The determination of dose characteristics of ruthenium ophthalmic applicators using radiochromic film

Ruthenium ophthalmic applicators are energetic beta ray sources, supplied in several shapes and dimensions, and used in intraocular tumor therapy. Because of their small dimensions, the determination of dosimetric characteristics represents a technical challenge. We developed a semiautomatic method to define surface dose, dose distribution, and percentage depth dose of such applicators using radiochromic dosimetric media. These detectors consist of a thin (7 microns) radiation sensitive layer on polyester base (100 microns total thickness) changing color as a function of radiation exposure. Transmission images of exposed films were then grabbed with a TV-digitizer system to obtain a gray-level image from which dosimetric characteristics such as isodose distribution, dose values, and homogeneity of nuclide distribution were derived. Good agreement between experimental results and Monte Carlo simulation performed using the GEANT 3 code, appear to be a confirmation of the validity of the method. Moreover while manufacturer specifications of absolute and relative dose rates present a standard deviation error of +/- 30% on dose rate and +/- 6% on accuracy of relative dose values, the proposed method reduces the errors to +/- 10% and +/- 4%, respectively.     

Mapping dose distributions

Clinical dose calculations are often performed by scaling distances from a dose distribution measured in one medium to calculate the dose in another. These perturbation calculations have the mathematical form of a mapping. In this paper we identify five conditions required for particle transport to reduce to this form and develop a new mapping for electrons which approximately satisfies these conditions. This continuous scattering mapping is based on two parameters, the scattering power of the medium which determines the shape of the scaling paths, and the stopping power of the medium which determines where the energy is deposited along these paths. Pencil beam dose distributions are calculated with EGS4 in one medium and mapped to other media. The resultant distributions are compared with EGS4 calculations done directly in the second medium. The accuracy of the mapping algorithm is shown to be superior to both linear density scaling and the MDAH electron pencil beam algorithm [Kenneth R. Hogstrom, Michael D. Mills, and Peter R. Almond, "Electron beam dose calculations," Phys. Med. Biol. 26, 445-459 (1981)] for pencil beams in homogeneous media and inhomogeneous phantoms (both slab and nonslab geometries) for a variety of materials of clinical interest.     

Study of dosimetry consistency for kilovoltage x-ray beams

In this paper, the consistency of kilovoltage (tube potentials between 40 and 300 kV) x-ray beam dosimetry using the "in-air" method and the in-phantom measurement has been studied. The procedures for the measurement of the central-axis depth-dose curve, which serve as a link between the dose at the reference depth to the dose elsewhere in a phantom, were examined. The uncertainties on the measured dose distributions were analyzed with the emphasis on the surface dose measurement. The Monte Carlo method was used to calculate the perturbation correction factors for a photon diode and a NACP plane-parallel ionization chamber at different depths in a water phantom irradiated by 100-300 kV (2.43 mm Al-3.67 mm Cu half-value layer) x-ray beams. The depth-dose curves measured with these two detectors, after correcting for the perturbation effect (up to 15% corrections), agreed with each other to within 1.5%. Comparisons of the doses at the phantom surface and at 2 cm depth in water for photon beams of 100-300 kV tube potential obtained using the "backscatter" method and those using the "in-phantom" measurement have shown that the "in-air" method can be equally applied to this energy range if the depth-dose curve can be measured accurately. To this end, measured depth ionization curves require depth-dependent correction factors.     

