PHYSICAL CHARACTERIZATION OF MEGAVOLTAGE THERAPY BEAMS

Megavoltage beams of electrons and bremsstrahlung x-rays from linear accelerators have found increasing use in cancer radiotherapy over the last 20 years. Our knowledge of the physical properties of these beams, however, is still incomplete. Fundamental macroscopic parameters such as beam energy and beam contamination, for example, are in general not accurately known. On the microscopic level, the mechanisms of dose deposition at the cellular level and the Relative Biological Effectiveness (RBE) for these beams is also poorly understood. The proposed projects is intended to continue our work on providing a comprehensive set of data at both the macroscopic and microscopic level of the physical parameters of x-ray and electron therapy beams. A newly designed and completed fast Compton Spectrometer will be used to accurately measure the energy distributions of megavoltage therapy x-ray beams, as well as the photon contamination in electron beams. Design modifications on the linear accelerator have been made which lower the dose rate to levels which enable the spectrometer to be placed directly in the therapy beam. The importance of flattening filters, collimators, field size, and depth in tissue on altering the beam energy distributions will be explored. Such measurements will provide useful information for purposes of determination of absolute doses, as well as optimum target, filter, and collimator design. Experimental results will be complemented by Monte Carlo computer calculations (using the program Electron Gamma Showers) of these same parameters. Concurrent measurements of microdosimetric (i.e., linear energy, and Linear Energy Transfer, or LET) spectra using a Rossi type gas proportional counter will complement these energy measurements. Correlations between the macroscopic and microscopic parameters of the beams will provide information on the mechanisms of energy deposition for therapy beams. These correlations will also be useful in testing the predictions of various theoretical models of energy deposition, and radiobiological effectiveness.