Journal of Medical Physics
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   1994| July-September  | Volume 19 | Issue 3  
    Online since April 24, 2009

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Specifications For Photon Beam Quality From Linacs
Anil Kumar Sharma, Sanjay S Supe, K Subbarangaiah
July-September 1994, 19(3):98-103
The photon beam quality is normally determined along the central axis of a symmetrical beam of a defined field size. Today when there are at least three different methods for field flattening, giving significantly different energy distributions over the field area, it would certainly be important to have some more detailed information about the off-axis photon energy. The treatment techniques are further becoming more and more complex. Irregular and heavily off-centered beams, dynamic wedges etc. are often used in clinical practice today. Another drawback with the commonly used methods for beam quality specifications, TPR20/10and D20 / D10, is the large change in stopping power ratio relative to TPR20/10 when used for ionization chamber dosimetry. Further, it was recently shown that high energy electrons may affect these ratios in high energy photon beams. To obtain more detailed information about the energy distribution over the whole field, a method based on narrow beam penetration of water (HVL in water) was used to study the beam quality of a dual photon energy linac (Clinac - -1800) employing a simple and reliable device specially constructed for these measurements. Both the photon beams (6 and 18 MV) showed marked softening away from the beam central axis. It is very important to apply corrections for the observed beam quality change in the cross-section of the beam, otherwise it may result in erroneous patient dose while using large or off-centered fields.
[ABSTRACT]   Full text not available  [PDF]
  731 176 -
Biochemical Reasoning For Radiation Protection And Screening methods For Radiation Sensitivity And Potential Carcinogenicity
Emanuel Riklis, Richard B Setlow, Ingrid Emerit
July-September 1994, 19(3):108-117
The problem of resistance or sensitivity to radiation, ionizing and non-ionizing alike, is of importance both to healthy individuals and to cancer patients. The possibility of severe sensitivity to radiation should be particularly recognized in the case of radiation workers and in patients undergoing radiotherapy. The sensitivity to ultraviolet light is of concern to the general public, but in particular to individuals who have a deficiency in their ability to repair damages in their DNA, a deficiency which is expressed in several diseases, in notably Xeroderma pigmentosum (XP), Ataxia telangiectasia (AT) and other autoimmune diseases. DNA repair capacity and the possiblity of affecting it by various modifying agents is a basic consideration in planning radiotherapy or phototherapy. The measurement of DNA repair capacity of cells, and the study of the effect of modifying agents on DNA repair is of importance. For these reasons, a relatively simple and accurate method for determining DNA repair has been developed, and it enables also to measure effects of modifying compounds on repair. The method is based on inhibition semiconservative DNA synthesis by treatment of cells with trimethylpsoralen + near UV light, and measuring repair synthesis as uptake of labelled thymidine after exposure to radiation. With this method an improved protection by the phosphoaminothiol WR-2721 was demonstrated, and shown to be a result of both reducing the damage and enhancement of DNA repair. Similar results were obtained with nicotinamide at low concentrations, acting in improving repair through the polyADPribose system. Recently, a clastogenic plasma factor which causes chromatid breaks in plasma cells has been found and isolated from plasma of individuals who have been exposed to radiation in the Chernobyl reactor accident. The breakage factor is known to exist also in plasma of individuals who suffer from a variety of autoimmune diseases, such as AT, Bloom's syndrome, polyarthritis and other inflammatory conditions. The clastogenic factor (CF) acts like a long-lived free radical, as it can be found years after the exposure to radiation, and may be eliminated by the antioxidant enzyme superoxide dismutase (SOD). This provides hope to individuals carrying the breakage factor, and this approach requires further research. The methods for measuring DNA repair capacity and the presence of CF in irradiated persons are important contributions to the determination of risk of carcinogenesis.
[ABSTRACT]   Full text not available  [PDF]
  773 102 -
Radiation And Pregnancy : A Self - Teaching Computer Program
Pratik Kumar, M.M Rehani
July-September 1994, 19(3):127-131
Two self-interacting computer programs have been developed. The first program which consists of fifteen topics aprises the users with broad spectrum of radiation risks to the unborn during pregnancy and the status of various views in this regard. Another program estimates the dose to uterus in sixteen radiological examinations depending upon the radiographic parameters used. The dose to uterus and hence to the fetus calculated by computer program in different radiographic views have been found to be in agreement with that reported in NRPB-R200 survey report. The two programs combined provide a better understanding of the rather confusing situation regarding dilemma about termination of pregnancy following inadvertent radiation-exposure, apprehension about radiation-effect in the minds of prescribing doctor and patients, dose estimation and advice to pregnant workers and like.
[ABSTRACT]   Full text not available  [PDF]
  737 122 -
Controversies In External Beam And High Dose Rate Brachytherapy Of Oesophageal Cancer
Ranjan K Sur, Victor C levin, Simon Malas, Bernard Donde
July-September 1994, 19(3):104-107
Various controversies in the treatment of oesophageal carcinoma with external beam radiotherapy and high dose rate intracavitary irradiation have been reviewed. Conflicting results from different parts of the world has made it difficult to optimize the radiation dose that may give the best results. More studies and longer follow-up are needed before a definite conclusion can be made on the optimization of dose.
[ABSTRACT]   Full text not available  [PDF]
  676 107 -
Radioactive Seed Localizations From Computed Tomography Imaging Technique
Komanduri M. Ayyangar, Sailesh Gupta, Ralph R Dobelbower, Cheng B Saw, Leroy J Korb
July-September 1994, 19(3):121-126
A method that combines radiographic localization of seeds in brachytherapy implant with CT images has been developed to provide accurate seed location and dose distribution data, in relation to the three dimensional localization of target volume and critical organs at risk. The accurate determination of radioactive seed locations in permanent implants from computed tomography (CT) scans is limited by the magnitude of the pixel size and the slice thickness. The image of a single seed may appear in more than one consecutive CT image. A method of removing this artifact using a tolerance diameter about a seed center determined in the previous CT slice is described. To improve further the accuracy of defining the seed positions and orientations, the reconstructed seed coordinates derived from the three film technique were transformed into CT coordinates for dose distribution computations. The differences between this technique and the technique of using CT scans to reconstruct seed coordinates directly were assessed. This method has been applied in the case of prostate implants. It allows for the generation of dose volume histograms for the prostate, bladder and the rectum.
[ABSTRACT]   Full text not available  [PDF]
  677 82 -
Interstitial Dosimetry Concepts - An Overview
A Shanta
July-September 1994, 19(3):93-97
Interstitial therapy with 192Ir wires or seeds has gained considerable importance with the introduction of afterloading techniques. The interstitial afterloading techniques, the distribution of sources do not conform to that of conventional Paterson-Parker system or Quimby system. The concepts followed for specifying the reference dose rate vary from centre to centre. Analysis of some of the most popular dosimetry systems and their comparison with Paterson-Parker System of dosage are discussed in this paper.
[ABSTRACT]   Full text not available  [PDF]
  632 89 -
Quality Assurance In Transport Of Radioactive Material The Medical Physicist's Role
A.N Nandakumar
July-September 1994, 19(3):118-120
Currently, over 25,000 packages are being transported within India every year. Of these, nearly, 75% are for medical applications. Quality Assurance in design and fabrication is the responsibility of the designer /consignor. Quality Assurance standards in the use of packages are apparently less demanding and its importance is liable to be undermined. Off-normal incidents with potential exposure to transport workers and public can be averted by proper QA techniques. The medical physicist can contribute to the overall QA by carrying out certain checks.
[ABSTRACT]   Full text not available  [PDF]
  589 123 -
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