CT assisted computation of collimator angle and importance of off-axis planning of inclined lesions.

In the radiation treatment of inclined lesions the longitudinal axis of beam should be always parallel to the longitudinal axis of lesion in the treatment plane. Hence the choice of correct collimator angle is of prime importance in the oblique field off-axis treatment planning of inclined lesions. In this paper a graphical method is discussed to determine the collimator angles from the CT scans taken at different off-axis planes. A phantom was constructed which can simulate conditions like inclined lesion in the patient and CT scans were taken at different levels. The collimator angles estimated from CT scans are found to match with the collimator angles obtained from simulator within +/- 1 degree for various gantry angles. The method is applied to plan a case of cancer of the oesophagus (upper third), which is a typical example of inclined lesion.