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Errors in off‐axis treatment planning for a 4‐MeV machine
Author(s) -
Lam WingChee,
Lam KamShing
Publication year - 1983
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 - 10.1118/1.595317
Subject(s) - beam (structure) , perpendicular , computation , separable space , physics , monitor unit , symmetry (geometry) , field (mathematics) , radiation treatment planning , dosimetry , optics , mathematics , computational physics , geometry , mathematical analysis , nuclear medicine , algorithm , medicine , pure mathematics , radiation therapy
Computer treatment planning systems allow dose computation in planes parallel to the central one (off‐axis plans). The beam data may consist of, e.g., percentage depth doses along the central axis plus off‐axis ratios (OAR) at several depths. In some systems, the calculation of an off‐axis plan is based on the assumption that the OAR can be represented by a separable function: OAR( x , y )= f ( x )⋅ g ( y ), where x and y are the symmetry axes perpendicular to the beam axis and the functions f and g are equal for a square open field. The errors of this assumption for a 4‐MeV machine were measured for open fields and wedged fields at five different depths. The measured dose was compared with that predicted by the above equation for points at 50%, 75%, and 88% of the half field width from the beam axis. Maximum deviation of more than 10% was observed with the probable error of the measurement being 1%.