Room: Exhibit Hall
Purpose: Quantify the variation in magnitude of lateral penumbra for aperture-shaped and non-aperture-shaped proton pencil beam scanning (PBS) fields in homogenous geometry.
Methods: Beam specific target volumes were generated using a commercial treatment planning system (TPS). Aperture and non-aperture PBS fields were optimized on these targets for varied range shifter thicknesses (0cm, 4.0cm, and 7.5cm) and air gap (5-30cm) using Monte Carlo dose calculation. Lateral penumbra (80/20) was measured at 2cm intervals across each target volume to characterize penumbra variation with depth for each range shifter thickness. Next, penumbra was measured at four depths (7cm, 11cm, 15cm and 19cm) for each variation of airgap to characterize the dependence of penumbra on this quantity.
Results: The TPS-calculated penumbra values increase approximately linearly with depth across all combinations of range shifters at constant air gap. At 2 cm depth, penumbra reduction was 2.7mm, 3.7mm and 4.2mm when using range shifter thicknesses of 0cm, 4.0cm and 7.5cm respectively. At a depth of approximately 20cm, the variation in penumbra 80/20 between aperture-shaped and non-aperture-shaped beams is less than 1mm across all three range shifter thicknesses using a constant air gap of 5cm. The average linear slope of penumbra reduction versus depth across all variations of range shifter thicknesses was 0.13 penumbra reduction [mm] per depth increase [cm]. Analysis of the variation of penumbra with airgap shows that reduction is largest at shallow depths and small air gaps.
Conclusion: This work serves to provide clinical guidance on the usefulness of apertures in reducing the penumbra of PBS treatments. Results indicate that the use of apertures for PBS treatments is most beneficial for shallow targets. At approximately 20cm benefits of using apertures becomes negligible as penumbra reduction drops to less than 1mm.
Not Applicable / None Entered.