Evaluations of Penumbra Reduction Using Multi-Leaf Collimator and 3D Bolus in Scanned Proton Therapy

Proton therapy offers excellent distal tissue sparing; however, precise control of the lateral penumbra is critical for protecting adjacent healthy tissues. This study systematically evaluated improvements in lateral penumbra in proton pencil beam scanning (PBS) using a surface-contacted 3-dimensional printed bolus (3DBS), compared with conventional plans employing a nozzle-mounted range shifter (RS), for beams with and without a multi-leaf collimator (MLC).

A total of 460 uniform-dose PBS plans were generated for a cubic target (60 × 60 × 54 mm³) positioned at 9 different depths ranging from 0 to 280 mm in water. Plan configurations included plans with 3 different materials for 3DBSs (silicone, resin, and thermoplastic polyurethane) and were compared with plans with a polyethylene RS. Each configuration was further subdivided into plans with and without MLC. Air gaps ranging from 50 to 300 mm were evaluated. Lateral penumbra widths and mean doses to surrounding organs at risk (OARs) within a 30-mm margin were evaluated. Two-dimensional dose distributions for selected beams were experimentally verified using a 2D ionization chamber array and radiochromic films.

All plans with 3DBSs improved lateral penumbra and reduced mean OAR doses compared with plans with RS, under equivalent MLC conditions, except for a limited subset of plans at depths greater than 200 mm with minimal air gaps. Measured dose distributions demonstrated acceptable agreement with calculated plans. Comparisons among the 3 3DBS materials indicated feasibility in material selection based on required bolus geometry and clinical implementation considerations.

The combined use of MLC and 3DBS provides a systematic and effective strategy for lateral penumbra control in proton PBS. This approach enables improved dosimetric precision in clinically sensitive scenarios where even small reductions in dose to surrounding tissues are critical.