Output factors for squared, rectangular and elongated photon fi elds of linear accelerators : A supplement to Stirling’ formula.
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Date
2010-04-18
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Journal of Thai Society of Therapeutic Radiology and Oncology - วารสารมะเร็งวิวัฒน์
Abstract
For dose calculation in radiation therapy with photons a variety of basic beam data are required,such as percentage depth doses, dose profi les, and output factors (OF) for open as well as forwedged beams. In contrast to profi le measurements, output factors have to be measured for all squared,rectangular and also for extremely elongated fi eld sizes. To realize smooth output factor functions, atleast 100 different fi eld sizes with small increments in length and width have to be measured.A simple modification of the Stirling’ formula, taking into consideration the collimator exchangeeffect, allows an accurate prediction of the output factors for all fi eld sizes, based on the experimentaldata survey for the minimum, the maximum and the reference fi eld size (10 x 10 cm2) only. The ratio ofthe calculated OF and the measured one’s stays within 1.0025 ± 0.009, even for elongated and wedgedfi eld sizes. All major vendors of medical linear accelerators have been included in this study.
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Journal of Thai Society of Therapeutic Radiology and Oncology - วารสารมะเร็งวิวัฒน์; Vol. 16 No. 1 January – June 2010; 42 - 49