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Dosimetric Investigation of a Rotating Gamma Ray System for Intra- and Extra-Cranial SRS/SRT

C Ma

C Ma1*, A Eldib1 , O Chibani1 , G Mora2 , J Li3 , L Chen1 , (1) Fox Chase Cancer Center, Philadelphia, PA, (2) University of Lisbon, Lisbon, Portugal, (3) OUR United RT Group, Xian, China


SU-F-205-4 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 205

Purpose: Cobalt beams have exhibited superior dosimetric advantages for stereotactic radiosurgery/radiotherapy (SRS/SRT) of intra- and extracranial tumors compared to higher energy x-rays due to the sharper beam penumbra and the use of non-coplanar multiple source arrangement. This work investigates the potential clinical benefits of a novel rotating Gamma ray machine for SRS/SRT.

Methods: The new SRS/SRT system (CybeRay, OUR United RT Group, Xian, China) consists of a ring gantry and a focusing treatment head with 16 Φ6.22mm cobalt-60 sources. Each source has 7 collimators of 6, 9, 12, 16, 20, 25 and 35mm diameter. The treatment head can swing 35° superiorly, allowing a total of 43° non-coplanar beam incident. The treatment couch provides 6-degrees-of-freedom motion compensation and the kV cone-beam CT system has a spatial resolution of 0.4mm. Monte Carlo simulations were performed to compute plan dose distributions and to compare with 12 previously treated CyberKnife plans.

Results: The CybeRay system had a 0.3mm isocenter accuracy. The low-dose acquisition mode of the CBCT system provided fluoroscopy and 3D imaging at a dose level of <1cGy. The maximum dose rate was >3Gy/min at the center of a 16cm diameter PMMA spherical phantom. The output factor varied from 1 to 0.739 for intracranial treatment using Φ20-Φ6 collimators, and from 1 to 0.698 for extracranial SRS/SRT using all 7 collimators. The beam penumbra (20%/80%) was 3.3mm and 4.5mm for the Φ6 and Φ35 collimators. Superior treatment plans were obtained with CybeRay for intracranial SRS/SRT with much reduced near target brain doses. CybeRay also produced favorable dose distributions for peripheral lung tumors using a partial-arc technique to spare the opposite lung and critical structures.

Conclusion: The unique dosimetric properties of cobalt beams and the accurate stereotaxy/dose delivery make the new cobalt design an ideal system for advanced SRS/SRT of intra- and extracranial targets.

Funding Support, Disclosures, and Conflict of Interest: This study was supported in part by OUR United RT Group, Xian, China.

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