PubMed · 5293595
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K W Beckman. 1971. Computer program for intracavitary dose planning.. https://pubmed.ncbi.nlm.nih.gov/5293595/
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PURPOSE: Many articles have been published on the measurement of the dose to points outside the primary beam, often called the peripheral dose (PD), for instance, to the gonads, for specific treatment machines and/or techniques. We investigated the possibilities for developing a generalized method based on the data from several publications. METHODS AND MATERIALS: The data from several publications were recalculated for a reference situation, then averaged, and the frequency distributions around the mean were determined. Published data were available for 60Co, 4, 6, 8, and 10 MV, and 18 to 25 MV for a large variety of treatment machines. Furthermore, an analysis of possible corrections for depth dependence, field elongation, irregularly shaped fields, wedges, and shielding blocks was carried out. RESULTS: The frequency distributions of all published PD values for square fields for photon energies of 4 MV to 25 MV showed a standard deviation of 33%. The PD values of 60Co are significantly different with a standard deviation of 25%. A difference in the leakage radiation between cobalt machines and linear accelerators can possibly explain this difference, especially for large distances, where leakage radiation predominates. Taking the uncertainty of the risk factors into consideration, we conclude that the use of average values is justified. Although statistically not significant, the peripheral dose appears to be dependent on photon energy with a minimum around 6 MV. CONCLUSIONS: It is possible to estimate the peripheral dose for photon energies of 4 MV to 25 MV with an accuracy of +/- 33%; for 60Co, the accuracy is even better. The variation of the PD between different treatment machines is so small that it is justified to use average PD values, irrespective of the treatment machine.
Cobalt-57 flood sources are often used for system flood-field uniformity checks of scintillation camera performance. Such sources are now available in large sizes and with high activities. Uniformity images using new 57Co sources have shown artefacts which are not present in uniformity images obtained from technetium-99m flood sources of the same activity. The high-energy photons emitted by 57Co and cobalt isotope impurities appear to influence the images obtained. This was investigated in three cameras using three 57Co sources of different age. The flood-field images obtained with the 57Co sources showed non-uniformity patterns that were dependent on the age of the cobalt source, the distance of the source to the collimator, and the specific camera type. Quantification of the uniformity images reflected these findings. Energy spectra of a new 57Co source, obtained with an external 1024-channel analyser connected to the camera, showed a broad tail of high-energy photons above the 122-keV photopeak, due to Compton scatter and collimator penetration. This tail diminished with older sources and with increased source to collimator distance, indicating that in both situations fewer high-energy photons were being measured by the camera system. This tail of high-energy photons contributes to the total count rate, but because this is not obvious from the counts observed in the photopeak window, the camera can unsuspectingly be operated at too high a total countrate where pile-up effects become significant. Caution is therefore advised when using 57Co sources. Problems may be minimized by purchasing low-activity sources (certainly no more than 370 MBq), by placing the source at a distance from the collimator, e.g. 50 cm, by allowing a new source to decay, and by manufacturers producing 57Co sources without detectable radioactive contaminants.
The mechanistic aspects of the spectrophotometric method of analysis of lipid hydroperoxides (LOOH) based on the oxidation of ferrous to ferric ion and subsequent complexation of the latter by thiocyanate are considered. The method of analysis, as revised by us, was carried out in the same solvent that had been used for the extraction of lipids from the sample, a deoxygenated chloroform:methanol or a dichloromethane:methanol (2:1, v/v) mixture, and used a single solution containing both reagents, Fe2+ and SCN-, for developing the response. In that solvent, total lipids up to 5 mg/ml did not interfere, and linear increase of the absorbance of ferric thiocyanate complex was obtained up to 2 x 10(-5) M LOOH. Molar absorptivity of the ferric thiocyanate complex expressed per mol of LOOH was determined as 58,440 M-1 cm-1, based on the average of four ferric ions produced by each LOOH molecule. The estimated lowest detectable limit was about 170 pmol LOOH/ml of analyzed solution, which corresponded to about 50 mumol LOOH/kg lipid in complex natural mixtures. In addition to good sensitivity, and in contrast to some other more popular spectrophotometric assays for LOOH, the method is responsive also to hydroperoxides of mono- and di-unsaturated fatty acids. The method, thus, provides an easy, rapid, sensitive, and complete measure of hydroperoxidation of lipids.