Tectal Low-Grade Glioma with H3 K27M Mutation.

Background: In the revised World Health Organization 2016 classification of central nervous system tumors, "diffuse midline glioma, H3 K27M-mutant" has been added as a new diagnostic entity. However, some confusion exists concerning this diagnostic entity because H3 K27M-mutant diffuse midline glioma is diagnosed with grade IV regardless of morphologic phenotype. Furthermore, the significance of H3 K27M mutation in tumors that aren't typical "diffuse midline glioma, H3 K27M-mutant," such as those with an unusual location and nontypical histology, remains unclear.

Case Description: To elucidate further such unusual tumors, we describe here a rare case of pediatric low-grade glioma located in the tectum, which was morphologically a pilocytic astrocytoma (PA) with genetically H3 K27M mutation but no microvascular proliferation, necrosis, mitoses, or other genetic alterations, insofar as we were able to observe. At the latest follow-up, 28 months after surgery, radiotherapy, and chemotherapy, the patient was found to be free from any neurologic deficits and MRI demonstrated that the tumor was stable without tumor regrowth. This case might be identified as "diffuse midline glioma, H3 K27M-mutant", grade IV, when applying only the current World Health Organization 2016 classification. In addition, we discuss the morphologically benign gliomas harboring the H3 K27M mutation based on the literature.

Conclusions: We describe here a rare case and present a short literature review of circumscribed/nondiffuse gliomas, particularly in PA with H3 K27M mutation. However, the significance of H3 K27M mutation for PA remains unclear, so further studies and clinical data are needed to elucidate the biology and optimal treatment of such tumors.