Numerical calculation of energy deposition by high-energy electron beams: III-B. Improvements to the 6D phase space evolution model

The phase space evolution model of Huizenga and Storchi, Morawska-Kaczyńska and Huizenga and Janssen et al has been modified to (i) allow application on currently available computer equipment with limited memory (128 Megabytes) and (ii) allow 3D dose calculations based on 3D computer tomographic patient data. This is a further development aimed at the use of the phase space evolution model in radiotherapy electrons beam treatment planning. The first modification regards the application of depth evolution of the phase space state combined with an alternative method to transport back-scattered electrons. This depth evolution method requires of the order of 15 times less computer memory than the energy evolution method. Results of previous and new electron transport methods are compared and show that the new electron transport method for back-scattered electrons hardly affects the accuracy of the calculated dose distributions. The second modification regards the simulation of electron transport through tissues with varying densities by applying distributed electron transport through similarly composed media with a limited number of fixed densities. Results of non-distributed and distributed electron transport are compared and show that the distributed electron transport method hardly affects the accuracy of the calculated dose distributions. It is also shown that the results of the new dose distribution calculations are still in good agreement with and require significantly less computation time than results obtained with the EGS4 Monte Carlo method.     

Improving calibration accuracy in gel dosimetry

A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is presented which has intrinsically higher accuracy than current methods, and requires less gel. Two test-tubes of gel (inner diameter 2.5 cm, length 20 cm) are irradiated separately with a 10 x 10 cm2 field end-on in a water bath, such that the characteristic depth-dose curve is recorded in the gel. The calibration is then determined by fitting the depth-dose measured in water, against the measured change in relaxivity with depth in the gel. Increased accuracy is achieved in this simple depth-dose geometry by averaging the relaxivity at each depth. A large number of calibration data points, each with relatively high accuracy, are obtained. Calibration data over the full range of dose (1.6-10 Gy) is obtained by irradiating one test-tube to 10 Gy at dose maximum (Dmax), and the other to 4.5 Gy at Dmax. The new calibration method is compared with a 'standard method' where five identical test-tubes of gel were irradiated to different known doses between 2 and 10 Gy. The percentage uncertainties in the slope and intercept of the calibration fit are found to be lower with the new method by a factor of about 4 and 10 respectively, when compared with the standard method and with published values. The gel was found to respond linearly within the error bars up to doses of 7 Gy, with a slope of 0.233 +/- 0.001 s(-1) Gy(-1) and an intercept of 1.106 +/- 0.005 Gy. For higher doses, nonlinear behaviour was observed.     

Experimental grounds for using chlorin e6 in the photodynamic therapy of malignant tumors

The toxicity, pharmacokinetics and antitumour effect of chlorin e6 after light irradiation were studied. The LD50 value of chlorin e6 in C3H mice is 189 +/- 9 mg kg-1 and in Wistar white rats is 113 +/- 18 mg kg-1 14 days after intraperitoneal injection. The concentration of chlorin e6 in blood, liver, kidney, spleen and tumors (sarcoma M-1 and sarcoma 45) of the rats was determined by a fluorescence method, 3, 6, 12, 18, 24, 48 and 72 h after administration at a dose of 10 mg kg-1. For this purpose, chlorin e6 was extracted from tissues by the detergent Triton X-100. The depth of necrosis in sarcoma 45, the regression rate of sarcoma M-1 and the animal cure rate were evaluated after chlorin e6 administration at doses of 1-10 mg kg-1 and subsequent irradiation with krypton laser light. Depending on the dose and the time interval between chlorin e6 injection and irradiation, the depth of necrosis in sarcoma 45 varied from 5.0 to 15.0 mm. The cure rate of the animals with sarcoma M-1 varied from 10% to 60%. The antitumor effect was directly proportional to the chlorin e6 dose and light energy exposure and inversely proportional to the time interval between photosensitizer injection and irradiation.     

