The procedures of radiation therapy consist of a number of steps, one of which is the calculation of the radiation dosage pattern that will result when a particular arrangement of radiation beams or sources is applied to a patient. The purpose of this calculation is two-fold. One is to predict, as part of the dose planning process, what dose distribution can be achieved with a selected beam arrangement. The second is to record what treatment has been given so that post-treatment analysis can be carried out. Both of these are important.

Electrons↗

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Activation of nicotinic receptors on GABAergic amacrine cells in the rabbit retina indirectly stimulates dopamine release.

The retina possesses subpopulations of amacrine cells, which utilize different transmitters, including acetylcholine (ACh), GABA, and dopamine. We have examined interactions between these neurones by studying the effects of nicotinic agonists on GABA and dopamine release. Isolated rabbit retinas were incubated with [3H]dopamine and then superfused. Fractions of the superfusate (2 min) were collected and the [3H]dopamine in each sample was measured. Endogenous GABA release was examined by incubating retinas in a small chamber. At 5-min intervals, the medium was changed and the GABA measured by high-pressure liquid chromatography (HPLC). Exposure of the retina to nicotine, epibatidine, and other nicotinic agonists increased the release of both GABA and dopamine. The effects of nicotine and epibatidine were blocked by mecamylamine, confirming an action on nicotinic receptors. The action of epibatidine on dopamine release was unaffected by glutamate antagonists but was blocked by picrotoxin and gabazine. These results suggested that nicotine might increase dopamine release indirectly by stimulating the release of GABA, which in turn inhibited the release of an inhibitory transmitter acting tonically on the dopaminergic amacrines. Exposure of the retina to GABA caused a small increase in dopamine release. This hypothetical inhibitory transmitter was not GABA, an opioid, adenosine, glycine, nociceptin, a cannabinoid, or nitric oxide because appropriate antagonists did not affect the resting release of dopamine. However, metergoline, a 5HT1/5HT2 receptor antagonist, and ketanserin, a 5HT2A receptor antagonist, but not the 5HT1A antagonist WAY100635, increased the resting release of dopamine and blocked the effects of nicotine. The 5HT1A/5HT7 agonist 8-hydroxy DPAT inhibited both the nicotine and GABA-evoked release of dopamine. We conclude that nicotinic agonists directly stimulate the release of GABA, but the evoked release of dopamine is indirect, and arises from GABA inhibiting the input of an inhibitory transmitter, which we tentatively identify as serotonin.

Animals↗

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Corrections to absorbed dose calculations for tissue inhomogeneities.

Traditional methods for correcting for the presence of tissue inhomogeneities may produce errors as great as 10% at points within or closese to the inhomogeneities. Measurements were made in phantoms containing aluminum or cork inhomogeneities. Agreement between measured and predicted results was usually within 2%-3%.

Methods↗

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Air versus water calibration for 60Co gamma rays.

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Atmosphere↗

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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.

Elementary Particles↗

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Displacement corrections used in absorbed dose determination.

When an (exposure) calibrated ionization chamber is used for the determination of absorbed dose from a photon beam, the reading of the instrument must be multiplied by a number of factors, one of which is an attenuation correction for phantom material displaced by the chamber. the magnitude of this correction must depend on the size and shape of the ionization chamber as well as the energy of the radiation beam. For cobalt-60 radiation, a single number, 0.985, has generally been used. Recent measurements, however, and "first scatter" calculations, of kerma suggest that a more appropriate value for a Farmer-type chamber used in a water phantom would be 0.975. Such a change is small but would be important when dose calculations based on "in phantom" measurements are compared to calculations that are based on in air measurements. Values for the attenuation factor for other beam energies have not been generally available. We have carried out "first scatter' calculations for a rather wide range of energies and spectra. Measurements in 60Co beams and in a high-energy (25 MV) photon beam support the calculations. A set of proposed displacement correction factors is presented.

Cobalt Radioisotopes↗

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On the selection of stopping-power and mass energy-absorption coefficient ratios for high-energy x-ray dosimetry.

