Factors affecting the precision of TLD dose measurements using an automatic TLD reader

An increase in the variability of TLD dose measurements prompted a study of the precision of dose measurements with a Rialto automatic TLD reader. It was found that readings with the same chip measured using two different trays could differ by as much as 7%. In order to overcome this problem it was necessary to ensure that individual chips were correctly identified. Marking the chips with a hard graphite pencil was found to be a satisfactory method of identifying them, which, although it reduced the light output by approximately 3%, had no effect on the reproducibility of readings. Using marked chips and an individual tray for each chip, the reproducibility of a reading was reduced to a typical value of 1%. A system of quality assurance based on these findings is described.     

Dosimetry studies with TLDs for stereotactic radiation techniques for intraocular tumours

Between March 1993 and January 1997, stereotactic radiation techniques were used to irradiate 66 intraocular tumour patients with the Gamma Knife (Leksell Gamma Knife, model B unit) at the University of Vienna, Austria. This study investigates the dosimetry for stereotactic irradiation of ocular structures. For the dosimetry program KULA 4.4, Gamma Knife stereotactic irradiation of the eye represents an extreme frontal skull position. In addition, irradiation of the eye may be performed in the usual supine position in exceptional cases only. With the patient in the prone position, the dose planning program has to calculate with a significantly large number of single-beam extrapolations. In our first experiment we measured the isocentre dose for eight different gamma-angle positions, both in prone and supine positions, using TLD measurements in an Alderson head phantom. We found a maximum deviation of +/- 1.6% using these individually calibrated TLDs. In the second experiment we examined the dose cross profiles for the two most frequently used treatment positions (supine position, gamma = 65 degrees, and prone position, gamma = 140 degrees). For this purpose we implanted a specially designed TLD array into the orbit of a human cadaver head. We found excellent agreement of the dose values measured for the isocentre as well as the posterior part of the eye with orbit with deviations of less than -2.7%. However, for the anterior part of the eye, deviations between computer-generated calculations and the TLD measurements were found to range up to -30%. These differences were noticed both for supine and prone positions. For the Gamma Knife stereotactic irradiation of ocular tumours or pathologies, precautions should be taken to avoid significant underdosage in the anterior part of the radiation field.     

TLD extrapolation for skin dose determination in vivo

The conformational features of copper-free ceruloplasmin (CP), as compared to the holo-protein, were evaluated utilizing far- and near-UV circular dichroism and fluorescence spectroscopy. The results obtained indicate that apo-CP maintains the secondary structure of the holo-protein, while the tertiary interactions are much weaker. In addition, the removal of copper from the holo-protein leads to the exposure of hydrophobic patches to solvent, as shown by the fact that apo-CP, at variance from the holo-protein, binds the hydrophobic probe ANS. It is proposed that the CP molecule, upon copper removal, acquires the conformational features typical of a molten globule, which might be the conformational state of CP during its biosynthesis before metal incorporation.     

Conversion of ionization measurements to radiation absorbed dose in non-water density material

The radiation absorbed dose to non-water equivalent materials of interest in radiotherapy is the dose to lung and the dose to bone. The measurement and calculation of dose to the lung has been of great interest and much effort has gone into the development of accurate lung dose calculation methods. The radiation absorbed dose to the bone is usually not calculated and most absorbed dose calculations have been done without correcting for the presence of bone. For the lower megavoltage photon beams this may be appropriate, however, as the energy of the photon beam increases, the region of electronic disequilibrium becomes larger and pair production which depends on the atomic number of the material becomes significant. Therefore the bone will produce greater perturbations of the dose distribution. The dose to lung-equivalent material is uniquely obtained from ionization measurements. However, in bone-equivalent materials two different calculations of absorbed dose are possible: the absorbed dose to soft tissue plastic (polystyrene) within bone-equivalent material and the dose to the bone-equivalent material itself. Both can be calculated from ionization measurements in phantoms. These two calculations result in significantly different doses in a heterogeneous phantom composed of polystyrene and aluminium (a bone substitute). The dose to a thin slab of polystyrene in aluminium is much higher than the dose to the aluminium itself at the same depth in the aluminium. Monte Carlo calculations confirm that the calculation of dose to polystyrene in aluminium can be accurately carried out using existing dosimetry protocols. However, the conversion of ionization measurements to absorbed dose to high atomic number materials cannot be accurately carried out with existing protocols and appropriate conversion factors need to be determined.     

