Analysis of a manual calculation technique for isocentric dose determination

A method is presented for manual calculation of the dose to points off the central ray for isocentric radiation therapy. A simplified technique for manual calculation is derived from a more general formula used in the computer dosimetry program developed by Sloan-Kettering Memorial Hospital in New York. The manual method assumes certain variables in the general case to be constant. The technique is evaluated by comparing its results to those of the computer program in three hypothetical examples.