Summary
A six-year-old boy presented with abdominal pain and was found to have a large mediastinal mass. Biopsy revealed a spindle cell sarcoma, and molecular testing identified an NTRK1 rearrangement, confirming the diagnosis of NTRK1-rearranged spindle cell sarcoma. This finding is significant as it allows for targeted therapy with TRK inhibitors, highlighting the importance of molecular testing in pediatric sarcomas.
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Haiyan Qiu, MD, PhD
Kotaro Takeda, MD, PhD
February 2026—We report the case of a six-year-old boy with no significant past medical history who presented with a two-month history of intermittent left flank and abdominal pain. Initial abdominal radiography performed by his primary care provider demonstrated a moderate stool burden, and the patient was started on a two-week course of polyethylene glycol with minimal relief. Five days prior to admission, he developed daily tactile fevers, one episode of non-bloody, non-bilious emesis, and non-bloody diarrhea. Two days before presentation, the abdominal pain acutely worsened, became diffuse, and was associated with decreased oral intake, fatigue, and occasional night sweats.
The patient initially presented to the emergency department, where magnetic resonance imaging of the abdomen revealed small-volume free pelvic fluid without evidence of appendicitis. Subsequent review identified a partially visualized left pleural effusion, prompting dedicated chest imaging. Computed tomography of the chest demonstrated a bulky, lobulated, heterogeneously enhancing left posterior mediastinal mass measuring 7.7 × 8.0 × 8.7 cm (AP × TV × CC), crossing the midline and causing rightward mediastinal shift, without associated hilar or axillary lymphadenopathy. The thyroid was unremarkable.
Biopsy of the mediastinal mass revealed a highly cellular spindle cell neoplasm composed of uniform primitive spindle cells arranged in intersecting fascicles with hemangiopericytoma-like vasculature (Fig. 1A and 1B). Brisk mitotic activity and focal necrosis were present. Immunohistochemistry demonstrated diffuse cytoplasmic smooth muscle actin positivity (Fig. 1C) and nuclear/cytoplasmic S100 expression (Fig. 1D), with negativity for cytokeratin, desmin, CD45, CD34, and p16 (not shown). The Ki-67 proliferation index was approximately 40 percent.