Introduction

It is spindle cell tumor. Infantile fibrosarcoma is among the most common soft tissue sarcoma (STS) in the first year of life and usually presents before 2 years.

Pathology

The tumor has a primitive histologic appearance, sometimes with a classic fibrosarcoma herringbone fascicular pattern, but also sometimes with poorly defined fascicles, marked inflammatory infiltrate,
prominent myoid nodules, hemangiopericytoma-like vascular pattern, or sheets of spindle
cells and varying proportions of round or epithelioid tumor cells and zonal coagulative
necrosis. The immunohistochemical findings have not traditionally been helpful.

The distinctive ETV6-NTRK3 gene fusion associated with t(12; 15)(p13; q25) is present in
the majority of cases. Neurotrophic tyrosine receptor kinase (NTRK 1/2/3) genes encode tropomyosin-related kinase (TRK) receptors involved in growth regulation, differentiation, and apoptosis of neurons. NTRK fusion in infantile fibrosarcoma. cytogenetics will reveal t(12;15)(p13;q25) /ETV6-NTRK3, Trisomies 8, 11, 17, 20.

Sites

Most common is distal extremities.

Age

Most commonly present in < 2-year age

Clinical Presentation

• Most commonly presents as a deep, nontender soft-tissue mass in the extremities or trunk.
• The tumor may also arise in the head and neck or in visceral locations.
• Grow rapidly and impinge on neurovascular structures or produce compartment syndrome.
• Distant metastases are uncommon (<5%), but may involve lymph nodes, lung, and the peritoneal cavity
• Spontaneous regression in some neonates

Treatment

1. Surgical resection if feasible, consider delayed surgical resection for unresectable or rare metastatic tumors.
2. Vincristine/dactinomycin chemotherapy
3. Consider tyrosine kinase inhibitor-Larotrectinib, an NTRK 1/2/3 inhibitor, is FDA approved for adults and children with solid tumors that have NTRK gene fusions if the cancer is progressive, metastatic, or unresectable. The most common Adverse events were fatigue, nausea, dizziness, vomiting, increased hepatic transaminases, cough, constipation, and diarrhea. In children, larotrectinib 100 mg/m2 twice daily.The half-life of larotrectinib is 2.9 hours. It is metabolized primarily by CYP3A4 and is 70% plasma protein bound. Larotrectinib is also detectable in CSF. Dose modifications are recommended for patients with moderate or severe hepatic dysfunction. The overall response rate was 75% (95% CI 61, 85%).

Prognosis

Generally favorable, although rapid tumor growth may produce morbidity and mortality