Enhancing proton therapy for superficial lesions using patient-specific bolus and multi-leaf collimator.

In scanning proton therapy for superficial tumors, offset range shifters (ORS) may degrade the lateral penumbra as the air gap increases. This study evaluated the dosimetric benefit of replacing ORS with 3D-printed patient-specific boluses (PSBs) and assessed the effectiveness of combining PSBs with multi-leaf collimators (MLCs). Head and breast phantoms were CT-scanned, and 4-cm-thick PSBs made of thermoplastic polyurethane (TPU) and silicone were fabricated. PSBs were modeled in the treatment planning system using measured water-equivalent thickness (WET). Virtual planning target volumes were defined in superficial layers. Treatment plans were generated with ORS or PSB, with and without MLC, at air gaps of 5–20 cm. Dose conformity was assessed using the conformity index at the 50% isodose level (CI50%) and penumbra width (distance from 80% to 20% dose). To validate dose calculations with PSBs, measured lateral dose distributions were compared with calculations using gamma analysis (3%/2 mm). With ORS, CI50% was 2.45 ± 0.08/2.57 ± 0.05 and penumbra was 15.6 ± 1.2 mm/15.1 ± 1.38 mm for head/breast cases, showing inferior conformity. PSBs achieved higher conformity regardless of air gap. When combined with MLCs, PSBs provided the best results (CI50% = 1.93 ± 0.03/2.04 ± 0.02; penumbra = 3.7 ± 0.4 mm/4.1 ± 0.1 mm). Gamma pass rates exceeded 95% in all cases, confirming agreement between measurement and calculation. PSBs improved dose conformity compared with ORSs in superficial proton therapy, and their combination with MLCs further enhanced treatment quality.