Novel Operating methods of operation for the Rotating Gamma System for safer and more effective stereotactic Radiosurgery near critical organs at risk.

Stereotactic radiosurgery (SRS) with the Rotating Gamma System (RGS) has been used to effectively treat functional disorders of the brain, such as trigeminal neuralgia, arteriovenous malformations, and benign and malignant tumors within the brain with minimal dose spillage to the surrounding healthy tissue. Although under normal operation the RGS produces a sharp penumbra, treatment of lesions near critical organs at risk (OAR) is risky or impossible. Here we present two new operation modes of the RGS, the Intensity Modulated Radiosurgery (IMRS) and Speed Modulated Radiosurgery (SMRS) modes, which could further sharpen the penumbra of the RGS to target lesions near critical OARs. Geant4 based Monte Carlo simulations of the RGS dosimetry were performed for the normal, IMRS and SMRS operation modes of the RGS. Results from the normal and IMRS models were validated with comparisons to experimental data collected with EBT3 films. For the IMRS and SMRS modes the penumbra significantly sharpened along the semi-major axis of the dose profiles; however, the IMRS mode requires a longer dose delivery time compared to the SMRS mode. Nonetheless, both operation modes could be used to target lesions near critical OARs that are not currently possible with the RGS or other SRS devices.