Dosimetric effect of collimating jaws for small multileaf collimated fields.
The dosimetric effects from the jaw positioned close to the small field (0.5 x 0.5, 1 x 1, and 2 x 2 cm2) 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 x 0.5 cm2) 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.
Chow
James C L
JC
Medical Physics Department, Grand River Regional Cancer Center, Grand River Hospital, PO. Box 9056, 835 King Street West, Kitchener, Ontario N2G 1G3, Canada. james.chow@grhosp.on.ca
Seguin
Melanie
M
Alexander
Andrew
A
D004868:Equipment Failure; D019544:Equipment Failure Analysis / Q000379:methods*; D011785:Quality Assurance, Health Care / Q000379:methods*; D011835:Radiation Protection / Q000379:methods*; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods*; D018570:Risk Assessment / Q000379:methods*; D012307:Risk Factors; D012542:Scattering, Radiation