Dose calculation in static or dynamic off-axis fields.

X-ray fields that are shifted away from the conventional central axis exhibit alterations in output, beam profile, and beam energy. An extension of a conventional dose calculation algorithm to correct for these perturbations is presented. This algorithm employs the product of three functions describing the dose at the center of the field at dmax, the relative off-center dose at dmax, and the relative dose at depth along a fanline passing through the point of interest. The nature of the problem is characterized, a basic data set necessary to support the algorithm is described and an abridged set of data that may assist in the development of an independent collimator dose calculation capability is presented. The appropriateness of this technique for both conventional and dynamically collimated fields is illustrated.

Analysis of tissue-maximum ratio/scatter-maximum ratio model relative to the prediction of tissue-maximum ratio in asymmetrically collimated fields.

The tissue-maximum ratio/scatter-maximum ratio model has been examined as an empirical model capable of predicting the variation in tissue-maximum ratio observed in fields shifted off-axis by independent collimators. This model has been assessed for its sensitivity to the necessary extrapolation of percentage depth dose data and phantom-scatter factors to zero field size and to large field sizes. This model was found to yield good results for both 6- and 24-MV x rays and to be relatively insensitive to the assumptions made regarding unmeasurable beam parameters.

Beam profile generator for asymmetric fields.

A boundary factor technique for predicting beam profiles has been described by Chui and Mohan [Med. Phys. 13, 409 (1986)]. Boundary factors are calculated as the ratio of the intensity measured in the central plane of a collimated field to the intensity measured in the same location in a 40 X 40 cm field. However, significant discrepancies arise if these factors are applied to the 40 X 40 cm intensity function at off-axis points to predict the beam profile for independently collimated fields. These discrepancies are primarily due to the assumption that the 40 X 40 cm profile approximates an unperturbed intensity function that would exist in the absence of the collimators or scattering media. Two techniques to reduce the residual perturbation are presented. The resulting refined boundary factors allow further factoring so as to permit the calculation of beam profiles for any field size and offset from three discrete functions.

Interaction of hyperthermia and radiation: radiation quality.

Cell-survival data were collected to determine the survival response of asynchronous CHO cells subjects to radiation and hyperthermia. The irradiation was at room temperature 100 minutes before exposure to hyperthermia at 42 degrees C. The survival response to the combination of these two agents is expressed by means of a survival surface, a three-dimensional concept relating cell survival to heat dose and radiation dose. The survival surface could be approximately described by a survival model comprising three components of cell killing: the unperturbed radiation component, the unperturbed hyperthermia component, and the interaction component. The dependence of the radiation component and the interaction component on radiation quality were investigated by irradiating with either 60Co gamma rays, 250 kV X rays or 14.7 MeV neutrons. An analysis suggests that the interaction component and the radiation component exhibit similar dependencies on radiation quality both for the deposition of damage and the repair or accumulation of that damage.

Interaction of hyperthermia and radiation: the survival surface.

The interaction of hyperthermia and radiation on cell survival is illustrated by the increase in sensitivity to radiation caused by hyperthermia and the increase in sensitivity to hyperthermia caused by radiation. A method of expressing the total response to a combination of these two agents by means of a survival surface in three dimensions is explained and discussed. A simple mathematical model of the suface is formulated in which is a first order interaction term is separated from the two terms representing unperturbed radiation survival and unperturbed hyperthermia survival. An experiment is described in which survival data points were obtained for 77 combinations of radiation and hypertermia doses in the form of a matrix. These data points are plotted as 11 radiation survival curves and seven hyperthermia survival curves. The results are also analysed to give the parameters of the survival surface model. The model with its three components is shown to be a good approximation to the experimental results.

Interaction of hyperthermia and radiation: temperature coefficient of interaction.

Cell survival data were collected to determine the survival surface for asynchronous CHO cells X-irradiated at room temperature 100 min prior to a hyperthermic exposure. The survival surface could be approximately described by a survival model comprising three components of cell killing; the unperturbed radiation component, the unperturbed hyperthermia component and the interaction component. The temperature sensitivities of the hyperthermia component and of the interaction component were investigated by using different hyperthermic temperatures, revealing that the interacting component is significantly less sensitive to a change in temperature than the unperturbed hyperthermia component.