WE-D-17A-01: A Dynamic Collimation System for Spot Scanned Proton Therapy: Conceptual Overview

Authors


Abstract

Purpose:

In the absence of a collimation system, the lateral penumbra in pencil beam scanning (PBS) proton therapy delivered at low energies is highly dependent on the spot size. This dependence, coupled with the fact that spot sizes increase with decreasing energy, reduces the benefit of the PBS technique for treating shallow tumors such as those found in the head and neck region. In order to overcome this limitation, a dynamic collimation system (DCS) was developed for sharpening the lateral penumbra of low energy proton therapy dose distributions delivered by PBS.

Methods:

The proposed DCS consists of two pairs of orthogonal trimmer blades which intercept the edges of the proton beam near the target edge in the beam's eye view. Each trimmer blade is capable of rapid motion in the direction perpendicular to the central beam axis by means of a linear motor, with maximum velocity and acceleration of 2.5 m/s and 19.6 m/s2, respectively. Two-dimensional treatment plans were created both with and without the DCS for in-air spot sizes (σ_air) of 3, 5, 7, and 9 mm, representing a wide array of clinically available equipment.

Results:

In its current configuration, the snout of the DCS has outer dimensions of 22.6 × 22.6 cm2 and is capable of delivering a minimum treatment field size of 15 × 15 cm2. Using off the shelf components, the constructed system would weigh less than 20 kg. The treatment plans created with the DCS yielded a reduction in the mean dose to normal tissue surrounding the target of 26.2–40.6% for spot sizes of 3–9 mm, respectively.

Conclusion:

The DCS can be integrated with current or future proton therapy equipment and we believe it will serve as a useful tool to further improve the next generation of proton therapy delivery.

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