: the dose error, which is the difference between the interpolated and true dose at a given point; and the position error, which is the distance between the point of interest and the nearest point which has, in fact, the dose value estimated for the point of interest. We examine the problem using both an analytical beam profile (a Fermi function) and measured 60Co, x-ray and proton beam profiles. Our analysis showed that the interpolation errors are proportional to the curvature of the dose distribution and are relatively high in regions on either side of, but not including, the steepest part of the penumbra. Our results showed how big an interpolation error one should expect for a given size of the calculation grid. The specification of accuracy should be cast in the form of a pair of requirements, one for dose and the other for position. At a given point, only one of the two requirements needs to be satisfied. The position requirement is almost always the less demanding in clinical practice and permits the use of a larger grid spacing than if only a dose requirement is applied.(ABSTRACT TRUNCATED AT 250 WORDS)

Surface Dose Measurements in Clinical Photon Beams

Abstract: Surface dose measurements have been made with different dosemeters routinely used in clinical practice such as thermoluminescence (TL) dosemeters of different thicknesses or Si-diodes. The results obtained are compared with surface dose measurements made with an extrapolation chamber. TL-dosemeters with a thickness of 0.13 mm are shown to be suitable for skin dose estimations and accurate within 5% if appropriate correction factors are applied. The measured dose obtained for 6oCo, 6 MV and 21 MV x-rays should, for these dosemeters, be multiplied by 0.82, 0.90 and 1.0 respectively to obtain the correct surface dose. Thicker dosemeters are inaccurate because they overestimate the surface dose inside the beam and underestimate it outside the beam and the deviation from the correct dose varies with irradiation geometry.

Analysis of the DS86 atomic bomb radiation dosimetry methods using data on severe epilation

Abstract: This report presents a reanalysis of the Hiroshima and Nagasaki data on severe epilation as an acute radiation effect using both the new DS86 and the old T65D dosimetries. The focus of the report is on several aspects of the data which have previously been examined by Jablon et al (ABCC TR 12-70, 1970) and Gilbert and Ohara [Radiat. Res. 100, 124-138 (1984)]. The report examines the uniformity of epilation response across shielding category, across sex and age, and in terms of interactions between city, sex, age, and shielding category; it also investigates the apparent relative biological effectiveness (RBE) of neutrons in the DS86 dose compared with the T65D dose, using both within- and between-city information. In addition the report discusses evidence for nonlinearity in epilation response. The epilation response function exhibits nonlinearity in terms of both a marked increase in slope at about 0.75 Gy, and then, beginning at about 2.5 Gy, a leveling off and eventual decrease in response. The principal conclusions of the report are as follows. The use of the DS86 dosimetry rather than T65D increases the apparent RBE of neutrons compared with gamma dose from approximately 5 to 10. At these values of RBE the slope of the dose response, in a middle range from 0.75-2.5 Gy, is about 165% greater using DS86 than T65D. With respect to the interactions of sex, city, and shielding method, the size and significance of virtually all nonuniformities in epilation response seem using T65D are also evident with DS86. Additionally it seems difficult to find any evidence that DS86 is an improved predictor of epilation response over T65D. Finally, the fact that the nonlinearity in dose response and apparent actual downturn in epilation occurrence rate at the high end of dose is more striking with DS86 than with T65D is found to be due primarily to the common practice of truncating all T65D doses to 600 rad.