A method for the selection of average stopping-power (L/rho)medair and energy-absorption coefficient (mu en/rho)medair ratios has been developed. The quality of the x-ray beam is characterized by the ratio of ionization chamber readings at depths of 20 and 10 cm in water (TMR)2010. For convenience, a relationship is established between experimental (TMR)2010 and the nominal accelerating potential (MV) of the accelerator. Experimental (TMR)2010 are related to (L/rho)medair and (mu en/rho)medair in a three-step process. First, using experimental and theoretical spectra in the range 60Co to 45 MV, (TMR)2010 were calculated for primary and first-scatter photons, and a graph of experimental versus calculated (TMR)2010 for these same spectra was constructed. Second, (L/rho)medair and (mu en/rho)medair were calculated for a large number of primary spectra [for most of which experimental (TMR)2010 were not available] and a graph constructed that related these quantities and (TMR)2010 calculated as above for this group of spectra. Third, using the graphs from the preceding steps, graphs relating the calculated (L/rho)medair and (mu en/rho)medair with experimental (TMR)2010 were constructed. Data are presented for water, polystyrene, acrylic, graphite, A-150, C-552, Bakelite, and nylon for beams with nominal accelerating potentials in the range 2-45 MV.

Humans↗

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Lung dose corrections for 6- and 15-MV x rays.

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

Animals↗

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Comments on "Validity of the concept of separating primary and scatter dose".

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Biophysical Phenomena↗

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The dependence of mass energy absorption coefficient ratios on beam size and depth in a phantom.

The Monte Carlo computer code "electron gamma shower" (EGS) has been used to determine photon spectra in a water phantom. Spectra used by Johns and Cunningham and for the AAPM dosimetry protocol have been used as input data and ratios of average mass energy absorption coefficients have been calculated for a number of depths and field sizes. The results show that there is a slight dependence on both of these parameters. For example, (mu en/P) water graphite for cobalt-60 varies from a value of 1.111 for the primary spectrum in air, to 1.135 at a depth of 20 cm in a phantom for a beam approximately 1 m2 in area. This variation of over 2% is relevant for dosimetry. The variation is less than this for high-energy radiation beams and in most cases can be ignored. The effect is greater for high atomic materials such as bone, where the range of variation of (mu en/P)bone water, again for cobalt radiation, may be as great as 15%. This too is less for high-energy bremsstrahlung spectra.

Cobalt Radioisotopes↗

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The optimization of pencil beam widths for use in an electron pencil beam algorithm.

Pencil beam algorithms for the calculation of electron beam dose distributions have come into widespread use. These algorithms, however, have generally exhibited difficulties in reproducing dose distributions for small field dimensions or, more specifically, for those conditions in which lateral scatter equilibrium does not exist. The work described here has determined that this difficulty can arise from the manner in which the width of the pencil beam is calculated. A unique approach for determining the pencil beam widths required to accurately reproduce small field dose distributions in a homogeneous phantom is described and compared with measurements and the results of other calculations. This method has also been extended to calculate electron beam dose distributions in heterogeneous media and the results of this work are presented. Suggestions for further improvements are discussed.

Algorithms↗

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The validity of the density scaling method in primary electron transport for photon and electron beams.

In the convolution/superposition method of photon beam dose calculations, inhomogeneities are usually handled by using some form of scaling involving the relative electron densities of the inhomogeneities. In this paper the accuracy of density scaling as applied to primary electrons generated in photon interactions is examined. Monte Carlo calculations are compared with density scaling calculations for air and cork slab inhomogeneities. For individual primary photon kernels as well as for photon interactions restricted to a thin layer, the results can differ significantly, by up to 50%, between the two calculations. However, for realistic photon beams where interactions occur throughout the whole irradiated volume, the discrepancies are much less severe. The discrepancies for the kernel calculation are attributed to the scattering characteristics of the electrons and the consequent oversimplified modeling used in the density scaling method. A technique called the kernel integration technique is developed to analyze the general effects of air and cork inhomogeneities. It is shown that the discrepancies become significant only under rather extreme conditions, such as immediately beyond the surface after a large air gap. In electron beams all the primary electrons originate from the surface of the phantom and the errors caused by simple density scaling can be much more significant. Various aspects relating to the accuracy of density scaling for air and cork slab inhomogeneities are discussed.

Computer Simulation↗