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PubMed1986

Chamber-dependent wall correction factors in dosimetry.

The calculation of the response of ionisation chambers to different photon beam qualities used in radiotherapy requires electron stopping-power data. These data have recently been revised and are used here to derive a consistent set of correction factors for ionisation chambers of different wall and build-up cap composition. Theoretically derived parameters km and katt which relate the exposure calibration of an ionisation chamber to the absorbed dose to the air of the cavity, are compared with the experimentally derived product kmkatt showing generally good agreement but also significant discrepancies for plastic-walled chambers with inner graphite coatings. A table of km values is given for a large number of commercial ionisation chambers. The new stopping-power data are also used to evaluate the wall-dependent correction factor (pwall) that enters into the determination of the absorbed dose to water in photon beams, results being given as a function of the quality of the beam. Our theoretical calculations of pwall are consistent with existing experimental data.

Cobalt Radioisotopes↗

PubMed2000

Energy correction factors of LiF powder TLDs irradiated in high-energy electron beams and applied to mailed dosimetry for quality assurance networks.

Absorbed dose determination with thermoluminescent dosimeters (TLDs) generally relies on calibration in 60Co gamma-ray reference beams. The energy correction factor fCo(E) for electron beams takes into account the difference between the response of the TLD in the beam of energy E and in the 60Co gamma-ray beam. In this work, fCo(E) was evaluated for an LiF powder irradiated in electron beams of 6 to 20 MeV (Varian 2300C/D) and 10 to 50 MeV (Racetrack MM50), and its variation with electron energy, TLD size and nature of the surrounding medium was also studied for LiF powder. The results have been applied to the ESTRO-EQUAL mailed dosimetry quality assurance network. Monte Carlo calculations (EGS4, PENELOPE) and experiments have been performed for the LiF powder (rho = 1.4 g cm3) (DTL937, Philitech, France), read on a home made reader and a PCL3 automatic reader (Fimel, France). The TLDs were calibrated using Fricke dosimetry and compared with three ionization chambers (NE2571, NACP02, ROOS). The combined uncertainties in the experimental fCo(E) factors determined in this work are less than about 0.4% (1 SD), which is appreciably smaller than the uncertainties up to 1.4% (1 SD) reported for other calculated values in the literature. Concerning the Varian 2300C/D beams, the measured fCo(E) values decrease from 1.065 to 1.049 +/- 0.004 (1 SD) when the energy at depth in water increases from 2.6 to 14.1 MeV; the agreement with Monte Carlo calculations is better than 0.5%. For the Racetrack MM50 pulsed-scanned beams, the average experimental value of fCo(E) is 1.071 +/- 0.005 (1 SD) for a mean electron energy at depth Ez ranging from 4.3 to 36.3 MeV: fCo(E) is up to 2% higher for the MM50 beams than for the 2300C/D beams in the range of the tested energies. The energy correction factor for LiF powder (3 mm diameter and 15 mm length) varies with beam quality and type (pulsed or pulsed-scanning), cavity size and nature of the surrounding medium. The fCo(E) values obtained for the LiF powder (3 mm diameter and 15 mm length) irradiated in water, have been applied to the EQUAL external audit network, leading to a good agreement between stated and measured doses, with a mean value of 1.002 +/- 0.022 (1 SD), for 170 beam outputs checked (36 electron beam energies) in 13 'reference' radiotherapy centres in Europe. Such fCo(E) data improve the accuracy of the absorbed dose TLD determination in electron beams, justifying their use for quality control in radiotherapy.

Calibration↗

PubMed1998

Investigation of the chamber correction factor (k(ch)) for the UK secondary standard ionization chamber (NE2561/NE2611) using medium-energy x-rays.

This paper evaluates the characteristics of ionization chambers for the measurement of absorbed dose to water for medium-energy x-rays. The values of the chamber correction factor, k(ch), used in the IPEMB code of practice for the UK secondary standard (NE2561/NE2611) ionization chamber are derived and their constituent factors examined. The comparison of the chambers' responses in air revealed that of the chambers tested only the NE2561, NE2571 and NE2505 exhibit a flat (within 5%) energy response in air. Under no circumstances should the NACP, Sanders electron chamber, or any chamber that has a wall made of high atomic number material, be used for medium-energy x-ray dosimetry. The measurements in water reveal that a chamber that has a substantial housing, such as the PTW Grenz chamber, should not be used to measure absorbed dose to water in this energy range. The value of k(ch) for an NE2561 chamber was determined by measuring the absorbed dose to water and comparing it with that for an NE2571 chamber, for which k(ch) data have been published. The chamber correction factor varies from 1.023 +/- 0.03 to 1.018 +/- 0.001 for x-ray beams with HVL between 0.15 and 4 mm Cu. The values agree with that for an NE2571 chamber within the experimental uncertainty. The corrections due to the stem, waterproof sleeve and replacement of the phantom material by the chamber for an NE2561 chamber are described.

