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Session: Therapy General ePoster Viewing [Return to Session]

Investigating Shot Within Shot Technique with a Rotating Gamma Machine

A Eldib1*, H Liu1, L Chen1, Mahmoud H. Abdelgawad2, M Sherif3, C Ma1, (1) Fox Chase Cancer Center, Philadelphia, PA, USA (2) Physics Department, Faculty of science, Al-Azhar University, Cairo, Egypt ,(3) Kuwait Cancer Control Center, Hawally 32097,KW, Kuwait


PO-GePV-T-422 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Gamma Knife treatments are mostly forward planned using sphere packing techniques. Shot within shot is a method used by some planners, where two shots having different collimator sizes are assigned same stereotactic coordinates but with different beam-on times. Novel rotating gamma machines that provide a fast automated system for changing collimator sizes have been developed with one added feature, i.e., the swinging angle. This could extend the solution space of the planning techniques. The aim of the current work is to investigate the dosimetric benefit of this new planning feature.

Methods: The CybeRay system (OUR United RT Group, Xian, China) was used in the study. It consists of a ring gantry containing 13 Co-60 sources focusing at the isocenter and 7 changeable collimators. The treatment head can also swing 35° in the superior direction. Superimposed shots with various time weightings with and without swinging angles were calculated in 2 cylindrical phantoms of 15cm and 30cm diameters. Dose distributions were calculated using the Prowess treatment planning system (Prowess, Concord, CA). The shot-within-shot technique was also compared to the optimization algorithm in Prowess using real patient CT data.

Results: It was shown that the width of the prescription isodose diameter can be changed at sub-millimeter increments depending on the superimposed cone sizes. For example, a shot delivery of 50% from 2.5cm cone and 50% from 0.6cm cone resulted in a 0.6cm change in diameter in superior-inferior direction. The swinging angle increased the shot diameter for the 2.5cm cone in the superior-inferior direction by 0.4cm. The case study showed that shot within shot can achieve better conformity index compared to the optimizer for some target shapes.

Conclusion: Shot within shot plus a swinging angle can add more abilities in shaping the isodose lines for better target dose conformity.


Not Applicable / None Entered.


TH- External Beam- Photons: gammaknife

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