Figure Legends:
Figure 1. Brain imaging. A) T1-weighted brain MRI without intravenous contrast at 1-day of age showed a large extra-axial cystic mass in the right convexity and temporal lobe signal alteration. Also present was intraventricular hemorrhage with fluid-fluid levels in the dilated lateral ventricles. B)-E) T1-weighted brain MRI images with intravenous contrast. B) At 7-months of age, the previously noted cystsic lesion demonstrated an extensive and avidly enhancing solid component at the periphery. C) At 8-months of age, following surgical resection, there is residual enhancing mass in the posterior aspect of the tumor bed. D) At 13-months of age, 2 months after completion of conventional chemotherapy, there is progressive residual mass. E) At 15-months of age, 2 months after starting larotrectinib, rapid reduction of the residual tumor is seen.
Figure 2. Tumor histopathology. The desmoplastic portion of the tumor is comprised of spindle cells (upper portion of panel A) with abundant reticulin (B). Spindle cells are polyphenotypic. The expression of S100 protein (C) and smooth muscle-specific actin (SMSA; D) are shown here. Arrows in A-D mark the glia limitans border between abnormal cerebral cortex and desmoplastic neoplasm. Away from the desmoplastic components, the cerebral cortex contains nodular layer I clusters of neurons (E-H; arrows in E-G) and cortical dysplasia (I-K), with disorganized lower layers of the cortex (I) containing dysmorphic neurons (J and K). Arrows in K point to neuronal cell bodies positive for neurofilament. A minority of the tissue resection consisted of high-grade neuroepithelial tumor. Malignant neuroepithelial neoplasm (L-O) is hypercellular with foci of necrosis (M), vascular proliferation (arrows in N and O), diffuse S100 protein expression (O), and high Ki67 proliferative index (P). There was no immunoreactivity for the mutant proteins BRAF V600E or H3K27M. Clusters of malignant cells were present in postoperative drain fluid, presumably CSF (Q).