Air↗

PubMed1998

The significance of variations in the angular correction factor in in situ gamma spectrometry.

In situ gamma spectrometry is a powerful method of assessing radioactive contamination in soil. The most widely adopted calibration methodology relates the overall sensitivity of the detector system to the product of three calibration factors: (a) the flux at the detector per unit activity in the ground phi/S(A), (b) the detected count-rate per unit flux incident normally at the detector N0/phi and (c) a correction factor to take into account the angular non-uniformity in response of the detector (Nf/N0). The dependence of the latter factor on the activity distribution with soil depth is generally neglected despite the lack of published evidence to support this. By (i) modelling and (ii) use of published experimental profiles, this work examines the range of Nf/N0 values likely to be encountered in the field. It was found that the use of a fixed angular correction factor is justified given that the maximum errors in the derived activity concentration do not exceed 5% and are far outweighed by other uncertainties.

Calibration↗

PubMed1984

A method for estimating the population at risk in primary care practices by applying correction factors to the active patient census.

Epidemiological research based in primary care practices has been hindered by the inability to estimate the number of persons served by individual practices. This study examines one method proposed as a means of estimating practice populations. The effectiveness of this "correction factor" method depends on uniformity in the proportion of persons visiting a physician in a two-year period across various sociodemographic subpopulations in the United States. The National Health Interview Survey data were examined for evidence of such uniformity. Within broad age-sex categories there was little variation by race, educational level, income, Spanish ancestry, or location of residence in the proportion reporting a physician visit within the previous two years. Although these results are encouraging, a number of problems remain before the "correction factor" method can be credibly used to estimate practice denominators.

Adolescent↗

PubMed2006

Monte Carlo correction factors for a Farmer 0.6 cm3 ion chamber dose measurement in the build-up region of the 6 MV clinical beam.

Reference dosimetry of photon fields is a well-established subject and currently available protocols (such as the IAEA TRS-398 and AAPM TG-51) provide methods for converting the ionization chamber (IC) reading into dose to water, provided reference conditions of charged particle equilibrium (CPE) are fulfilled. But these protocols cannot deal with the build-up region, where the lack of CPE limits the applicability of the cavity theorems and so the chamber correction factors become depth dependent. By explicitly including the IC geometry in the Monte Carlo simulations, depth-dependent dose correction factors are calculated for a PTW 30001 0.6 cm(3) ion chamber in the build-up region of the 6 MV photon beam. The corrected percentage depth dose (PDD) agrees within 2% with that measured using the NACP 02 plane-parallel ion chamber in the build-up region at depths greater than 0.4 cm, where the Farmer chamber wall reaches the phantom surface.

Calibration↗

PubMed1998

Determination of CT scanner radiation output: correction factors for partial irradiation of thimble and pencil ionisation chambers by collimated fan beams.

CT radiation output (mGy mAs-1) measured free-in-air on axis is required for compliance testing protocols. It can also be used as an input parameter for computer programs for patient dose assessment for a range of CT examinations. Output correction factors have been investigated as a function of collimation width for two chambers; an in-beam, 6 cc general diagnostic chamber, and a 3 cc pencil CT chamber. For irradiation by a fan beam perpendicular to the chamber axis, a correction factor given by the ratio, chamber active length to beam collimation width, is applicable to within +/- 5% of a measured ratio for the pencil CT chamber. However, this is not the case for the short 6 cc thimble chamber where an exponential function was found to fit the data to within similar limits. It is recommended that a pencil chamber be used for determining radiation output in air for a CT scanner. For a given scan condition static measurements are only required, obviating a need for sequential movements of an occluded small volume chamber to sample a radiation profile.

Radiation Dosage↗

PubMed1994

Aeq and other factors in a 60Co beam for a spherical or cylindrical minimal phantom and for an ionization chamber with a known A(wall) correction factor.

Three types of tissue-air ratio (TAR) are summarized. These TARS differ in their definition of the in-air absorbed dose. The first defines it as the absorbed dose at the centre of a spherical or cylindrical minimal water phantom in free space. The second defines it as the maximum primary absorbed dose in a semi-infinite water phantom. The third defines it as the absorbed dose in an imaginary infinitesimal mass of water within the cavity of a chamber in free space, where the absorbed dose is averaged within the cavity. It is concluded that the 60Co TAR data compiled by Godden and the 60Co TAR data of Johns and Cunningham should be reviewed. Aeq and other factors are evaluated for spherical and cylindrical minimal water phantoms in a 60Co gamma-ray beam. A method of obtaining Aeq and other factors for an ionization chamber with a known Awall correction factor is also reported. The work indicates some discrepancies with previously published material.