Accelerated radiotherapy regimen for malignant gliomas using stereotactic concomitant boosts for dose escalation

Parvovirus B19 (PV B19) infection was investigated in 29 pregnant women with fetal hydrops, after exclusion of feto-maternal incompatibility within red blood cell antigens, TORCH infections, feto-maternal hemorrhage and genetics reasons. The active viral infection was detected in 9 women (31%) by PCR amplification of DNA B19; in 2 of them IgM and IgG, in 1 IgM and in 4 IgG antibodies were also present. In 6 women (20%) IgG antibodies were only found, but not IgM and DNA B19, which confirmed infection in the past. In addition in 9 cases DNA B19 was evaluated in the fetal blood. The results in the mothers and their fetuses were concordant (4 positive, 5 negative). Our conclusion is that in nonimmune hydrops fetalis, PV B19 infection should be based on the viral DNA evaluation in the blood of mother (or fetus). IgM antibodies, in time of fetal disorders, might not be detected.     

An efficient method for small field treatment dose calculation for stereotactic radiosurgery using a LINAC

The normal procedure for a physician-physicist team designing a treatment plan for multiarc stereotactic radiosurgery is the trial-and-error approach of changing the collimator size and the location of the isocenter of radiation and viewing the isodose curves on two-dimensional computed tomography (CT) or magnetic resonance imaging (MRI) image planes. Automatic optimization procedures have also been used to optimize beam weight or beam size. However, either process is very time consuming. To improve the speed of the dose calculation, a random sampling method has been proposed. Unfortunately, the sampled values of an objective function are different from one sample to another. Such a sampling method cannot be used in automatic optimization because the next move in an optimization process is based on the current and past objective function values. To this end, an adaptive method based on the size of the collimators is proposed and used to determine a small volume in the shape of a hollow sphere for which the dose is calculated. With an appropriate choice of an adaptive hollow sphere, the objective function calculated based on such a hollow sphere is the same as that calculated with the traditional three-dimensional (3-D) cube matrix. However, with the new adaptive method, the speed of calculating a dose can be improved by a factor of 4 to a factor of 100. Because of the improvement in the speed of calculating a treatment dose, the new adaptive hollow sphere method for calculating a treatment dose can be used routinely in designing a treatment plan.     

Mantle treatment. Absorbed dose measurements in patients compared with dose planning

In vivo dose measurements were performed in the hypopharnyx-oesophagus for 60 adult patients receiving mantle treatment with 60Co. The measurements of absorbed dose in more than 1 600 points were analysed and compared with dose plans for the same patients. For the technique used the mean difference between in vivo measured and planned absorbed dose is within +0.4 and -1.3 per cent with a standard error of the mean of 0.4 per cent.     

Magnetic resonance imaging changes after stereotactic radiation therapy for childhood low grade astrocytoma

BACKGROUND: Stereotactic radiotherapy (SRT) is fractionated radiotherapy delivered under stereotactic guidance to produce highly focal and precise therapy. We studied the incidence of imaging changes that can mimic tumor progression after completion of SRT for childhood low grade astrocytoma. METHODS: Between June 1992 and September 1994, we prospectively treated 28 children with low grade astrocytomas with SRT. The patients ranged in age from 2 to 22 years (median: 10 yrs) and none had received prior radiation therapy or radiosurgery. Routine fractionation was employed (180-200 centigray[cGy]) to a total dose of 5220-6000 cGy over 5 to 6 weeks. All of the patients underwent initial and follow-up magnetic resonance imaging (MRI) according to protocol. RESULTS: Median clinical follow-up for the 28 patients was 24 months (range, 5-32 mos) with a median radiographic follow-up of 15 months (range, 3-26 mos). Fifteen patients had reduction in tumor size, one patient had stable disease. Twelve patients (43%) developed increased size of the lesion, increased signal intensity or enhancement, cysts or cavitations, and an increase in edema or mass effect on follow-up MRI. Most of these changes occurred between 9 and 12 months after the start of SRT and resolved or decreased by 15 to 21 months. All but one patient had normal or stable neurologic examinations. CONCLUSIONS: Treatment-related MRI changes are common after conventionally fractionated schedules using stereotactic radiation techniques for patients with low grade astrocytomas. These changes can be distinguished from tumor progression by their transient nature as well as the general absence of clinical symptoms.     

