H. E. Johns

THYMINE HYDRATE FORMED BY ULTRAVIOLET AND GAMMA IRRADIATION OF AQUEOUS SOLUTIONS

Abstract— Evidence is presented for the formation of a thymine hydrate upon ultraviolet (UV) or gamma irradiation of aqueous solutions. The UV quantum efficiency exhibits a dependence on pH similar to that shown for uracil hydration, but the yield is three orders of magnitude smaller than for uracil. Hydration is not affected by wavelength, oxygen, or concentration of thymine. The reversal rate of the photohydrate to thymine is similar to the reversal rates of both isomers of the thymine hydrate formed by γ radiolysis, and depends on pH in the same way as the rate for the uracil photohydrate. The photohydrate of thymine is chromatographically identical to the cis isomer of 6‐hydroxy‐5, 6‐dihydrothymine.

DOSIMETRY IN PHOTOCHEMISTRY

Abstract— A method of dosimetry which is useful in analyzing complex photochemical reactions involving more than one photoproduct is described. This involves determining a photochemical cross section for each of the products, σB, σc…., and an absorption cross section σ. for the reacting species. The ratio σB/σa is the quantum yield for production of product B. Where a number of photoproducts are produced simultaneously, it is convenient to express ‘dose’ in terms of the average number of photons per cm2‘seen’ by each molecule in the vessel. This average can be calculated from the number of incident photons, the volume of the liquid, the absorbance and the path length. Experimental details of the irradiation facility which is convenient for such studies are presented. The method of dosimetry is illustrated using photochemical data for uracil and orotic acid in solution.

COBALT-60 THERAPY UNIT FOR LARGE FIELD IRRADIATION

Abstract A cobalt-60 therapy unit designed and constructed by The Ontario Cancer Institute is described. This unit is capable of treating large fields up to 50 × 160 cm 2 at a conventional source to surface distance of 90 cm by using large collimator openings. Three selectable filters were incorporated. They all act as electron filters to improve the build-up characteristics of these beams. Two of the three filters also act as beam flatteners. The longitudinal collimators are individually driven so that it is possible to produce a vertical beam edge by closing one of the collimators completely. This condition is very useful for matching adjacent fields. The output of this unit, at its operating distance, ranges from 100–200 rad/min depending on which of the filters is chosen, which makes the treatment time shorter than most known facilities for very large field irradiation. This unit has been in routine use since 1977.

Xenon ionization detectors for fan beam computed tomography scanners.

Some of the physical characteristics of pressurized xenon ionization chambers as X-ray detectors for fan beam computed tomography scanners are presented. These include energy absorption efficiency, linearity of response, and sensitivity to scattered and off-focus radiation. A chamber 20 cm in length, pressurized with 5 atm xenon, will usefully absorb about 60% of the incident radiation. Its output can be madess linear over a range of X-ray intensity of 103 or more. We have also found that, with proper detector collimation. scattered radiation is insignificant. A design for a large (239 chamber) detector for use in an experimental fan beam scanner is described.

Gas ionization methods of electrostatic image formation in radiography.

Abstract The principles governing the use of an ionization chamber as an image-forming device for radiography are discussed. The proposal to use electron-avalanche amplification (Reiss, 1965) to enhance sensitivity is shown to be unsatisfactory owing to the wide range of avalanche size and the consequent “noise” in the picture. Good sensitivity and resolution can, however, be achieved by using a wide-gap chamber containing a high-z gas at a pressure of several atmospheres. Neither the range of the photoelectrons liberated nor the lateral diffusion of the ions in crossing the chamber prevent a resolution of some ten line pairs per mm from being obtained. A theoretical analysis is given first and this is confirmed by experimental pictures.

Calculation of the average energy absorbed in photon interactions

High energy electrons set into motion by photon interactions with matter lose some of their energy by bremsstrahlung. This loss must be evaluated before energy absorption coefficients may be calculated. Recent extensive tables of data published by Plechaty et al. contain an appreciable error in this quantity. The error results from two simplifying assumptions and for the case of very high photon energies interacting with high atomic number materials can be as much as a factor of two. This has important implications for the evaluation of quantities used in radiation dosimetry.

Effect of small inhomogeneities on dose in a cobalt‐60 beam

The effect of small ring-shaped cavities on the dose delivered by a cobalt-60 beam to a homogeneous medium was studied experimentally and theoretically. Changes as small as 1 x 10(-4) of the total dose were measured. Experimental results show that, depending on the position of the cavity, replacing water with a small cavity can either increase or decrease scatter dose to a point in the medium. The increase in scatter dose was not anticipated and to our knowledge is not predicted by any of the presently available inhomogeneity dose correction algorithms used in treatment planning. Calculations were based on the perturbations of first and second scatter dose contributions and show that the presence of a cavity in the medium introduces three processes that decrease scatter dose and five that increase it. The calculated net effect is in good agreement with experiment. Additional calculations show that the effect of a single small inhomogeneity cannot be easily extrapolated to larger inhomogeneities and that multiple inhomogeneities do not act independently. Results from this study demonstrate two constraints that must be satisfied by future dose calculation algorithms: (i) they must correctly determine dose in a homogeneous non-unit density material, and (ii) they must account for the change in dose due to small inhomogeneities in the medium.

FLASH PHOTOLYSIS STUDIES OF OROTIC ACID

Abstract— The triplet state of orotic acid has been studied by flash photolysis. The rate for dimerization has been observed to vary from 2 × 109M‐1 sec‐1 at pH 1 where both the triplet and ground state molecules are neutral, to under 108 M‐1 sec‐1 above pH 9 where both the triplet and ground state molecules are doubly ionized. The pK of the triplet state has been measured as 4.6. The rate of oxygen quenching for the triplet is 2–3 × 109 M‐1 sec‐1 while the rate of radiationless decay in solution is 0.73 × 104 sec‐1. The triplet absorption spectra have been measured for the two ionic forms of the triplet.

Second scatter contribution to dose in a cobalt-60 beam.

Semi-analytic and direct numerical integration approaches have been used to study the contribution of doubly scattered photons to dose at a point P in a homogeneous water medium irradiated by a cobalt-60 point source. The semi-analytic analysis was used for an infinite field radius to calculate the variation of second scatter dose with depth, the angular dependence of the second scatter dose, and the spectra. Direct numerical integration was used to calculate the second scatter dose at a depth of 10 cm for various finite field radii ranging from 0 to 25 cm. Maps were made of the locations of scattering sites that were important to the second scatter dose. These calculations show that the second scatter contribution is generally less than the first scatter dose with the exception of a few sites remote from the point of interest. In many ways, the pattern of dose deposition by second scatter is similar to first scatter with less pronounced features. In other respects, the second scatter appears more isotropic. The implications of these calculations for approximate radiotherapy dose calculations are discussed.

Use of electron filters to improve the buildup characteristics of large fields from Cobalt‐60 beams

It has been shown that for Cobalt-60 units capable of producing large fields (up to 40 X 40 cm2)at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1-2 mm instead of 5 mm; and second, the maximum does in some extreme cases is 15%-20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%-20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.