Computer generated isodose curves for high energy x-ray machines. Experimental verification and time-saving...

Calculation of isodose patterns for high energy therapy units using Cunningham's BEAM program requires accurate information on the beam profile in air. This may be obtained by scanning across the x-ray beam using a very small silicon diode detector. Data obtained on a particular unit may not be applicable to any other unit. Computer calculations of isodose curves for high energy x-ray machines must use a 0.25 cm. or smaller lateral grid of dose calculation points to ensure accurate reproduction of the sharp fall-off of the dose at the beam edge. Computer calculation time to obtain the isodose curves using this size lateral grid is prohibitively long. The computing time can be appreciably reduced by: (1) changing the width of the annular sectors used in calculating the contribution of scattered radiation to the dose to 0.5 cm.; and (2) calculating the dose at 0.25 cm. lateral intervals within 1 cm. of the beam edge, but changing the spacing of the grid to 1 cm. inside the field. These programming strategies do not produce any significant change in the isodose curves but lead to a significant reduction in computer time.