Analytical Formula for the total scatter factor of the Small Radiation Field Size

Background & Aims : We present an analytical formula (AF) to calculate the total scatter factor of the MLC-shaped small radiation beams. Materials & Methods : Pinpoint, Semiflex, and Farmer ion chambers were used for MLC/Jaw-shaped fields extending from (10x10 cm 2 ) down to (1x1 cm 2 ). The total scatters factor, beam profiles, and penumbra was investigated for MLC/Jaw-shaped beams. Results : We found that the total scatters factor and the penumbra was clearly higher for MLC-shaped beams due to the MLC leakage, which becomes more and more significant for the small fields. The Analytical Formula was introduced as a least-square fit equation and verified against to measure data. The maximum deviation at small field size (< 4 cm 2 ) was within 3.6% and 43.9% from both MLC and Jaw-shaped beams, respectively. Conclusion : Our analytical formula shows a very good agreement with measured data at a large field size (> 4 cm 2 ), and had small deviation with MLC-shaped small field size, emphasizing that our analytical formula is valid and reasonably good for the total scatters factor calculation of the MLC-shaped beams, especially the ultra-small field size.