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Dosimetric effect of collimating jaws for small multileaf collimated fields
Author(s) -
Chow James C. L.,
Seguin Melanie,
Alexander Andrew
Publication year - 2005
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 - 10.1118/1.1861413
Subject(s) - collimated light , dosimetry , multileaf collimator , optics , medical physics , physics , nuclear medicine , linear particle accelerator , medicine , beam (structure) , laser
The dosimetric effects from the jaw positioned close to the small field ( 0.5 × 0.5 , 1 × 1 , and 2 × 2cm 2 ) side‐edge generated by a single‐focused multileaf collimator (MLC) were measured and studied. The measurement is important in intensity modulated radiotherapy (IMRT) because generally the jaw cannot perfectly cover all the leaf‐ends in a segment of irregular field. This leads to additional dose contributed by (1) the end surface of the jaw, (2) the leaf‐end, and (3) the inter‐ and intraleaf leakage/transmissions during the dosimetric measurement. Moreover, most of the conventional treatment planning systems ignore these effects in the dose calculation. In this study, measurements were made using a Varian 21 EX linear accelerator with 6 MV photon beam through a MLC containing 120 leaves. Percentage depth dose, beam profile, and output for small fields were measured by varying the jaw at different positions away from the leaf‐ends in the field side‐edge. Moving the jaw away from the leaf‐ends increases the output and penumbra width for the small fields. Such increase is particularly significant when the field size is small ( 0.5 × 0.5cm 2 ) and the degree of increase changes quickly when the jaw‐end is at about 1–2 cm from the leaf‐end. It is suggested that measurements should be carried out in the IMRT commissioning to provide information to physicists in reviewing the treatment planning system's accuracy regarding leaf leakage/transmission and jaw effects.