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H & D curves were obtained for various film/screen combinations exposed to 60Co, 4-MV, and 45-MV photons. Data indicate that contrast is enhanced more by light-emitting screens than by lead screens or direct exposure. Flexible film cassettes are more comfortable for the patient, and extra-large film can be used for larger patients.
Seven new and two conventional screens were investigated. Measurements were performed at different energies of the photon fraction interacting in the screens, and energy losses due to escape of K-fluorescent radiations were calculated. The speed of the screens was dependent on the energy; this was primarily due to the fact that x-ray absorption is dependent on energy. Rare earth and other new screens absorb a maximum of 1.5 times as much energy and emit twice as much light as comparable calcium tungstate screens. Reabsorbed K-fluorescent radiations might contribute to image unsharpness in the new screens.
Squamous carcinomas in WHT/Ht mice were exposed to the biomedical pion beam at TRIUMF. The biological effects of the pion beam were compared to those of 250 kVp x rays. The dose rates were 2-3 rad/min. (.02-.03 Gy/min.) for pions and 56 rad/min. (.56 Gy) for x rays. The surviving fraction of epithelioma cells irradiated in vivo was studied using the TD50 assay. RBE values for pions were calculated from the ratio of the x-ray and pion D0 values and were found to be 1.6 +/- 0.3 for oxic cells and 2.1 +/- 0.4 for hypoxic cells. OER values were 1.8 +/- 0.5 for pions and 2.4 +/- 0.4 for 250 kVp x rays.
Although low entrance dosea are desirable for cosmetic and tolerance reasons, they may leave the superficial nodes underdosed. Surface dose dependence on area and distance from the shadow tray for 10 MV photons was studied and comparisons made with 60Co and 4 MV energies. The use of 10 MV photons would leave superficial nodes underdosed unless a bolus or supplementary treatment, such as electrons, is used. Treatment planning implications for a mantle field are discussed.
The effect of Adriamycin (doxorubicin hydrochloride) (10 mg/kg intraperitoneally) on the response of mouse gut to irradiation with 6-MVp photons or fast neutrons (67 MeV p--Be), was assessed with the six-day death endpoint. The drug reduced the LD50/6 by the same factor (approximately 1.25) for both types of radiation. Thus, the radiobiological effectiveness (RBE) (photons/neutrons) for gut damage is unaltered by the addition of Adriamycin. The data indicated no significant effect of drug scheduling if given within 16 hours before or after irradiation.
The authors questioned whether more sophisticated density correction algorithms than those presently used for photon-beam dose calculations are necessary to take full advantage of CT in treatment planning. Predictions obtained with correction equations based on the radiological thickness (effective SSD) and generalized power-law TAR methods, as well as on the more complex equivalent-TAR method, we compared with ionization-chamber measurements in phantoms containing stimulated lung and soft tissues. Various geometric configurations were investigated for 60Co and 8-MV radiations. While the equivalent-TAR method gave the best general agreement, the power-law method appears to be accurate enough for regular treatment-planning purposes. The effective SSD method has a limited range of validity. At present, complicated methods of density correction do not seem to be justified.
The 15-MV photon beam of a linear accelerator (Siemens Mevatron 20) was studied for electron and scattered photon contamination. The surface dose, attributable almost entirely to contamination electrons, has a Gaussian lateral distribution, a linear dependence on field width for square fields, and an inverse square dependence on distance from the bottom of the fixed head assembly. This geometrical dependence is consistent with the proposal that the field flattening filter is the main source of electron contamination when accessories are absent. A tissue-maximum-ratio curve in the build-up region for the electron and photon contamination was produced utilizing the linearity of dose with respect to field width. The derived contamination curve inside was similar to the measured build-up curve outside the field. The primary photon component, obtained by subtracting the contaminant contribution, showed no dependence on field size, source-to-probe distance, or presence of accessories.
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The authors describe a useful bolus material for therapy with high-energy photons and electrons (greater than 1 MeV). Qualities evaluated included flexibility, transparency, tissue equivalence (re radiation interactions), stability at high doses, durability, lack of toxicity, ease in cleaning, low flammability, and cost. Using a parallel-plate ionization chamber and 60Co, 4-, and 10-MV photons as well as 6- and 18-MeV electrons, maximum deviation from water-equivalent values was approximately 2 mm for 6-MeV electrons. The material comes in large sheets of precise thickness (+/- 0.5 mm) which can be cut and layered to form more complex shapes.
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To improve the accuracy of scintigraphic ventricular volume determination, which is limited by photon attenuation between the heart and the gamma camera, a method was developed for directly measuring the photon attenuation of radioactivity delivered as a bolus through a Swan-Ganz catheter into the right atrium. Comparison of the count rate recovered from this bolus with the total ex vivo measured activity determined by imaging an aliquot of the administered activity allowed calculation of the attenuation factor. Left ventricular stroke volumes determined scintigraphically by the count method in gated blood pool studies and then corrected with this attenuation factor correlated well with stroke volumes determined from thermodilution cardiac output and heart rate (r = 0.92; SEE, 6.1 ml). The agreement between the two measurements was markedly less when an average attenuation factor was employed for correction of scintigraphic volumes (r = 0.52; SEE, 14.8 ml). The results indicate that correction for photon attenuation is needed for accurate measurement of left ventricular volumes. Correction of left ventricular counts based on body weight or body surface area improves the accuracy of volume estimates.
Positron-computed tomography with [18F]fluorodeoxyglucose to measure glucose metabolism has shown changes in the brain distant to a focal area of infarction, demonstrating that what appears as a focal abnormality represents a more widespread functional process. Several approaches are presented to better understand quantitative metabolic data and focus on caudate and basal ganglia function. Area-to-area correlations in Parkinson's and Huntington's diseases showed decreases in the number of cortical relationships compared with control subjects, suggesting that the basal ganglia are involved with the ability of cortical regions to function together. In aphasia, caudate metabolism correlated with several language measures that suggested a role in some undefined basic process, seemingly related to Broca's area function. The studies presented suggest that the caudate may involve integrating the processing of language and cognition with the execution of the resulting response. Motor and cognitive function seem related to similar and overlapping brain systems. The disruption of such systems may result in loss of both cognitive and motor aspects of a function.
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Relative error functions have been calculated for the various single and double photon energy techniques used for the measurement of linear bone mineral and fat mass in vivo. By considering only those errors arising from statistical fluctuations in photon counting useful information for guidance in the most efficient operation of the method is obtained. The results indicate that statistically the single energy method is always significantly better than the double energy method for measurement of both bone and fat fractions in soft tissue at total thicknesses up to 15 g/cm-2. In those circumstances where it may be necessary to use the double-photon energy technique there is an optimum sample thickness of 5 g/cm-2 for minimum error. Values of photon energy which permit measurement with the maximum precision in the minimum count time are deduced for a wide range of practical conditions and are presented along with a table of potentially suitable radioisotopes.
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