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Unequal weighting of given doses in opposed fields in treatment of cancer of the tonsillar region using 60 Co, 4‐, 8‐, 15‐, 24‐MVp photons
Author(s) -
Wollin M.,
Gilbert H. A.,
Kagan A. R.
Publication year - 1976
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.594276
Subject(s) - photon , weighting , physics , nuclear medicine , dosimetry , medicine , optics , medical physics , radiology
Weighting of given doses from parallel opposed fields is a widespread practice in radiation therapy of cancer in the tonsillar region. In order to determine when weighting is useful, dose distributions on the central axis were calculated for various energies ( 60 Co, 4‐, 8‐, 15‐, and 24‐MVp photons), weighting factors, and field separations. Criteria for judging the usefulness of weighting were (i) that variation in the dose across the tumor volume not exceed 10%, and (ii) that the dose to critical tissues outside the target volume be at least 20% less than target volume dose. The clinical situation that met these criteria was a lateral lesion that extended to less than one‐third of the transverse diameter of the head, treated with either 60 Co, 8, or 4 MVp with 4:1 weighting. Under these conditions, weighting was considered to be justified.