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Determination of the quality index ( Q ) for photon beams at arbitrary field sizes
Author(s) -
Sauer Otto A.
Publication year - 2009
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 - 10.1118/1.3197062
Subject(s) - tomotherapy , dosimetry , collimator , imaging phantom , field (mathematics) , photon , optics , beam (structure) , laser beam quality , physics , field size , energy (signal processing) , quality (philosophy) , computational physics , mathematics , statistics , nuclear medicine , radiation therapy , quantum mechanics , medicine , laser , pure mathematics , laser beams
Purpose: A commonly used beam quality index ( Q ) for high‐energy photon beams is the tissue phantom ratio ( TPR 20 , 10 ) for a square field of 10 × 10cm 2and SDD of 100 cm . On some specialized radiotherapy treatment equipment such a reference collimator setting is not achievable. Likewise a flat beam profile, not explicitly required in dosimetry protocols, but certainly influences the measurement of Q , is not always produced. In this work, a method was developed in order to determine Q at any field size, especially for small and nonflattened beams. Methods: An analytical relationship was derived between TPR 20 , 10for arbitrary field sizes and Q [theTPR 20 , 10( 10 × 10cm 2 ) ] as quality index. The proposed model equation was fitted to the measured and published data in order to achieve three general fit parameters. The procedure was then tested with published data from TomoTherapy and CyperKnife treatment devices. Results: For standard flattened photon fields, the uncertainty in Q measured at any field size using the parameters derived from this study is better than 1%. In flattening‐filter free beams, the proposed procedure results in a reliable Q for any field size setting. Conclusions: A method is introduced and successfully tested in order to measure the beam quality under nonstandard conditions. It can be used, e.g., to get energy dependent correction factors as tabulated in dosimetry codes of practice even if standard conditions are not adjustable.