>BACKGROUND: We report two cases of infantile tectal gliomas initially concerning on biopsy for high-grade features, but which are proving over time to have indolent clinical courses. Case I. A three-months-old female presented with hydrocephalus and a small tectal mass; she underwent an endoscopic third ventriculostomy (ETV). The first follow-up MRI performed 5 months later showed relief of hydrocephalus but marked increase (three- to four-fold) in mass size, likely due to decompression post-alleviation of hydrocephalus. A biopsy yielded a tiny fragment of diffuse low-grade glioma, low Ki-67; IHC for H3.3 K27M was initially reported as mutant, but on more recent review as indeterminate, in view of the tiny tumor fragment and high background staining. Therapeutic recommendations included proton-beam irradiation, marrow-ablative chemotherapy, conventional-dose chemotherapy - and observation only. The parents elected careful follow-up. Now 16 months following biopsy, the child is developing well and the MRI is stable, with advanced imaging characteristics of a low-grade glial lesion. Case 2. A male was diagnosed in utero with hydrocephalus, a post-natal MRI confirming this and revealing a small tectal mass. An ETV and biopsy were performed, yielding a tiny tumor fragment; pathology was consistent with a diffuse glial tumor, with rare mitoses; IDH1 and ATRX intact, P53 negative, GFAP positive and Ki67 20–30%. The tumor was deemed a high-grade glioma. Insufficient tissue for sequencing was available. The infant has been followed with close follow-up MRI scans with advanced imaging, which remain consistent with a low-grade glioma, and continues stable at 3 months since birth; a repeat biopsy is planned concomitant with shunt revision. >CONCLUSION: Infants with tectal gliomas are rare, and anecdotally never undergo progression to high-grade. In the absence of definitive sequencing data, extreme caution should be exercised in offering treatment recommendations.
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