Electron arc therapy: Beam data requirements and treatment planning

In electron are therapy a long narrow beam defined by a secondary collimator is used to sweep across the treatment area of the patient. Central axis percentage depth doses and off-axis ratios measured with such a collimator show variations with the source-to-surface distance (SSD). Errors may be introduced into isodose distributions calculated with the effective SSD method when beam data measured at a fixed SSD are used. A treatment planning algorithm using beam data measured at two SSDs to allow an interpolation and extrapolation procedure to account for SSD variations has been implemented. For calculations in off-axis planes, off-axis ratios normal to the plane of rotation are also required. Results of the calculations were checked with film dosimetry measurements and found to be in good agreement.