Studies of the nature of the binding by albumin of platelet-activating factor released from cells

This report confirms that human umbilical vein endothelial cells activated by A23187 produce platelet-activating factor (PAF) (22.4 +/- 9.9 ng/10(6) cells/h; mean +/- S.E.). A proportion of the PAF produced (56%) was released by cells into the medium. The PAF released, however, was not detected without prior organic extraction, and the method of organic extraction was critical for detection. Extraction with 80% ethanol was not successful, but a modified methanol/chloroform extraction method was. These observations may explain some of the conflicting reports in the literature on release of PAF by activated endothelial cells. The requirements for organic extraction may reflect the nature of cell-released PAF's binding by albumin; it was observed that PAF added to identical media could be detected in a bioassay without the requirement for extraction. Such PAF was also readily degraded by PAF-acetylhydrolase added to media, while PAF released from cells was resistant to such degradation, suggesting that it was released in a "protected" configuration. Stimulation of cells was performed in media with albumin as the only extracellular macromolecule. Limited proteolytic digestion of the albumin with trypsin and pepsin showed that PAF released by cells was located exclusively between amino acids 240 and 386 (domain II), while no synthetic PAF added to media was located on this region. These results are identical to those described for the release of PAF by the early embryo. Albumin exposed to embryos had a higher thiol concentration (0.77 +/- 0.04 micromol of thiol/micromol of albumin; mean +/- S.E.) than control media to which an equivalent amount of synthetic PAF was added (0.59 +/- 0.02 micromol of thiol/micromol of albumin) (measured with Ellman's reagent). Furthermore, albumin from conditioned media was more susceptible to reduction by 10 mM dithiothreitol than control albumin, as assessed by its mobility on PAGE. The protected configuration of released PAF was caused by cell-dependent conformational changes to albumin involving cysteine-cysteine disulfide bonds. Partial reduction with dithiothreitol of albumin exposed to cells resulted in released PAF being able to be detected directly in a bioassay without the requirement for prior organic extraction.     

Intracranial epidermoid and dermoid cysts. Results of microneurosurgery

A number of radon-related properties have been surveyed in underground shopping centers in Hong Kong. These parameters include the radon concentration, the total potential alpha energy concentration of radon progeny, the equilibrium factor and the unattached fraction of radon progeny. The mean values recorded for these were 29.2 +/- 7.8 Bq/m3, 3.60 +/- 1.53 mWL, 0.46 +/- 0.16 and 0.05 +/- 0.03, respectively. Based on these figures, we have calculated the average radon dose received by employees in an underground shopping center in Hong Kong to be 0.22 mSv/yr, which represents an approximate increase of 8% over the total dose of about 2.7 mSv/yr received by the average person in Hong Kong.     

Test procedures for verification of an electron pencil beam algorithm implemented for treatment planning

The calculation of an electron dose distribution in a patient is a difficult problem because of the presence of tissue and surface inhomogeneities. Verification of the dose planning system is therefore essential. In this investigation, a novel method is used to evaluate a commercially available system (Helax-TMS), at electron energies between 10 and 50 MeV, both for a conventional treatment unit and an MLC-collimated scanned beam unit with a helium-filled treatment head. First, the experiments were designed to verify the local beam database and some fundamental characteristics of the electron beam calculations. Secondly, a number of generalised situations that would be encountered in the clinical treatment planning were evaluated oblique incidence, field shaping with multi-leaf collimator, bolus edges, and air cavities. Dose distributions in two generalised anatomical phantoms simulating a neck and a nose were also analysed. The results have, when so possible, been presented as the dose ratio within the 'flattened area' for dose profiles and down to the 'treatment depth' (80% dose level) for depth doses. In the penumbra region and in the dose fall-off region, the comparison has been represented by the distance deviation between calculated and measured dose profiles or depth doses. A new tool, 'volume integration', was used to evaluate the deviations from a more clinical point of view. Most results were within +/- 2% in dose for volumes larger than a sphere with a diameter of 15 mm, or +/- 2 mm in position. Dose deviations were generally found for oblique incidences and below heterogeneities such as small air cavities and bolus edges in limited volumes.     

