title: Gapping fields on sloping surfaces.
abstract: 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.
authors.affiliation: Radiation Oncology Center, St. Louis, MO 63110.
authors.forename: R
authors.initials: R
authors.lastname: Keys
authors.forename: P W
authors.initials: PW
authors.lastname: Grigsby
mesh_terms: D006689:Hodgkin Disease / Q000532:radiotherapy; D006801:Humans; D011878:Radiotherapy / Q000379:methods*; D013679:Technology, Radiologic