The geometry of avoiding beam intersection in oblique field treatment technique with mesh frames

In the radiotherapy of cancer of oesophagus and cancer of bladder, the necessity of three field arrangement is very common. If the patient is positioned in supine position, then during posterior oblique field treatments the mesh frame of the table top in telecobalt machines such as Th-780, Th-780 C and PHOENIX, intercepts the field. As this is only a geometrical situation, whether the mesh frame intersects the beam or not is given in the form of a mathematical condition. In case the aluminium frame intercepts the radiation field then an alternative arrangement has to be employed. (author). 2 refs., 4 figs