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An empirical relationship for determining photon beam quality in TG‐21 from a ratio of percent depth doses
Author(s) -
Followill David S.,
Tailor Ramesh C.,
Tello Victor M.,
Hanson William F.
Publication year - 1998
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 - 10.1118/1.598396
Subject(s) - dosimetry , calibration , photon , beam (structure) , laser beam quality , optics , photon energy , computational physics , physics , quality (philosophy) , nuclear medicine , mathematics , statistics , laser beams , medicine , laser , quantum mechanics
A key component of the Radiological Physics Center's (RPC) on‐site dosimetry review visits are photon beam calibrations for which determination of the energy of the x rays is a key element. The ratio of ionizations, TPR 20/ TPR 10 , for a 10 cm × 10 cm field at depths of 20 and 10 cm for a constant SCD is used as a quantitative measure of beam quality in the Task Group 21 protocol. The RPC has measured both TPR 20/ TPR 10and the corresponding ratio of percent depth dose( D 20/ D 10 ) at a constant SSD for 685 photon beams (4–25 MV) for most makes and models of accelerators. A strong correlation between TPR 20/ TPR 10and D 20/ D 10is presented which allows the determination of the TPR ratio from the measurement of the ratio of percent depth doses. An analysis of the uncertainty introduced in the TG‐21 factors ( L ̄ / ρ ,P wall ,P repl ) caused by the spread in the measured data and translated into the determination of the TPR ratio results in an insignificant error ( < 0.3 % ) . This empirical relationship provides an alternate technique for quantifying the beam quality defined in the TG‐21 protocol without surrendering any loss of precision in output calibration. This technique may be found by those who calibrate at a fixed SSD to be an easier and quicker method.