Impact of Spot Size and Beam-Shaping Devices on the Treatment Plan Quality for Pencil Beam Scanning Proton Therapy

Purpose This study aimed to assess the clinical impact of spot size and the addition of apertures and range compensators on the treatment quality of pencil beam scanning (PBS) proton therapy and to define when PBS could improve on passive scattering proton therapy (PSPT). Methods and Materials The patient cohort included 14 pediatric patients treated with PSPT. Six PBS plans were created and optimized for each patient using 3 spot sizes (∼12-, 5.4-, and 2.5-mm median sigma at isocenter for 90- to 230-MeV range) and adding apertures and compensators to plans with the 2 larger spots. Conformity and homogeneity indices, dose-volume histogram parameters, equivalent uniform dose (EUD), normal tissue complication probability (NTCP), and integral dose were quantified and compared with the respective PSPT plans. Results The results clearly indicated that PBS with the largest spots does not necessarily offer a dosimetric or clinical advantage over PSPT. With comparable target coverage, the mean dose (D mean ) to healthy organs was on average 6.3% larger than PSPT when using this spot size. However, adding apertures to plans with large spots improved the treatment quality by decreasing the average D mean and EUD by up to 8.6% and 3.2% of the prescribed dose, respectively. Decreasing the spot size further improved all plans, lowering the average D mean and EUD by up to 11.6% and 10.9% compared with PSPT, respectively, and eliminated the need for beam-shaping devices. The NTCP decreased with spot size and addition of apertures, with maximum reduction of 5.4% relative to PSPT. Conclusions The added benefit of using PBS strongly depends on the delivery configurations. Facilities limited to large spot sizes (>∼8 mm median sigma at isocenter) are recommended to use apertures to reduce treatment-related toxicities, at least for complex and/or small tumors.