Gapping fields on sloping surfaces

The accurate positioning of treatment fields has always been difficult, and becomes even more important when two fields are adjacent and a gap must be calculated. The standard formula (based on similar triangles) assumes a flat surface. In Hodgkin's patients the gapping between the mantle and para-aortic fields involve sloping surfaces and the patient thickness can vary as much as 5 or 6 cm between central axis and the lower edge of the mantle field. Due to the divergence of a large mantle field, using the standard formula and gapping on the skin surface can lead to an overlap of 7 to 9 mm and a region which can receive 140% of the midplane dose. A modification of the existing formula which takes into account sloping surfaces has been formulated for both source-skin-distance and isocentric treatments. In addition, problems in positioning related to mantle fields when treating the patient prone and supine have been investigated.