Application of 3D-printed compensators for proton pencil beam scanning of shallowly localized pediatric tumors.

Background: In modern proton radiotherapy facilities with pencil beam scanning technology, the lowest energy of a proton beam typically ranges between 60 and 100 MeV, corresponding to a proton range in water of 3.1-7.5 cm. The irradiation of superficial lesions usually requires the application of a range shifter (RS) to further reduce the proton range. A certain distance from the patient to the RS increases the spot size, causing worse plan conformity. As an alternative solution, a patient-specific 3D-printed proton beam compensator (BC) can be applied to reduce the air gap and beam scattering.

Materials and methods: This study is based on treatment planning system simulations using retrospectively selected data from six pediatric patients with diagnosed sarcomas located in the head and neck area. For three of these patients, 3D-printed compensators were utilized during the treatment phase, prior to the retrospective analysis. Treatment plans for children with shallow lesions treated using RSs and BCs were compared. Planning target volume constraints (D_98% >95%, D_2%< 107%) and organs-at-risk (brainstem, spinal cord, visual organs, chiasm, cochlea) constraints (D_2%, D_max and D_Mean) were applied. The entire process of using a BCs in the treatment of pediatric superficial tumors is presented, including 3D printing procedure (via fused filament fabrication method), dosimetric verification of the material (Water Equivalent Ratio measurements) and assessment of its homogeneity, print quality and Hounsfield Unit specification. Beam parameters analysis including spot sizes and penumbras, were performed. Treatment plans were compared in terms of plan conformity and sparing of critical organs.

Results: The application of BCs reduced the low-dose irradiation areas, improved conformity and reduced critical organs exposure. BCs decreased the lateral spot size by approximately 57% and the penumbras by 41-47% at different depths in the cube target. The variation in BC homogeneity was less than 3.5%, meeting the criteria for plan robustness evaluation.

Conclusions: Compared with RS placement at the nozzle, the placement of 3D-printed BCs in the near vicinity of the patient for the treatment of superficial tumors led to a more conformal dose distribution.