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Gregory J. Riely, MD, PhD


Unique Presentation and Spread of Congenital Infantile Fibrosarcoma With NTRK::ETV6 Fusion

By: Emily Rhode
Posted: Tuesday, August 30, 2022

Mark Luquette, MD, of the University of Minnesota Twin Cities, Minneapolis, and colleagues identified a case of congenital infantile fibrosarcoma with NTRK3::ETV6 that presented in both the soft-tissue site and the placenta. According to the investigators, although the exact mechanism of the placental metastasis is unknown, it appears to be unique in that the metastases were subendothelial and limited to the perivascular space in the stem villi. The findings were published in the journal Pediatric and Developmental Pathology.

A female patient born at term presented with swelling in the left inguinal region and thigh; ultrasound and MRI revealed a mass suggesting lymphatic malformation or tumor necrosis. The tumor was initially identified as a kaposiform hemangioendothelioma because of bleeding diathesis. The patient was started on sirolimus and methylprednisone but developed complications including right heart failure and respiratory distress, leading to a differential diagnosis of kaposiform lymphangiomatosis.

Upon examination of the placenta, the researchers found a subendothelial tumor that appeared intravascular, and a working diagnosis of myofibromatosis was made. When the patient was 1 month old, needle biopsies of the thigh tissue and subsequent next-generation sequencing found a rearrangement of intron 5 of ETV6 and intron 14 of NTRK3, and fluorescence in situ hybridization (FISH) of the placenta showed rearrangement of the ETV6 locus in 12p13, confirming infantile fibrosarcoma in both the thigh mass and the placenta.

Therapy with 100 mg/m2 of larotrectinib twice daily was started, and resection of the tumor site was performed after 7 months of treatment. Larotrectinib was continued for 12 months postoperatively. At 6 months after treatment, imaging showed almost complete resolution, and treatment continues with no toxicity.

“Neither the clinical team nor pathologists considered [infantile fibrosarcoma] in the differential diagnosis prior to the biopsy of the leg tumor, a consideration that would have allowed pan-TRK IHC and ETV6 FISH to arrive at an answer considerably faster,” the researchers concluded.

Disclosure: For full disclosures of the study authors, visit

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