A stereotactic radiation therapy device for retinoblastoma using a noncircular collimator and intensity filter

The proximity of the lens to the retina makes the treatment of retinoblastoma a challenge for external beam radiation therapy. The approximately 1 mm separation between the posterior edge of the lens and the anterior region of the retina causes a trade-off between coverage of the entire retina and excessive dose to the lens. A stereotactic, LINAC based, lens sparing technique for treating retinoblastoma is presented. The technique uses noncoplanar arcs with the lens at isocenter. A special noncircular collimator blocks the lens but it also causes the dose distribution to vary across the retina. A fluence modulation filter is used to reduce the dose inhomogeneity across the target. The resulting dose distribution is roughly hemispheric, providing both anterior coverage of the retina and lens blocking unlike conventional techniques. The method used to develop the collimator and filter assembly is presented. Dosimetry of the assembly was carried out using radiochromic film, and the results were entered in a treatment planning system. The dose distribution as measured in a phantom is provided and compared to calculations.     

Comparison of high dose rate, low dose rate, and high dose rate fractionated radiation for optimizing differences in radiosensitivities in vitro

In an attempt to clarify the biological nature of a human endogenous retrovirus (HERV), HERV-R, which is a single-copy type of HERVs and is conserved as a full-length viral sequence, the expression of HERV-R mRNA in normal autopsied systemic organs was examined by Northern blot analysis. The expression showed different levels among individuals, with the adrenal glands expressing the highest level of HERV-R among all organs tested, except for the placenta. In various adrenal tumors, HERV-R was expressed at high levels in all cortical adenomas but less so in pheochromocytomas. In situ hybridization revealed the expression of HERV-R to be localized in all layers of the adrenal cortex, but not in the medulla. This high-level expression of HERV-R in the adrenal cortex may possibly relate to differentiation and/or steroid production by adrenocortical cells.     

Digital radiography in paediatrics: radiation dose considerations and magnitude of possible dose reduction

The purpose of this study was to evaluate the radiation doses received by paediatric patients examined using a digital radiography unit, and to compare these doses with those received from conventional screen-film systems. In this way, guidelines could be drawn up concerning the magnitude of possible dose reductions achievable using digital radiography. The study was undertaken on approximately 900 patients undergoing abdomen, chest, pelvis and skull examinations. Patients were categorized into the following age groups: 0-1 month, 1-12 months, 1-5 years, 5-10 years and 10-15 years. Approximately half were X-rayed using a Fuji computed radiography system and half using a conventional screen-film system. Entrance surface dose was calculated from the recorded exposure parameters and measured X-ray tube outputs. Dose-area product was recorded directly. Image quality was assessed clinically using criteria recommended by a working group of the Commission of the European Communities. Apart from chest examinations, it was found possible to reduce doses by about 40% on average, by using a computed radiography system instead of a 600 speed screen-film combination. There was no significant difference in the dose for chest examinations. Satisfactory image quality can therefore be achieved by using computed radiography as a 1000 speed system for abdomen, pelvis and skull examinations, and as a 600 speed system for chests. Since very few departments appear to use screen-film systems of speeds greater than 400, then, for most departments, the use of computed radiography would result in dose reductions of at least 60%, or 33% for chests.     

Importance of pre-treatment radiation absorbed dose estimation for radioimmunotherapy of non-Hodgkin''s lymphoma

We have been developing a new type of centrifugal pump for long-term use. The magnetically suspended centrifugal pump (MSCP) contains no shaft and seal so that long life expectancy is predicted. Paracorporeal left ventricular (LV) assist circulation between the left atrium and the descending aorta was instituted using sheep. The flow rates ranged from 2.5-5.5 L/min. The sheep that lived the longest (46 days) died of an embolism as a result of the thrombus in the pump. No thrombus formation was observed in other pumps. Plasma free hemoglobin levels ranged from 9 to 18 mg/dl, which led to the conclusion that the hemolysis level remained within an acceptable range. Two driving modes were compared. The slope of the pressure-flow relationship plot under a constant motor current mode was steeper than that under a constant rotational speed mode, and thus, the flow fluctuation decreased. In conclusion, the MSCP is durable for more than a month at the current stage of development and is a promising device for long-term ventricular assist.     

