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

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
cm2) down to (1x1 cm2). 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 cm2) 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 cm2), 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.