A Monte Carlo study of the quality dependence of diamond thermoluminescent dosimeters in radiotherapy beams

Monte Carlo simulations with the EGS4 code system have been performed to determine the quality dependence of diamond TLDs in photon beams ranging from 25 kV to 25 MV x-rays and also in megavoltage electron beams. It has been shown that diamond TLDs in the form of discs of thickness 0.3 mm and diameter 5.64 mm show no significant dependence on the incident energy in clinical electron beams when irradiated close to dmax, but require an energy correction factor of 1.050 +/- 0.008 compared with diamond TLDs irradiated in 60Co gamma-rays. The correction factor increases with depth of irradiation and this effect is greater for thicker detectors. The Monte Carlo predicted sensitivity in x-ray beams is constant within 2.5% over the energy range 250 kV to 25 MV. However the sensitivity decreases by about 60% for 25 kV x-rays compared with 60Co gamma-rays.     

Heterochromatin effects on the frequency and duration of LCR-mediated gene transcription

Organ transplantation is associated with an early bone loss that subsequently increases the risk of osteopenia and bone fractures. It is not known whether bone loss continues in long-term survivors, but persistent bone demineralization could further jeopardize an already diminished bone mass. To better define long-term bone status of kidney transplant recipients (KTR), we conducted cross-sectional and longitudinal evaluations of bone mineral density (BMD) in 70 KTR with a mean posttransplantation time of 8.1 years. BMD was determined by dual-energy X-ray absorptiometry and was repeated in 55 of the patients after a mean follow-up period of 22 +/- 5 months. Lumbar and femoral osteopenia, defined as a BMD lower than 2 standard deviations from mean value of sex- and age-matched controls, was present in 28.6% and 10.5% of patients, respectively. There was a significant negative correlation between cumulative prednisone dose and adjusted lumbar as well as femoral BMD (R = 0.45, P < 0.001 and R = 0.29, P < 0.05, respectively). Five patients had a vertebral BMD below a fracture threshold of 0.777 g/cm2. Vertebral fractures (VF) were found in four men and were associated with higher prednisone dosage (P < 0.05), larger cumulative prednisone dose (P < 0.05), and significantly lower BMD values. According to World Health Organization recent criteria for women, prevalences of lumbar and femoral osteopenia and lumbar and femoral osteoporosis in female patients reach 75%, 65%, 33%, and 10%, respectively. The longitudinal part of the study showed a persistent pathological lumbar demineralization process. Over the study period, BMD, expressed as a percentage of that of controls, decreased from 89 +/- 14% to 86 +/- 14% (P < 0.001). Annual change of bone mass was -1.7 +/- 2.8% per year. Accelerated vertebral bone loss (defined as a decrease of BMD, expressed as a percentage of that of controls, of more than 1% per year) was present in 56% of patients and was associated with higher prednisone dosage (P < 0.01). In conclusion, although VF are relatively infrequent in long-term survivors of renal transplantation, osteopenia is a frequent finding, and a substantial proportion of women present lumbar osteoporosis. An ongoing demineralization process of the spine is also demonstrated and probably contributes to long-term spinal osteoporosis incidence. Prednisone dosage remains the most constantly isolated risk factor.     

Changes in the norms governing practices for the manufacture of pharmaceutical products: implications for the MERCOSUR

It is done a comparative study between the "Recommended rules for drug products manufacturing and inspection", approved in 1975 by the World Health Organization (and still in force in the MERCOSUR); and the standards published in 1992 by the WHO Expert Committee on Specifications for Pharmaceutical Preparations 32nd Report, named "Good Manufacturing Practices for pharmaceutical products". The correspondence between the regulation in force in the MERCOSUR and the Good Manufacturing Practices Inspection Guide for pharmaceutical industry, used by Health Authorities in the Common Market Member States, is analysed. It is noticed a disagreement between the rule in force and the instrument for verifying its fulfillment. The proposal of this article is the adoption by the Common Market Group, of the rules published by the WHO in 1992, and the establishment of an inspection guide which absolute agrees with it.     

