Dosimetric Evaluation of a Gamma Ray System for Advanced Radiation Therapy Treatments

Abstract

Gamma ray systems have exhibited superior dosimetric advantages for stereotactic radiosurgery/radiotherapy (SRS/SRT) of intracranial tumors due to the use of non-coplanar multiple sources and the rapid dose fall-off outside the treatment target as a result of lower secondary electron energies. In this work we performed a dosimetric planning comparison study on a novel Gamma ray system to evaluate its potential clinical benefits for SRS of intracranial tumors and stereotactic body radiotherapy (SBRT) of other body sites. The new Gamma ray system (CybeRT, OUR United RT Group, China) consists of a ring gantry with two treatment heads: a focusing head with 16 cobalt-60 sources and an adjustable (1-2cm) cobalt-60 source equipped with a multileaf collimator (MLC). The MLC has 60 paired leaves, and the maximum field size is 40cmx40cm (40 pairs of 0.5cm central leaves, 20 pairs of 1cm outer leaves). The treatment heads can provide a total of 35° non-coplanar beam incidence. 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 obtain dose distributions and compare with measurements. A retrospective study of 27 previously treated patients was performed using the Prowess RT Pro (a prototype version), Accuray MultiPlan and Varian Eclipse TPS to compare cobalt plans with 6 MV x-ray plans on the CybeRT, CyberKnife and linac systems. Monte Carlo results confirmed the new Gamma ray system design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient was similar to or better than that of 6MV photon beams with adjustable source sizes that also resulted in variable dose rates required for treatment optimization and VMAT delivery. CybeRT had a 0.3mm 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 Gamma ray 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 with CybeRT for head and neck treatments was similar to that with CyberKnife and conventional linacs and only 2-3% higher for abdominal and pelvic treatments. Superior treatment plans were obtained using the focusing Gamma treatment head for small (up to 50cc) tumor volumes and using the Gamma MLC treatment head for large (> 30cc) tumor volumes for intracranial, head and neck, breast, lung, liver and prostate patients investigated in this study. The unique dosimetric properties of cobalt beams and the accurate stereotaxy/dose delivery techniques make the new Gamma ray system an ideal RT system for advanced SRS/SBRT as well as conventional radiation therapy.