Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:1.3.1.51 (HDR)
605 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Practical dosimeters in brachytherapy, such as thermoluminescent dosimeters (TLD) and diodes, are usually calibrated against low-energy megavoltage beams. To measure absolute dose rate near a brachytherapy source, it is necessary to establish the energy response of the detector relative to that of the calibration energy. The purpose of this paper is to assess the accuracy of Monte Carlo photon transport (MCPT) simulation in modelling the absolute detector response as a function of detector geometry and photon energy. We have exposed two different sizes of TLD-100 (LiF chips) and p-type silicon diode detectors to calibrated 60Co, HDR source (192Ir) and superficial x-ray beams. For the Scanditronix electron-field diode, the relative detector response, defined as the measured detector readings per measured unit of air kerma, varied from 38.46 V cGy-1 (40 kVp beam) to 6.22 V cGy-1 (60Co beam). Similarly for the large and small chips the same quantity varied from 2.08-3.02 nC cGy-1 and 0.171-0.244 nC cGy-1, respectively. Monte Carlo simulation was used to calculate the absorbed dose to the active volume of the detector per unit air kerma. If the Monte Carlo simulation is accurate, then the absolute detector response, which is defined as the measured detector reading per unit dose absorbed by the active detector volume, and is calculated by Monte Carlo simulation, should be a constant. For the diode, the absolute response is 5.86 +/- 0.15 (V cGy-1). For TLDs of size 3 x 3 x 1 mm3 the absolute response is 2.47 +/- 0.07 (nC cGy-1) and for TLDs of 1 x 1 x 1 mm3 it is 0.201 +/- 0.008 (nC cGy-1). From the above results we can conclude that the absolute response function of detectors (TLDs and diodes) is directly proportional to absorbed dose by the active volume of the detector and is independent of beam quality.
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PMID:Accuracy of Monte Carlo photon transport simulation in characterizing brachytherapy dosimeter energy-response artefacts. 879 80

The relationship between cellular radiosensitivity and normal-tissue response to radiotherapy in individual cancer patients has attracted increasing attention over the last few years. Recent work has suggested that a correlation exists between fibroblast sensitivity and normal-tissue reactions. We have examined the radiosensitivity of fibroblasts grown from skin biopsies of four normal individuals and three patients identified as having suffered unexpectedly severe reactions to clinical radiotherapy, called here 'over-reactor' (OR) patients. Clonogenic survival was measured after high (HDR) and low dose-rate (LDR) irradiation. By comparing the two, and LDR Recovery Factor was derived. Potentially-lethal damage repair was examined in 4 cell strains. After HDR the OR strains were indistinguishable from the normals. At LDR the range of sensitivity was expanded. The OR strains fell at the sensitive end of the range and were characterized by a lack of LDR recovery, which clearly distinguished them from the normal strains. Experimental errors were estimated by considering all the data sets together rather than viewing each experiment individually. Duplicate strains from several patients were tested, and the differences between them were found to be within the estimated experimental errors, suggesting that these differences were not biologically significant. The data are consistent with the hypothesis that normal-tissue response is linked to individual cellular radiosensitivity. Our data confirm the importance of using LDR irradiation in clinical investigations of cellular sensitivity.
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PMID:Low dose-rate fibroblast radiosensitivity and the prediction of patient response to radiotherapy. 880 Feb

To correct for the influence of source configuration on the measured activity of spherical and cylindrical brachytherapy sources, a geometric correction factor was calculated for the Standard Imaging HDR-1000 well-type ionization chamber. A Fortran program modelled each source as a lattice of point sources. Because of the cylindrical symmetry of the well chamber, it could be uniquely modelled by point detectors along the perimeter of the radial plane of the detection volume. Path lengths were calculated and attenuation factors were applied to each source-detector point combination individually. The total dose rate at each detection point was found through a Sievert summation of the point source contributions. For 137Cs sources with identical activities, a correction factor of 0.965 +/- 0.005 was calculated, equal to the ratio of the dose rate of the cylindrical source to that of the sphere. Experimental verification using a Nuclear Associates 67-809 series cylindrical sources and an Amersham spherical 137Cs source yielded a correction factor of 0.958 +/- 0.016.
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PMID:A well-type ionization chamber geometric correction factor. 882 81