Osteoporotic fracture rate, bone mineral density, and bone metabolism in Taiwan

Taiwanese people have spinal bone mineral density (BMD) values similar to those of Caucasians, whereas their hip BMD values are 10% to 15% lower. In 1992, the prevalence of vertebral fractures, diagnosed according to the -3 SD morphometric criteria, was 18% for women and 12% for men older than 65 years in the major cities of Taiwan. Despite this high prevalence of vertebral fractures, the incidence of hip fractures in the elderly of both sexes was only 203 per 100,000 in 1996, which was lower than in Caucasians and similar to that in mainland Chinese. Hip and vertebral fractures are both associated with lower BMD values. The risk factors for low BMD in Taiwan include a lighter body weight and aging in both sexes, and menopause for women. An increased bone turnover rate is associated with a lower BMD in both men and postmenopausal women, although the rate seems to increase in women but decrease in men with aging. In Taipei City, daily calcium intake is relatively low (mean intake +/- SD; 640 +/- 240 mg), but the vitamin D stores seem to be generally adequate for middle-aged and elderly women. There was a significant association between a higher daily calcium intake and a higher BMD/lower bone turnover rate for women in this age group. Vitamin D receptor allelic polymorphism was not an important factor in low BMD and rapid bone turnover.     

The effect on bone mass and bone markers of different doses of ibandronate: a new bisphosphonate for prevention and treatment of postmenopausal osteoporosis: a 1-year, randomized, double-blind, placebo-controlled dose-finding study

The present article describes the results from a phase II dose finding study of the effect of ibandronate, a new, third generation bisphosphonate, in postmenopausal osteoporosis. One hundred and eighty postmenopausal, white women, at least 10 years past a natural menopause, with osteopenia defined as a bone mineral density (BMD) in the distal forearm at least 1.5 SD below the premenopausal mean, entered and 141 (78%) completed a 12 months randomized, double-blind, placebo-controlled study. The women received 0.25, 0.5, 1.0, 2.5, or 5.0 mg ibandronate daily or placebo. All women received a daily calcium supplementation of 1000 mg Ca2+. Bone mass and biochemical markers of bone turnover were measured every 3 months throughout the study period. The average changes in bone mass showed positive outcome in all regions in the groups receiving ibandronate 2.5 and 5.0 mg. The responses in the two groups were not significantly different, although there was a tendency toward a higher response in bone mass in the group receiving ibandronate 2.5 mg, where the increase in BMD was 4.6 +/- 3.1% (SD) in the spine (p < 0.001), 1.3 +/- 3.0% (SD) to 3.5 +/- 5.3% (SD) in the different regions of the proximal femur (p < 0.03 to p < 0.002), and 2.0 +/- 1.9% (SD) in total body bone mineral content (BMC) (p < 0.001). There was no significant changes in bone mass in the group receiving calcium (placebo) and ibandronate 0.25 mg. Dose-related responses were found in all biochemical markers of bone turnover. In average, serum osteocalcin decreased 13 +/- 14% (SD) (placebo) and 35 +/- 14% (SD) (5.0 mg). Urinary excretions of breakdown products of type I collagen decreased 35 +/- 21% (SD) (placebo) and 78 +/- 28% (SD) (5.0 mg), p < 0.001 in all groups. In conclusion, the results suggest that ibandronate treatment increases bone mass in all skeletal regions in a dose dependent manner with 2.5 mg being the most effective dose. Ibandronate treatment reduces bone turnover to premenopausal levels and is well tolerated.     

