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A case of NTRK-rearranged Spindle cell tumor in a Pediatric patient


 
 
 Recent studies continue to demonstrate that NTRK fusions occur more frequently in pediatric than in adult patients involving a broader panel of fusion partners as well as a wider range of pediatric tumors than previously recognized. The identification of these NTRK fusions has facilitated precision cancer diagnosis and TRK inhibitor targeted therapy. With the recent FDA approval of larotrectinib and entrectinib for the treatment of adult and pediatric NTRK-positive, unresectable solid tumors, identification of these fusions directly impacts patient care.
 
 
 
 Our patient, a 10 year old female presented with a large right sided buttock mass and pressure effects from the tumor. An incisional biopsy showed a moderately moderately cellular tumor with a collagenous and partially myxoid stroma. The atypical cells had ovoid nuclei with vesicular chromatin, minimal to no atypia, and rare mitotic activity (<2/30 high-power fields), as well as fibrous tissue that appeared as ropy collagen. Some of the blood vessels were rimmed by a hyalinized cuff. A mild inflammatory component, namely scattered lymphocytes and fewer plasma cells were noted. Immunohistochemistry showed: SMA(faint+), S100(+), CD34(+), CD31(+), FLI1(+), NTRK(+). Negative for ALK1, desmin, SOX10, EMA, keratin AE1/3, CAM5.2, D2-40, myogenin, MUC4, TLE1, STAT6, BCOR, ERG. Both INI1 and H3K27me3 were retained. Proliferative rate by Ki-67 was low, showing <2% positivity.
 Next generation sequencing revealed the following: LMNA-NTRK1 fusion; CD36 N53fs*24 and CDKN2A/B CDKN2A loss exon 1. Thus, the histologic, immunophenotypic, and molecular findings together supported a diagnosis of NTRK-rearranged spindle cell tumor. This entity has alternately been termed lipofibromatosis-like tumor. Following confirmation of NTRK fusion, she was treated with oral TRK inhibitor with near total response. With this NTRK-rearranged spindle cell tumor’s minimal mitotic activity, absence of necrosis, and low cellularity, the behavior of this tumor was expected to be indolent rather than aggressive. However, the patient was presented for assessment and management at a recent tumor board about 8 months after her initial diagnosis as she had residual/recurrent tumor.
 
 
 
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 Our case highlights the clinical utility of screening for NTRK fusions in all pediatric tumors.
