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Dosimetric considerations of symmetric and asymmetric 60 Co teletherapy split fields
Author(s) -
Wrede Don E.,
Givens Stephen B.
Publication year - 1977
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 - 10.1118/1.594330
Subject(s) - dosimetry , ionization chamber , imaging phantom , physics , backscatter (email) , field size , field (mathematics) , electromagnetic shielding , optics , computational physics , ionization , nuclear medicine , mathematics , quantum mechanics , pure mathematics , medicine , ion , telecommunications , computer science , wireless
Split fields are commonly used for a variety of treatments for which local shielding is desired; a single field is split into two fields by means of lead blocks placed in the beam. The condition for assuming independence of the two fields is investigated by using the area over perimeter calculational method and comparing this to experimental depth‐dose data measured in a water phantom using a Shonka ionization chamber. Two principal variations affecting dose distributions are examined: block width and position of the block expressed as the fractional perpendicular distance between the central ray of the overall field and the center of the block. An intercomparison between calculation and experimental data shows that the right and left sides of the split field behave as independent fields giving TAR values in agreement with experimental data to within ±1%. If the fields are asymmetric and if the larger field is less than 70% of the total area including the blocked part, then the dosimetry should be based on average TARs, off‐center ratios, and backscatter factors for the two fields. If the larger area is greater than 70% of the total area, then the treatment time can be calculated from the larger field only.