The use of antibiotic-impregnated cement in infected reconstructions after resection for bone tumours

Bone mobilization, lowering of bone mineral density (BMD), and osteoporotic fractures are recognized in postmenopausal women with weight loss. Because a high-calcium intake suppresses bone loss in peri- and postmenopausal women, the present randomized, double-blind, placebo-controlled study was designed to test the hypothesis that calcium supplementation prevents net bone mobilization and consequent bone mineral loss during voluntary weight reduction in obese postmenopausal women. Subjects were placed on a moderate energy-restricted diet and either calcium supplementation (1 g/day) or placebo for 6 months. Body weight, bone turnover markers (pyridinium cross-links), osteocalcin, and parathyroid hormone (PTH) were measured at treatment weeks 1-5, 7, 10, 13, 16, 20, and 25. Total body BMD, insulin-like growth factor, 25-hydroxyvitamin D, and sex hormone binding globulin (SHBG) were measured at baseline and week 25. The calcium supplemented (n = 15; age 60.9 +/- 9.4 years, body mass index [BMI] 33.2 +/- 4.6 kg/m2) and placebo (n = 16; age 55.8 +/- 8.3 years, BMI 32.9 +/- 4.5 kg/m2) groups lost similar amounts of weight over the study interval (10.2 +/- 5.3% vs. 10.0 +/- 5.2%) and both groups increased SHBG (p < 0.001). There was a statistical effect of calcium supplementation during weight loss to suppress pyridinium cross-links, osteocalcin, and PTH (p < 0.05, < 0.01, and < 0.05, respectively). Loss of BMD tended to be greater in the placebo group by 1.4% (p < 0.08) after weight loss. One gram per day calcium supplementation normalizes the increased calcium-PTH axis activity and the elevated bone turnover rate observed during moderate voluntary energy restriction in postmenopausal women.     

Expression and regulation of vascular endothelial growth factor in osteoblasts

Bone formation is linked closely to angiogenesis. Because prostaglandin E2 (PGE2) is a potent stimulator of bone formation, its effects were evaluated on vascular endothelial growth factor, a secreted endothelial cell-specific mitogen, and a potent angiogenic protein. Prostaglandin E2 increased vascular endothelial growth factor protein in conditioned media of osteoblastic RCT-3 cells within 3 hours. Prostaglandin E2 also increased the steady-state levels of vascular endothelial growth factor mRNA in a dose-dependent manner. The increased expression of vascular endothelial growth factor mRNA produced by PGE2 was rapid (maximal at 1 hour) and was enhanced by the protein synthesis inhibitor cycloheximide (5 micrograms/ml). The increase in vascular endothelial growth factor mRNA by PGE2 was inhibited strongly by pretreatment for 3 hours with dexamethasone (10(-7) M). Stimulation of vascular endothelial growth factor by PGE2 and its suppression by dexamethasone implicate the involvement of vascular endothelial growth factor in bone metabolism.     

Quantification of mean energy and photon contamination for accurate dosimetry of high-energy electron beams

The scientific background of the standard procedure for determination of the mean electron energy at the phantom surface (E0) from the half-value depth (R50) has been studied. The influence of energy, angular spread and range straggling on the shape of the depth dose distribution and the R50 and Rp ranges is described using the simple Gaussian range straggling model. The relation between the R50 and Rp ranges is derived in terms of the variance of the range straggling distribution. By describing the mean energy imparted by the electrons both as a surface integral over the incident energy fluence and as a volume integral over the associated absorbed dose distribution, the relation between E0 and different range concepts, such as R50 and the maximum dose and the surface dose related mean energy deposition ranges, Rm and R0, is analysed. In particular the influence of multiple electron scatter and phantom generated bremsstrahlung on R50 is derived. A simple analytical expression is derived for the ratio of the incident electron energy to the half-value depth. Also, an analytical expression is derived for the maximum energy deposition in monoenergetic plane-parallel electron beams in water for energies between 2 and 50 MeV. Simple linear relations describing the relative absorbed dose and mass ionization at the depth of the practical range deposited by the bremsstrahlung photons generated in the phantom are derived as a function of the incident electron energy. With these relations and a measurement of the extrapolated photon background at Rp, the treatment head generated bremsstrahlung distribution can be determined. The identification of this photon contamination allows an accurate calculation of the absorbed dose in electron beams with a high bremsstrahlung contamination by accounting for the difference in stopping power ratios between a clean electron beam and the photon contamination. The absorbed dose determined using ionization chambers in heavily photon contaminated (10%) electron beams may be too low--by as much as 1.5%--without correction.     

