Presentation
Lump noted in the arm, within the triceps muscle.
Patient Data
There is a well-circumscribed intramuscular mass within the distal aspect of the lateral head of the triceps muscle, probably arising from an intramuscular branch of the radial nerve. It is T1 isointense (to muscle) and T2 hyperintense, with a central T2 low signal ('target sign') and a thin rim of surrounding fat ('split-fat sign'). It demonstrates poor central enhancement. No other mass lesions. The median and ulnar nerves are unremarkable.
Small elbow joint effusion noted, with no evidence of synovitis. Small pseudo-defect of the capitellum noted. Prominent red marrow within the imaged humeral shaft and proximal ulnar and radius.
Lignocaine 1 % for local anesthesia. 3 x 14 G biopsies were obtained from the oblong lesion ?peripheral nerve sheath tumor within the left triceps under ultrasound control. Needle approach was longitudinal to the lesion/humerus, with skin entry point from 19 mm proximal to the superior margin of the lesion.
Case Discussion
The lesion imaging aspects suggest an intramuscular nerve sheath tumor within the triceps.
MICROSCOPIC DESCRIPTION: Sections show fragments of skeletal muscle together with tumor composed of loose short collagen fibers separated by myxoid stroma. The tumor cells have elongate spindle nuclei with pointed ends, normochromic chromatin staining and small intranuclear vacuoles. No mitoses are identified. No necrosis is seen. Immunohistochemical are difficult to interpret focal staining with CD34 and s-100 is present. Neurofilament shows focal staining within the lesion.
DIAGNOSIS: Left intramuscular (triceps) lesion: Low-grade myxoid tumor. The differential diagnosis is between an intramuscular myxoma or nerve sheath myxoma.
Case courtesy of Dr Patsy Robertson and Dr Beng Ghee Lim, the Royal Melbourne Hospital.
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