HELSINKI, Finland – April 4, 2013 -- Clinicians at leading neurosurgery and radiation oncology departments in Italy have published a new paper describing the feasibility of integrating navigated brain stimulation (NBS) mapping results and neuroimaging data into treatment planning for therapeutic stereotactic radiosurgery. It has earlier been suggested that information from non-invasive NBS mapping and other non-invasive functional methods, including functional MRI (fMRI) and diffusion tensor imaging-based tractography (DTI) might be used to improve planning of radiosurgery by allowing for adjustment of the orientation of the radiation beams to decrease the radiation dose to critical cortical and subcortical areas. Integration of functional neuroimaging in CyberKnife radiosurgery: feasibility and dosimetric results, by Conti et al,. is the first publication to support the concept.
For lesions lying close to critical structures it is not always possible to stay within recommended irradiation safety limits while delivering biologically-effective radiation doses. For this reason, spatial resolution of the functional mapping method is important in order to avoid overestimation of critical volumes. The authors found that "The spatial resolution of NBS is actually extremely high, since we can record the differences in cortical responses within a few millimeters". Additionally, the authors observed that although non-invasive, NBS was a direct measurement and not a passive recording of brain activity during voluntary patient movements, relying on cerebral blood flow or metabolism which could be altered in the presence of brain tumors and vascular malformations, as is the case with fMRI.
In a series of 25 patients treated using the Accuray Cyberknife, treatment planning was enhanced by incorporating functional NBS mapping and functional neuroimaging data. Ten of the patients had AVMs, twelve patients had malignant brain tumors and three patients had meningiomas. After treatment plan optimization, the clinicians achieved an average 17% reduction in the radiation dose to critical functional volumes. The authors reported that no patient harbored neurological deficits attributable to irradiation at follow-up.
Comparison of fMRI and NBS mapping
The clinicians calculated the volume of functional motor cortex defined by fMRI and NBS mapping separately. The cortical surface considered functionally relevant was 30% smaller based on NBS mapping, as compared to fMRI. The clinicians reported that they achieved greater dose reductions (average 25%) to cortical functional areas because of the smaller functionally relevant volumes obtained with NBS mapping.
In their conclusion, the authors state: "Radiosurgery treatments rely on the quality of neuroimaging. The integration of functional data allows a reduction in radiation doses to functional organs at risk, including critical cortical areas, subcortical tracts, and vascular structures. The relative simplicity of integrating functional neuroimaging into radiosurgery warrants further research to implement, standardize and identify the limits of this procedure." The authors also state that the functional imaging and mapping data sets could be easily and reliably integrated into the Cyberknife treatment planning.