Stage specificity, dose response, and doubling dose for mouse minisatellite germ-line mutation induced by acute radiation

Germ-line mutation induction at mouse minisatellite loci by acute irradiation with x-rays was studied at premeiotic and postmeiotic stages of spermatogenesis. An elevated paternal mutation rate was found after irradiation of premeiotic spermatogonia and stem cells, whereas the frequency of minisatellite mutation after postmeiotic irradiation of spermatids was similar to that in control litters. In contrast, paternal irradiation did not affect the maternal mutation rate. A linear dose-response curve for paternal mutation induced at premeiotic stages was found, with a doubling dose of 0.33 Gy, a value close to those obtained in mice after acute spermatogonia irradiation using other systems for mutation detection. High frequencies of spontaneous and induced mutations at minisatellite loci allow mutation induction to be evaluated at low doses of exposure in very small population samples, which currently makes minisatellite DNA the most powerful tool for monitoring radiation-induced germ-line mutation.     

"Exact" and approximate methods for analysis of total error in radiation measurements

BACKGROUND: It is documented that truncal vagotomy and Billroth II gastroenterostomy disturbs the emptying of the gallbladder. The aim of the present prospective study was to assess the emptying of the gallbladder after Roux-en-Y gastroenterostomy. STUDY DESIGN: There were 34 patients, who had undergone either truncal vagotomy with pyloroplasty (TVP, 14 instances) or Billroth II gastrectomy (20 instances), and were subsequently subjected to Roux-en-Y gastroenterostomy. The emptying of the gallbladder was assessed before and after the Roux-en-Y procedure, by milk-technetium-99m labeled hepatoiminodiacetic acid (milk-99mTc-HIDA) scintigraphy. Milk-99mTc-HIDA scintigraphy was also performed on twenty-eight healthy subjects, who served as the control group. RESULTS: After excluding the subjects having spontaneous gallbladder evacuation before milk ingestion, there remained 26 subjects in the control group, 12 patients with TVP, and 19 with Billroth II gastrectomy. Truncal vagotomy with pyloroplasty was associated with delayed onset (p < 0.001) and decreased rate (p < 0.01) of emptying of the gallbladder compared with the control group. Truncal vagotomy with pyloroplasty also changed the normal pattern of emptying in two patients (sequential emptying and refilling events). Billroth II gastrectomy was associated with decreased extent and abnormal pattern of emptying compared with subjects in the control group (p < 0.0001) and patients having TVP. Roux-en-Y gastroenterostomy, performed upon patients with TVP, significantly increased lag phase duration (p < 0.001), decreased ejection fraction (p < 0.01), and changed the pattern of emptying of the gallbladder (p < 0.01). Roux-en-Y procedure performed upon patients with Billroth II gastrectomy significantly increased lag phase duration (p < 0.0001). CONCLUSIONS: Roux-en-Y gastroenterostomy severely disturbs all parameters of the emptying of the gallbladder.     

Ionization chamber shift correction and surface dose measurements in electron beams

Cylindrical ionization chambers produce perturbations (gradient and fluence) in the medium, and hence the point of measurement is not accurately defined in electron beam dosimetry. The gradient perturbation is often corrected by a shift method depending on the type of ion chamber. The shift is in the range of 0.33-0.85 times the inner radius (r) of the ion chamber, upstream from the centre of the chamber, depending upon the dosimetry protocol. This variation in shift causes the surface dose to be uncertain due to the high dose gradient. An investigation was conducted to estimate the effective point of measurement of cylindrical ion chambers in electron beams. Ionization measurements were taken with the ion chamber in air and in a phantom at source to chamber distances of <100 cm and >100 cm respectively. The data in air and in the phantom were fitted with the inverse square and electron depth dose functions, respectively. The intersection of the two functions provides an accurate estimate of the ion chamber shift and the surface dose. Our results show that the shift correction for an ion chamber is energy dependent. The measured shifts vary from 0.9r to 0.5r between 6 MeV and 20 MeV beams respectively. The surface dose measured with the ion chambers and mathematically determined values are in agreement to within 3%. The method presented in this report is unambiguous, fast and reliable for the estimation of surface dose and the shift needed in electron beam dosimetry.     