Algorithms↗

PubMed1981

Mutagen-induced sister chromatid exchange rate in Bloom syndrome remains unaltered in the presence of Bloom corrective factor.

Fibroblasts of a patient with Bloom syndrome (GM-1492) were cultured in the presence of either mitomycin C, ethylmethanesulfonate, or 4-nitroquinoline-1-oxide, (4-NQ1-O) and sister chromatid exchange was determined. The mutagens enhanced the sister chromatid exchange rate to different degrees, 4-NQ1-O being the most potent substance. Bloom corrective factor, which is present in normal cell-conditioned culture medium, reduced the spontaneously increased SCE in Bloom syndrome cells by about 20 SCE per metaphase but failed to reduce the additional mutagen-induced SCE increase. These findings indicate that only spontaneously, but not mutagen-induced, SCE in Bloom syndrome fibroblasts can be decreased by the Bloom corrective factor.

4-Nitroquinoline-1-oxide↗

PubMed1996

Measurement and calculation of heterogeneity correction factors for an Ir-192 high dose-rate brachytherapy source behind tungsten alloy and steel shields.

Shields made of high atomic number material are commonly used in vaginal applicators with high dose-rate (HDR) 192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF approximately 0.4, for 20 x 4 mm disk) than for a steel disk with the same dimensions (HCF approximately 0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source-to-detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.

Alloys↗

PubMed2000

Home radon levels and seasonal correction factors for the Isle of Man.

Ionizing radiation dose levels due to home radon can rise to levels that would be illegal for workers in the nuclear industry. It is well known that radon levels within homes and from home to home, and also from month to month, vary considerably. To define an Isle of Man radon seasonal correction factor, readings were taken in eight homes over a 12 month period. An average island indoor exposure of 48 Bq m(-3) (range 4-518 Bq m(-3)) was determined from 285 homes selected from a cohort of 1300 families participating in the European Longitudinal Study of Pregnancy and Childhood (ELSPAC) in the Isle of Man. This compares with a UK home average of 20 Bq m(-3) and a European Union average (excluding UK) of 68 Bq m(-3). Ten homes of those measured were found to have radon levels above the National Radiological Protection Board 200 Bq m(-3) action level. There are 29,377 homes on the Isle of Man, suggesting that there could be some 900 or more homes above the action level. No statistical difference was found between the NRPB and Isle of Man seasonal correction factors.

Air Pollutants, Radioactive↗

PubMedDate not supplied

Scatter factor corrections for elongated fields.

Measurements have been made to determine scatter factor corrections for elongated fields of Cobalt-60 and for nominal linear accelerator energies of 6 MV (Siemens Mevatron 67) and 18 MV (AECL Therac 20). It was found that for every energy the collimator scatter factor varies by 2% or more as the field length-to-width ratio increases beyond 3:1. The phantom scatter factor is independent of which collimator pair is elongated at these energies. For 18 MV photons it was found that the collimator scatter factor is complicated by field-size-dependent backscatter into the beam monitor.

Health Physics↗

PubMed1996

10 MV x-ray SMRs obtained using zero-area S(p) correction factors derived by means of the Bjärngard-Petti method.

Based on the Khan concept, a new set of 10 MV x-ray scatter-maximum ratios (SMRs) has been produced. THese new SMRs reflect no electron contamination and are parametrized. A method has been developed for determining the minimum field size with lateral CPE (charged particle equilibrium) at the field centre. It is found that for 10 MV x-rays the minimum square field with lateral CPE at large depths on the central axis in water is 3.5 x 3.5 cm2. The zero-area phantom scatter correction factor (S(p)) is derived using the Bjärngard-Petti method. It is found that the effect of electron contamination is to reduce the zero-area Sp correction factor.

Electrons↗

PubMed1999

A single-variable method for the derivation of collimator scatter correction factors in symmetrical and asymmetrical X-ray beams.

Using a minimal set of measured data, the collimator scatter correction factor of an asymmetrical collimated rectangular field (X1,X2;Y1,Y2) can be calculated from the product of one-dimensional factors, in combination with a correction term: Sc(X1,X2;Y1,Y2) = S(cx1)(X1)S(cx2)(X2)S(cy1)(Y1)S(cy2)(Y2) + c delta(X;Y). Two forms of the function delta(X;Y) were investigated.

Algorithms↗

PubMed2003

A study of the correction factor for ultraviolet phototherapy dose measurements made by the indirect method.

