Presentation
Incidental finding.
Patient Data
MRI shows a fusiform mass with tapered ends located in the soft tissue of the middle third of the posterior right forearm, measuring approximately 15 x 8 x 10 mm. It has well-defined margins, restricted diffusion, low signal on T1W, and high signal on STIR and T2W sequences, with strong, homogeneous enhancement after contrast administration. A rim of high-signal fat (high on T1W and T2W) is visible at the proximal and distal ends of the lesion, consistent with a split fat sign.
Microscopic appearance
Red blood cells are interspersed with clusters of spindle cells and cells with atypical nuclei.
Conclusion
Atypical cells.
Although the imaging findings strongly support a diagnosis of peripheral nerve sheath tumor (PNST), the oncologists have not ruled out a malignant lesion such as sarcoma or metastasis. Therefore, a fine needle aspiration (FNA) was recommended to exclude malignant cells, despite it not being a technique commonly recommended for PNSTs and it being part of our hospital's protocol for quite some time. The FNA results showed atypical cells, leading to the decision to proceed with complete surgical excision of the tumor and histopathological examination.
Surgical report:
a tumor was observed on the posterior aspect of the distal third of the right forearm, measuring approximately 1 x 2 cm, with well-defined margins
an incision was made over the tumor, dissecting down to the forearm muscles. A round, smooth, well-circumscribed mass was identified, encasing the posterior interosseous nerve branch
the tumor was excised and sent for histopathological examination
Macroscopic appearance
specimen consists of one sample, measuring 0.2 x 1 cm, opaque white, with a soft cut surface.
Microscopic appearance
The tumor structure consists of uniform spindle-shaped Schwann cells arranged in intersecting fascicles.
Conclusion
Schwannoma (neurilemmoma)
Case Discussion
The findings on MR imaging and histopathological results confirm a definitive diagnosis of a peripheral nerve sheath tumor (schwannoma).
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