KEY POINTS
- This planning and experimental verification study generated 460 uniform-dose proton PBS plans for a cubic target at nine depths, comparing silicone, resin, and thermoplastic polyurethane surface-contacted 3D boluses with a conventional range shifter, with and without MLC collimation.
- Under equivalent MLC conditions, 3D bolus plans generally produced narrower lateral penumbrae and lower mean surrounding OAR doses than range-shifter plans; exceptions were limited mainly to depths beyond 200 mm with minimal air gaps.
- MLC collimation provided additional penumbra reduction, with the combined MLC and 3D bolus configuration delivering the greatest overall improvement. Silicone generally produced the narrowest penumbra among the tested materials.
- Experimental verification of 45 plans produced mean 2%/2 mm gamma pass rates of 94.7%, 95.4%, and 95.7% for silicone, resin, and thermoplastic polyurethane, respectively. Film-measured penumbra differences were within ±1.5 mm of calculations except for one profile.
CLINICAL TAKEAWAY
Replacing a nozzle-mounted range shifter with a surface-contacted 3D bolus may improve lateral conformity in proton PBS, particularly for shallow targets near critical structures, and MLC collimation may provide further gains. However, this was a phantom-based study using rectangular boluses, so patient-specific geometry, setup reproducibility, manufacturing workflow, cost, and clinical benefit still require evaluation.