Synchrotron radiation in the study of the variation of dose response in thermoluminescence dosimeters with radiation energy

Thermoluminescence dosimetry (TLD) is a versatile technique with many applications for dosimetry of ionising radiation. However, in the range of kilovoltage x-rays which is widely used for diagnostic and therapeutic medical applications, problems arise from the differing dose response of most TL dosimeters with the radiation energy. The dose response of various TL detector types was investigated in mono-energetic x-ray beams of 26.8, 33.2, 40, 80.4 and 99.6keV from a synchrotron radiation source at the National Laboratory for High Energy Physics in Japan. This response was studied as a function of TL material (LiF:Mg,Ti, LiF:Mg,Cu,P and Al2O3), the detector geometry and size, and their thermal history. Due to the asymmetric diffraction from a Si crystal employed to produce monoenergetic photons there was more than 50% dose inhomogeneity in some of radiation fields used. Therefore, the different TL dosimeter types were rotated around and the results related to the reading of a set of "standard" LiF:Mg,Ti ribbons which were included in all experiments as reference detectors. No significant influence of the detector shape (physical size, thickness) on the dose response with energy could be found. However, the pre-irradiation thermal history influences the dose response with radiation energy: a fast cool down of LiF:Mg,Ti after a high temperature anneal will increase the sensitivity by more than a factor of two. The relatively new TLD material LiF:Mg,Cu,P (GR-200, obtained from Solid Dosimeter & Detector Laboratories, Beijing) was found to be approximately 100 times more sensitive than the standard LiF:Mg,Ti. In addition it proved to be more tissue equivalent for photon radiation between 27keV and 40keV. The performance of LiF:Mg,Cu,P makes it a very interesting TL material deserving further evaluation for applications in diagnostic and therapeutic x-rays.     

Influence of dose reduction recommendations on changes in chest radiography techniques

A previous dosimetric study on chest radiography identified ways to reduce patient entrance surface dose (ESD). This present study was designed to monitor changes that had occurred in the use of applied potential and film-screen sensitivity, after a series of recommendations were issued. The study falls into two parts: (1) an assessment of the impact of the recommendations and (2) what factors were responsible for change. Where changes had occurred, exposure factors were collected for 30 patients per tube and the mean ESD was calculated for each tube. Intercomparison (r = 0.93, p < 0.001) was made between calculated and measured (TLDs) values of mean ESD for 10 X-ray units, to ensure that the calculated values provided accurate estimates of the new mean ESDs. 89% of units previously monitored for patient ESD now use average applied potentials greater than 90 kVp and 51% are using film-screen sensitivities of 400. The mean ESD has been reduced on average by 47%, from 0.15 mGy to 0.08 mGy. It has been estimated that the annual collective dose from diagnostic radiology procedures in 30 hospitals in the West Midlands has been reduced by a value in excess of 40 man Sv. Reasons for change could be attributed to some of the following factors: (a) a knowledge of dose levels in comparison with other centres; (b) personal contact with departments; (c) feedback in terms of results and dose savings and (d) positive encouragement to make changes.     

The need for accurate target and dose specifications in conventional and conformal radiation therapy--an introduction

Back pain in children has long been considered a serious condition with an organic cause. The staff of the Shriner's Hospital had been informally noting that diagnoses among children with back pain had been changing. We conducted a retrospective study of 648 patients with spinal disorders treated at the Shriner's Hospital for Crippled Children, Spokane Unit, between January 1991 and June 1993. The patients were divided into two groups: those with pain and those without pain. It was found that back pain with no organic cause was the primary diagnosis in 57% of the group having pain. Only one child had a diagnosis of malignancy, and there were no cases of active infection. In the group with pain, three interesting associations were found concerning psychosocial problems, disability, and litigation. It was found that the diagnoses in children with back pain treated at this referral center during the study period parallel those in the adult population with back pain.