BACKGROUND: Optimization of ultraviolet (UV) phototherapy for treatment of psoriasis and other skin conditions requires accurate dosimetry. One factor involved in whole body treatments is the correction that needs to be applied to radiometer measurements of irradiance made remotely without a person in the phototherapy cabin. OBJECTIVES: To evaluate the correction factor for cabins of different design and to consider whether different factors should be used for different phototherapy cabins and radiometers. METHODS: An automated UV dosimetry system capable of recording irradiances at positions around the circumference of a circle equating to a human trunk has been developed. The system has been combined with a phantom to derive values for the ratio between irradiance measurements made by the direct method with a person in a cabin, and indirect measurements recorded remotely. In addition, values for the ratio in UVA cabins have been derived from comparisons between measurements made directly by persons in a cabin and indirect measurements. RESULTS: Variations in direct to indirect ratio (DIR) with cabin type were less than between individual sets of measurements. The mean DIR obtained for cabins with TL01 lamps was 0.85 +/- 0.02, while that for UVA cabins was 0.80 +/- 0.05. The DIR for dual lamp (TL01/UVA) cabins, when TL01 lamps were illuminated, was higher (0.92). CONCLUSIONS: The DIR should be applied to any measurements made using radiometers without a person or equivalent phantom in a cabin. It is proposed that standard values are appropriate for groups of cabins with a single type of lamp and similar reflectors.

Electronic Data Processing↗

PubMed2006

A method for calculation of true coincidence summing correction factors for extended sources.

In gamma-ray spectrometry, true coincidence summing correction factors for an extended sample can be calculated from full-energy-peak and total efficiencies as if the sample were a point source, if the so-called linear-to-square- (LS) curve, introduced by Blaauw and Gelsema, is known and properly applied. A method is described for obtaining the efficiencies and the corresponding LS-curve for an arbitrary cylindrical sample from calibration measurements in a reference geometry. The approach is aimed at the analysis of samples measured on p-type HPGe detectors in environmental gamma-ray spectrometry and was successfully verified against experimental data.

Journal Article↗

PubMed2000

Ionizing radiation and genetic risks. XII. The concept of "potential recoverability correction factor" (PRCF) and its use for predicting the risk of radiation-inducible genetic disease in human live births.

Genetic risks of radiation exposure of humans are generally expressed as expected increases in the frequencies of genetic diseases over those that occur naturally in the population as a result of spontaneous mutations. Since human data on radiation-induced germ cell mutations and genetic diseases remain scanty, the rates derived from the induced frequencies of mutations in mouse genes are used for this purpose. Such an extrapolation from mouse data to the risk of genetic diseases will be valid only if the average rates of inducible mutations in human genes of interest and the average rates of induced mutations in mice are similar. Advances in knowledge of human genetic diseases and in molecular studies of radiation-induced mutations in experimental systems now question the validity of the above extrapolation. In fact, they (i) support the view that only in a limited number of genes in the human genome, induced mutations may be compatible with viability and hence recoverable in live births and (ii) suggest that the average rate of induced mutations in human genes of interest from the disease point of view will be lower than that assumed from mouse results. Since, at present, there is no alternative to the use of mouse data on induced mutation rates, there is a need to bridge the gap between these and the risk of potentially inducible genetic diseases in human live births. In this paper, we advance the concept of what we refer to here as "the potential recoverability correction factor" (PRCF) to bridge the above gap in risk estimation and present a method to estimate PRCF. In developing the concept of PRCF, we first used the available information on radiation-induced mutations recovered in experimental studies to define some criteria for assessing potential recoverability of induced mutations and then applied these to human genes on a gene-by-gene basis. The analysis permitted us to estimate unweighted PRCFs (i.e. the fraction of genes among the total studied that might contribute to recoverable induced mutations) and weighted PRCFs (i.e. PRCFs weighted by the incidences of the respective diseases). The estimates are: 0.15 (weighted) to 0.30 (unweighted) for autosomal dominant and X-linked diseases and 0.02 (weighted) to 0.09 (unweighted) for chronic multifactorial diseases. The PRCF calculations are unnecessary for autosomal recessive diseases since the risks projected for the first few generations even without using PRCFs are already very small. For congenital abnormalities, PRCFs cannot be reliably estimated. With the incorporation of PRCF into the equation used for predicting risk, the risk per unit dose becomes the product of four quantities (risk per unit dose=Px(1/DD)xMCxPRCF) where P is the baseline frequency of the genetic disease, 1/DD is the relative mutation risk per unit dose, MC is the mutation component and PRCF is the disease-class-specific potential recoverability correction factor instead of the first three (as has been the case thus far). Since PRCF is a fraction, it is obvious that the estimate of risk obtained with the revised risk equation will be smaller than previously calculated values.

Genetic Diseases, Inborn↗

PubMed1998

Polarity and ion recombination correction factors for ionization chambers employed in electron beam dosimetry.

Polarity and ion recombination correction factors for the NACP (type-02) design parallel-plate ionization chamber employed in a recent UK national electron beam dosimetry intercomparison are derived over the full range of energies and measurement conditions encountered. In addition, these effects have been studied for a further four NACP chambers, a Markus parallel-plate chamber, a Roos parallel-plate chamber and a NE2571 graphite walled cylindrical ionization chamber.

Electrons↗