Improvements of lateral penumbra at various depth regions in proton pencil beam scanning with a multileaf collimator: Dose verifications and plan comparisons

Purpose In scanned proton therapy, the current consensus is that the effective range of the collimator’s contribution to lateral penumbra improvement is up to approximately 150 mm depth. We characterized the penumbra variations for scanned proton beams with or without a new type of multileaf collimator (MLC) under various air gaps, depth, and with or without range shifter (RS). Methods Eighty-six uniform dose plans were created (38 RS-negative and 48 RS-positive plans) for nine box targets of 60 × 60 × 54 mm³ at 0–280 mm depths in water. They were created with or without MLC, with 50–300 mm air gaps. The penumbra and average doses of MLC-positive and MLC-negative plans at the organs at risk (OAR) region of each box plan were compared. Besides, several plan doses were validated by measurements with penumbra (with an average of 80–20 % dose point widths for both side profiles) differences and 2D gamma analysis. Results The MLC-positive plans reduced the penumbra and mean OAR doses by 1.0–5.1 mm and 3.3–13.5 %, respectively, compared to MLC-negative plans even at >150 mm depths. The penumbra differences in measurements were <±1.5 mm for all plans. The mean gamma scores at 2 %/2 mm were 97.9 ± 2.3 % and 97.4 ± 3.1 % for the MLC-negative and MLC-positive plans, respectively. Conclusions The MLC-positive beams improved the penumbra and reduced the OAR dose in every depth region and air gap. We have shown that PBS with MLCs can be useful at more than 150 mm regions, depending on the machine.