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A filtration method for improving film dosimetry in photon radiation therapy
Author(s) -
Yeo Inhwan J.,
Wang CK Chris,
Burch Sandra E.
Publication year - 1997
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 - 10.1118/1.598108
Subject(s) - dosimetry , medical physics , filtration (mathematics) , radiation therapy , medical imaging , nuclear medicine , photon , materials science , medicine , optics , physics , radiology , mathematics , statistics
Successful radiotherapy requires accurate dosimetry for treatment verification. Existing dosimeters such as ion chambers, TLD, and diodes have drawbacks such as relatively long measurement time and poor spatial resolution. These disadvantages become serious problems for dynamic‐wedged beams. Thus the clinical use of dynamic wedges requires an improved dosimetry method. X‐ray film may serve this purpose. However, x‐ray film is not clinically accepted as a dosimeter for photon beams, because it overresponds to photons with energies below about 400 keV. This paper presents and develops a method which was initially proposed by Burch to improve the dose response of x‐ray film in a phantom. The method is based on placing high‐atomic number foils next to the film. The foils are used as filters to preferentially remove low‐energy photons. The optimal film and filter configuration in a phantom was determined using a mathematical scheme derived in this study and a Monte Carlo technique (ITS code). The optimal configuration thus determined is as follows: the filter‐to‐film distance of 6 mm and the filter thickness of 0.15 mm for percent depth‐dose measurement; the distance of 1 cm and the thickness of 0.25 mm for off‐axis (dose) ratio measurement. The configuration was then tested with photon beams from a 4 MV linac. The test result indicates that the in‐phantom dose distribution based on the optimal configuration agrees well with those measured by ion chambers.