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Applying a polynomial formula to photon beam output and equivalent square field
Author(s) -
Chen FanShih
Publication year - 1990
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 - 10.1118/1.596485
Subject(s) - square (algebra) , collimator , physics , photon , linear particle accelerator , field (mathematics) , beam (structure) , polynomial , dosimetry , mathematics , monitor unit , mathematical analysis , quantum mechanics , optics , geometry , medicine , pure mathematics , nuclear medicine , radiology
The polynomial formula proposed by Chen [Med. Phys. 15 , 348 (1988)] in calculating the electron beam output from a Therac 20 linear accelerator has been applied to generating the output factor of various machines with photon energies ranging from 100 kVp to 18 MeV. The calculated outputs are within 1% of the measured values. This formula can be very useful to the physicist in preparing an output table of photon beams or electron beams for a therapeutic unit. An equation is derived from this formula to calculate the equivalent square. The derivation shows that only under special circumstances is the equivalent square field equivalent to 2 a b /( a + b ); otherwise the equivalent square field depends on the formula's parameters as well as the sides of the rectangular field. These parameters, in turn, are dependent on the photon energy, the medium irradiated, and the collimator design.