Cobalt beams have exhibited superior dosimetric advantages for stereotactic radiosurgery/radiation therapy (SRS/SRT) of intracranial tumors compared to higher energy megavoltage x-ray beams due to the noncoplanar multiple source arrangement and the rapid dose falloff as a result of lower secondary electron energies. This work investigates the potential clinical benefits of cobalt beams for stereotactic body radiation therapy (SBRT) of lung cancer by Monte Carlo simulations of a novel SRS/SBRT machine design with superior dose conformity/gradient and accurate stereotaxy. A new commercially available SRS/SBRT system consists of a ring gantry with 2 cobalt treatment heads: 1 is a focusing head with 16 gamma sources and the other is a diverging gamma source equipped with an adjustable primary collimator and a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is either 40 cm x 40 cm (40 pairs of 0.5-cm central leaves, 20 pairs of 1-cm outer leaves). The treatment heads can swing 35° superiorly and 8° inferiorly, allowing a total of 43° noncoplanar beam incident. The treatment couch provides 6-degrees-of-freedom motion compensation, and the kV cone beam CT (CBCT) system has a spatial resolution of 0.4 mm. Monte Carlo simulations were performed to obtain dose distributions and compare with measurements. A retrospective study of 79 previously treated patients was performed to compare cobalt beams with 6-MV x-ray beams on robotic radiosurgery and linear accelerator SBRT systems for thoracic treatments. Monte Carlo results confirmed the new cobalt system design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6-MV photon beams with variable primary collimator sizes that also resulted in adjustable dose rates required for treatment optimization and plan delivery. The new cobalt system had a 0.3-mm isocenter accuracy. The low-dose acquisition mode of the CBCT system provided fluoroscopy and 3D imaging at a dose level 3 times less than conventional CBCT systems. Since cobalt beams produced lower energy secondary electrons, the new cobalt SRS/SBRT system exhibited better dose properties in low-density lung tissues. Because of their rapid depth dose falloff, cobalt beams were favorable for peripheral lung tumors with partial-arc deliveries to spare the opposite lung and critical structures. The skin dose from cobalt beams was only 2% to 3% higher than that from 6-MV x-ray beams. Superior treatment plans were obtained using the focusing cobalt treatment head for small (up to 50 mL) tumor volumes and using the cobalt MLC treatment head for large (>30 mL) tumor volumes for more than 70% of the patients investigated compared to high-energy x-ray beams. The unique dosimetric properties of cobalt beams and the accurate stereotaxy/dose delivery make the new cobalt design an ideal system for advanced SRS/SBRT as well as conventional RT of lung cancers.
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