Gapping fields on sloping surfaces

Richard Keys, Perry W. Grigsby

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


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.

Original languageEnglish
Pages (from-to)1183-1190
Number of pages8
JournalInternational journal of radiation oncology, biology, physics
Issue number5
StatePublished - May 1990


  • Gapping fields
  • Hodgkin's disease
  • Mantle field


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