Well-known radiotherapeutic strategies for hypoxic cells include hypoxic radiosensitizers, heavy particles, and fractionated irradiation. This study attempted to obtain the ultimate effectiveness of these strategies by combining nicotinamide plus carbogen (N + C) as a hypoxic radiosensitizer with fractionated pions. In addition, the influence on the N + C effect of X-ray dose rate used as a reference radiation was evaluated. When SCCVII tumors in the dorsum of feet reached 50 mm3 in volume, they were irradiated with pions (0.2 Gy/min), the same dose rate (LDR; 0.2 Gy/min) X-rays, or high dose rate (HDR; 1.5 Gy/min) X-rays in 10 fractions over 11 days. Nicotinamide (0.5 mg/g) was administered i.p. one hour before irradiation, and normobaric carbogen (95% oxygen and 5% carbon dioxide) was breathed from 10 min before irradiation. The effect was evaluated by tumor growth time (TGT50) assay. The combination of N + C significantly enhanced the effect of 30 Gy LDR and 28 Gy HDR X-rays, with the effect corresponding to that of 39 Gy HDR X-rays: the enhancement ratios were 1.2 and 1.4, respectively. The effect of 20 Gy pions was equivalent to the effect of 33 Gy HDR X-rays (ratio of 1.65), or the effect of N + C combined with 28 Gy HDR X-rays. However, N + C did not enhance the effect of 20 Gy pions. This suggested that the fractionated pions had great biological effectiveness against hypoxic cells. In conclusion, N + C afforded no additional benefit with fractionated pions, but it was thought to be of value for fractionated X-rays, even in a dose rate of 0.2 Gy/min.
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PMID:Radiosensitizing effect of nicotinamide and carbogen combined in fractionated pions or x-rays in SCCVII tumors. 885 Mar 70

1. HDR Ir-192 intracavitary insertion in Foley's catheter for a boostered radiation dose to the nasopharynx is a very simple and safe technique and may increase the local control and survival of the patients. 2. Intracavitary brachytherapy should be done in all cases of NPC after the completion of external irradiation to increase local control that may lead to long term survival of the patients.
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PMID:Intracavitary irradiation of nasopharyngeal carcinoma using Ir-192 in Foley's catheter. 886 99

Between April 1988 and December 1992, 37 patients with small, previously unirradiated, primary non-small cell carcinomas of the bronchus causing symptoms due to endobronchial disease were treated at the Christie Hospital, Manchester, with a single fraction of high dose rate intraluminal radiotherapy (ILT) using the microSelectron-HDR machine. Small primary (SP) lesions were defined as being less than 2 cm in diameter in a direction perpendicular to the central axis of the iridium-192 treatment source. Fifteen patients (41%) were treated to a dose of 15 Gy and 22 patients (59%) to 20 Gy at a distance of 1 cm from the central axis of the source. At 6 weeks following ILT, improvement in symptoms was seen in the following percentages of patients: haemoptysis 96%, pulmonary collapse 69%, cough 55% and dyspnoea 52%. The magnitude of improvement in these symptoms was largely maintained in patients surviving to 4 months and then 12 months post-ILT. Median actuarial survival was 709 days, 2-year survival 49.4% and 5-year survival 14.1%. Overall, there was no significant difference in survival after treatment with 20 Gy compared with 15 Gy at 1 cm. At the close of study, there were four patients still alive without disease recurrence with survivals of 38, 48, 49 and 63 months. All had had biopsy-proven squamous cell carcinomas and all had been treated with 20 Gy at 1 cm. Five patients died from massive haemoptysis as a terminal event at 4, 9, 9, 10 and 11 months post-ILT, well below the median survival for this group of patients. Again, all had been treated with 20 Gy as opposed to 15 Gy at 1 cm. Over the same time period, 287 patients with non-small cell carcinomas of more than 2 cm in diameter (large primary lesions, LP), were treated with a single fraction of ILT only, as their initial treatment. A consistently greater percentage of patients with SP lesions showed an improvement in the symptoms of haemoptysis and pulmonary collapse when compared with patients with LP lesions. Patients with LP lesions demonstrated a decreased actuarial survival when compared with SP lesions, with median survival being 156 days, 2-year survival 3.1% and no survivors beyond 39 months. This study demonstrates that, in patients with small endobronchial carcinomas a single fraction of ILT can give efficient palliation of symptoms and lead to long term disease-free survival, but that a dose of 20 Gy may be at the limit of bronchial radiation tolerance for a single dose technique employing a high dose rate source.
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PMID:Long-term survival and symptom palliation in small primary bronchial carcinomas following treatment with intraluminal radiotherapy alone. 887 Oct 2

Intraoperative radiation therapy (IORT) has the obvious advantage of maximally irradiating the tumor bed while eliminating surrounding normal organs from the field of radiation. This approach has been especially useful when the required radiation dose exceeds the tolerance dose of the surrounding normal tissues. However, the application of IORT has been significantly limited by cost, logistic issues, and technical problems related to delivering treatment to difficult anatomic areas. We have developed a new approach to IORT that obviates the need for patient transport: In a dedicated, shielded operating room, the surgery is performed and IORT is delivered via HDR remote afterloading. We have found this approach to be cost effective, logistically sound, and suitable for a wide range of anatomic sites. The technical aspects of the procedure, as well our preliminary results in colorectal cancer, will be presented.
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PMID:High-dose-rate intraoperative radiation therapy for colorectal cancer. 892 80

