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Dosimetry for asymmetric x‐ray fields
Author(s) -
Kwa William,
Kornelsen Richard O.,
Harrison Richard W.,
ElKhatib Ellen
Publication year - 1994
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 - 10.1118/1.597260
Subject(s) - collimator , dosimetry , field (mathematics) , field size , linear particle accelerator , monitor unit , physics , optics , dimension (graph theory) , square (algebra) , beam (structure) , mathematics , geometry , nuclear medicine , medicine , pure mathematics
Conventional linear accelerators have four field‐defining jaws or collimators. Usually, one set of the two opposing jaws moves concurrently to define the field width and the other set defines the field length. The resultant square or rectangular field will have the field centerline coincide with the collimator axis. However, some modern linacs have independent collimators or jaws that can be set asymmetrically. In this case, one of the two opposing jaws can be closed down independently of the other one to define an asymmetric field of smaller dimension. The field center now does not coincide with the collimator axis. Asymmetric collimators have found many clinical applications, but have complicated the dosimetry for physicists. Data acquisition and treatment planning implementations are tedious and complicated. An algorithm has been developed to correct for the reduced dose in the smaller asymmetric field. The approach used is similar in principle to the Day's equivalent field calculation. The difference in dose between an asymmetric and a symmetric radiation field is accounted for by a correction factor that is a function of the asymmetric and symmetric field sizes, off axis distance, and depth of measurement. The correction method presented here applies only to the closing down of one independent jaw. Beam profiles for asymmetric fields are measured for both the 6 and 10 MV photon beams. For a 6 MV photon beam, the calculated doses at the asymmetric field center at depths of 2, 10, and 25 cm are within 0.2% of the measured doses for a 20×20 cm 2 field blocked to a 5×20, 10×20, and 15×20 cm 2 asymmetric field. The agreement between the calculated and measured doses for the 10 MV photon beam is, on average, within 0.5%. The calculated beam profiles for asymmetric fields are also found to be in good agreement with measurements for both the 6 and 10 MV photon beams. The calculation method has been implemented into an in‐house developed treatment planning system. Examples of isodose distributions for the 6 MV asymmetric fields show good agreement with the measured data.