Impact of spot size and beam-shaping devices on the treatment plan quality for pencil beam scanning proton therapy

Publication date: Available online 29 December 2015
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Maryam Moteabbed, Torunn I. Yock, Nicolas Depauw, Thomas M. Madden, Hanne M. Kooy, Harald Paganetti
PurposeThis study aims 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 upon passive scattering proton therapy (PSPT).Methods and materialsThe patient cohort included fourteen pediatric patients treated with PSPT. Six PBS plans were created and optimized for each patient using three spot sizes (∼12, 5.4, and 2.5 mm median sigma at isocenter for 90-230 MeV range), and adding apertures and compensators to plans with the two 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.ResultsThe 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 (Dmean) 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 Dmean 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 Dmean and EUD by up to 11.6% and 10.9% compared to 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.ConclusionsThe 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 in order to reduce the treatment-related toxicities, at least for complex and/or small tumors.

Teaser

The clinical impact of spot size and additional application of beam-shaping devices (apertures and range compensators) for pencil beam scanning proton therapy was investigated. No clinical advantage over passive scattering was realized when spot size was relatively large (∼12 mm median sigma at the isocenter). Plans superior to passive scattering were achieved when decreasing the spot size and/or adding beam-shaping devices to larger spots.


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