Surgery for diffuse low-grade gliomas (DLGG) oncological outcomes

Abstract

For many decades, surgery for DLGG was matter of controversy, mainly due to the fact that, in the classical literature, extent of resection (EOR) was not objectively assessed on post-operative MRI. EOR was usually based on the sole subjectivity of the surgeon, with no volumetric calculation of the residual tumor. In all modern series with objective measurement of the EOR on systematic postoperative T2/FLAIR-weighted MRI, a more aggressive resection predicted significant improvement in overall survival (OS) compared with a simple debulking or biopsy- by delaying malignant transformation. However, development of neuroimaging led neurosurgeons to achieve tumorectomy according to the oncological limits provided by preoperative or intraoperative structural and metabolic imaging. Yet, this principle is not coherent, neither with the infiltrative nature of DLGG nor with the limited resolution of current neuroimaging. Indeed, MRI still underestimates the actual spatial extent of gliomas, since tumoral cells are present several millimeters to centimeters beyond the area of signal abnormalities. Therefore, an extended removal of a margin beyond these MRI-defined abnormalities, i.e. a "supra-total" resection, was recently proposed, with a dramatic improvement of OS. Consequently, the actual aim is not to remove only the "top of the iceberg" visible on imaging, but to perform a radical resection of the brain invaded by a DLGG, on the condition that this part of the nervous system is not crucial for cerebral functions. Thus, biopsy should be reserved only in very diffuse lesions, such as gliomatosis-like, when at least a subtotal resection is not possible. Neurosurgeons should shift from a traditional view consisting of removing a tumor mass within the brain (image-guided resection according to oncological and/or anatomical limits) to the removal of a diffuse chronic tumoral disease invading neural networks. They should take the habit to perform a maximal resection up to the boundaries of brain functions, that is, functional guided-mapping resection by means of intraoperative electrostimulation in awake patients. To solve the traditional dilemma (OS versus quality of life) by optimizing the onco-functional balance (namely, increased OS and better quality of life), the new philosophy is to abandon the conservative wait-and-see attitude to evolve toward an early, radical, safe and invidualized "preventive functional surgical neurooncology".