Development of Compton lens design for increased dose rate in linear accelerator based SRS

To develop a fundamentally new stereotactic radiosurgery (SRS) collimator design which utilizes initially off-axis Compton scattered photons to increase the dose rate at isocenter for small field treatments.

The proposed design added a set of conical slits to a standard cylindrical collimator to allow for scattered photons within the collimator to still contribute to the overall target dose. The design optimization was broken down into two regions: a solid interaction plate and a Compton slit region. The interaction plate geometry was developed to facilitate Compton scattering towards the target, and the Compton slit geometry was optimized to allow for Compton scattered photons to travel unattenuated towards the target. A series of sensitivity studies were performed using Monte Carlo N-Particle (MCNP6) Transport Code to optimize the geometry of the collimator focusing on the material, thickness, cone size, number of slits and slit width.

An optimized collimator design incorporating 6 slits for a 4 mm target allowed for an increase in the dose rate of 3.5% while limiting off axis increases between 1 and 5 cm to an average of less than 1% relative to standard collimator designs.

Preliminary designs present a proof of concept and suggest the potential for increases in dose rate for linac-based SRS systems. These designs have been able to achieve increases while maintaining a relatively low dose rate outside of the target. Further exploration into non-linear optimization of the slits and interaction plate geometry may lead to further increases than presently demonstrated.This concept warrants further study with actual measurement and to be tested for its practicality in clinical use.