Effect of Focal Spot Size on the Beam Output between Circular and Square Small Fields
Abstract
Purpose
The source size (focal spot) of a photon beam from linacs is known to have effects on the output for small-field dosimetry. This study aims to determine quantitatively the sensitivity of beam field output factors (FOF) as a function of field sizes between circular and square fields.
Methods
Because experimental measurements could not separate effect between source occlusion and lack of lateral scatters on FOF for extremely small fields, we used Monte Carlo simulations to avoid experimental field size setup uncertainties and investigated FOF of circular fields defined by cones and square fields defined by jaws ranging from 4 mm to 30 mm dimensions for 6 MV-FFF, 6MV-SRS, 6 MV, and 10 MV-FFF beams.
Results
The results showed that FOF depends on focal spot size. For a small difference in FWHM of focal spot size of Gaussian distribution between 1.2 mm from Trilogy/TX and 1.7 mm from TrueBeam led to greater than 10% difference in FOF for a 5×5 mm2 jaw opening between Trilogy/TX and TrueBeam 6 MV beams, respectively. However, these focal spot differences only led to less than 4% difference in FOF for 5 mm cone field between Trilogy/TX and TrueBeam beams, respectively. The reason is because jaws are closer to the radiation source than cones, the effect of source occlusion becomes more prevalent for jaw defined fields for very small fields.
Conclusion
This study simulated an “ideal experiment” free of machine geometric uncertainties and measurement setup uncertainties to quantitatively determine effect of focal spot size on FOF between circular and square field sizes. It is found that the focal spot size has a non-trivial effect on the FOF for jaw defined square fields and an insignificant effect on cone fields due to the source occlusion and lack of it, respectively.