In order to establish the standard schedule of 192Ir-HDR brachytherapy, it is necessary to determine the optimal time-dose-fractionation. A new simulation model was used to determine the optimal schedule. This model, which is based on the LQ-model, includes the effect of incomplete repair between fractionations. The fraction surviving after irradiation is expressed as log (surviving fraction) = n (-alpha x -beta x2)-n beta x2 hn (theta), and the change in fractionation will influence the value of hn (theta). From analysis of this model, the therapeutic ratio will approach one if two fractionations (6-hour and 18-hour intervals) or three fractionations (4-hour, 4-hour and 16-hour intervals) in a day are carried out for 6 days. This simulation model will provide an optimal treatment schedule for tumor control, for example, two daily fractionations of 4.6Gy with 6-hour intervals between the daily fractionations (and 18 hours to the first fractionations of the next day), 12 fractionations to 55.2Gy in 6 days or three daily fractionations of 3.3Gy with a 4-hour interval between the daily fractionations (and 16 hours to the first fractionations of the next day), 18 fractionations to 59.4Gy in 6 days. Further clinical study should be carried out to confirm this simulation.
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PMID:[Therapeutic ratio of fractionated high-dose-rate against continuous low-dose-rate brachytherapy analyzed by simulation model]. 894 Aug 18

We investigated the reproducibility of the cytochalasin B micronucleus (MN) assay in irradiated human lymphocytes to assess its suitability in predicting cancer predisposition and response to radiotherapy by virtue of defects in the processing of clastogenic lesions. G0 lymphocytes were exposed to 3.0 Gy 60Co gamma-rays at high (HDR) or low dose-rate (LDR). Six healthy donors were assayed three times each in nine experiments and compared with six ataxia-telangiectasia (A-T) heterozygotes. In controls, significant interexperiment variability in MN yields was observed at HDR and LDR, also in dose-rate sparing (i.e. reduction in MN yield at LDR compared with HDR). Significant inter-individual variability was seen at HDR, but not at LDR or for sparing. Average sparing was 66.4 +/- 4.8%. In spite of the experimental variability, a significant difference between controls and A-T heterozygotes was detected at LDR, and 5/6 heterozygotes had sparing values below the control range. This gives encouragement for the use of this assay in predictive testing if sources of experimental variability can be identified so as to improve discrimination between individuals. HDR and to a lesser extent LDR irradiation induced significant mitotic inhibition, seen as a reduction in binucleate cells after cytocholasin treatment. A positive correlation between mitotic inhibition and MN frequency suggests that similar lesions may be involved in these effects.
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PMID:Dose-rate sparing for micronucleus induction in lymphocytes of controls and ataxia-telangiectasia heterozygotes exposed to 60Co gamma-irradiation in vitro. 894 33

Using theoretical models based on radiobiological principles for the design of new treatment schedules for HDR and PDR brachytherapy, it is important to realise the impact of assumptions regarding the kinetics of repair. Extrapolations based on longer repair half times in a continuous LDR reference scheme may lead to the calculation of dangerously high doses for alternative HDR and PDR treatment schedules. We used the clinical experience obtained with conventional ERT and LDR brachytherapy in head and neck cancer as a clinical guideline to check the impact of the radiobiological parameters used. Biologically equivalent dose (BED) values for the in clinical practice of LDR brachytherapy recommended dose of 65-70 Gy (prescribed at a dose rate between 30-50 cGy/h) are calculated as a function of the repair half time. These BED values are compared with the biological effect of a clinical reference dose of conventional ERT with 2 Gy/day and complete repair between the fractions. From this comparison of LDR and ERT treatment schedules, a range of values for the repair half times of acute or late responding tissues is demarcated with a reasonable fit to the clinical data. For the acute effects (or tumor control) the best fits are obtained for repair half times of about 0.5 h, while for late effects the repair half times are at least 1 h and can be as high as 3 h. Within these ranges of repair half times for acute and late effects, the outcome of "alternative' HDR or PDR treatment schedules are discussed. It is predominantly the late reacting normal tissue with the longer repair half time for which problems will be encountered and no or only marginal gain is to be expected of decreasing the dose rate per pulse in PDR brachytherapy.
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PMID:Constraints in the use of repair half times and mathematical modelling for the clinical application of HDR and PDR treatment schedules as an alternative for LDR brachytherapy. 896 28


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