Mean energy, energy-range relationships and depth-scaling factors for clinical electron beams

Using Monte Carlo simulations we have studied the electron mean energy, Eo, and the most probable energy, Eo,p, at the phantom surface and their relationships with half-value depth, R50, and the practical range, Rp, for a variety of beams from five commercial medical accelerators with an energy range of 5-50 MeV. It is difficult to obtain a relation between R50 and Eo for all electrons at the surface because the number of scattered lower-energy electrons varies with the machine design. However, using only direct electrons to calculate Eo, there is a relationship which is in close agreement with that calculated using monoenergetic beams by Rogers and Bielajew [Med. Phys. 13, 687-694 (1986)]. We show that the empirical formula Eo,p = 0.22 + 1.98Rp + 0.0025R2p describes accurately the relationship between Rp and Eo,p for clinical beams of energies from 5 to 50 MeV with an accuracy of 3%. The electron mean energy, Ed, is calculated as a function of depth in water as well as plastic phantoms and is compared both with the relation, Ed = Eo (1-d/Rp), employed in AAPM protocols and with values in the IAEA Code of Practice. The conventional relations generally overestimate Ed over the entire therapeutic depth, e.g., the AAPM and IAEA overestimate Ed at dmax by up to 20% for an 18 MeV beam from a Clinac 2100C. It is also found that at all depths mean energies are 1%-3% higher near the field edges than at the central axis. We calculated depth-scaling factors for plastic phantoms by scaling the depth in plastics to the water-equivalent depth where the mean energies are equal. The depth-scaling factor is constant with depth in a given beam but there is a small variation ( < 1.5%) depending on the incident beam energies. Depth-scaling factors as a function of R50 in plastic or water are presented for clear polystyrene, white polystyrene and PMMA phantom materials. The calculated depth-scaling factor is found to be equal to R50water/R50plastic. This is just the AAPM definition of effective density but there are up to 2% discrepancies between our calculated values and those recommended by the AAPM and the IAEA protocols. We find that the depth-scaling factors obtained by using the ratio of continuous-slowing-down ranges are inaccurate and overestimate our calculated values by 1%-2% in all cases. We also find that for accurate work, it is incorrect to use a simple 1/r2 correction to convert from parallel beam depth-dose curves to point source depth-dose curves, especially for high-energy beams.     

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.     

In-phantom dosimetry and spectrometry of photoneutrons from an 18 MV linear accelerator

A combination of three superheated drop detectors with different neutron energy responses was developed to evaluate dose-equivalent and energy distributions of photoneutrons in a phantom irradiated by radiotherapy high-energy x-ray beams. One of the three detectors measures the total neutron dose equivalent and the other two measure the contributions from fast neutrons above 1 and 5.5 MeV, respectively. In order to test the new method, the neutron field produced by the 10 cm X 10 cm x-ray beam of an 18 MV radiotherapy accelerator was studied. Measurements were performed inside a tissue-equivalent liquid phantom, at depths of 1, 5, 10 and 15 cm and at lateral distances of 0, 10, and 20 cm from the central axis. These data were used to calculate the average integral dose to the radiotherapy patient from direct neutrons as well as from neutrons transmitted through the accelerator head. The characteristics of the dosimeters were confirmed by results in excellent agreement with those of prior studies. Track etch detectors were also used and provided an independent verification of the validity of this new technique. Within the primary beam, we measured a neutron entrance dose equivalent of 4.5 mSv per Gy of photons. It was observed that fast neutrons above 1 MeV deliver most of the total neutron dose along the beam axis. Their relative contribution increases with depth, from about 60% at the entrance to over 90% at a depth of 10 cm. Thus, the average energy increases with depth in the phantom as neutron spectra harden.