Soft tissue

Contributed by Nicholas Frazzette, M.D.

• Intramuscular myxoma does not typically recur after complete excision
• Cellular variants may have a small risk of local recurrence; however, they are benign, indolent lesions
• References: Histopathology 2001;39:287, J Bone Joint Surg Am 2019;101:160

• Typically well circumscribed with occasional focally ill defined borders with infiltration into surrounding skeletal muscle
• Cut surface is gelatinous and may have fluid filled cystic spaces
• References: Am J Clin Pathol 1965;43:104, Mayo Clin Proc 1973;48:401, Am J Surg Pathol 1998;22:1222, Histopathology 2001;39:287

Contributed by Nicholas Frazzette, M.D.

• Cytologically bland / uniform and hypocellular lesion with spindle cells in an abundant extracellular myxoid stroma
• Nuclei are small and ovoid; atypia is minimal to absent
• Sparse, small capillaries may be seen
• Necrosis and mitosis are absent
• Cellularity may be increased at the periphery and focal nondestructive infiltration of surrounding tissue may be seen
• Cellular variant may have an increased number of similarly bland, uniform cells with more prominent vascularity; however, nuclei atypia remains minimal; prominent mitotic activity and tumor necrosis should not be identified
• References: Am J Clin Pathol 1965;43:104, Mayo Clin Proc 1973;48:401, Am J Surg Pathol 1998;22:1222, Histopathology 2001;39:287

Contributed by Nicholas Frazzette, M.D. and Syed T. Hoda, M.D.

• Left gluteus mass, excision:
  • Intramuscular myxoma, completely excised
  • Tumor size: 2.5 cm in greatest dimension

A 45 year old woman presents with a painless, slowly growing mass in her thigh. Magnetic resonance imaging (MRI) demonstrates a well defined lesion within the muscle with high T2 signal intensity and low T1 signal intensity. On resection, the histology above is seen. Immunohistochemical studies are negative for S100, SOX10 and CD34. Which of the following is the most likely diagnosis?

1. Intramuscular myxoma
2. Low grade fibromyxoid sarcoma
3. Myxoid liposarcoma
4. Neurofibroma
5. Schwannoma

A. Intramuscular myxoma. Intramuscular myxoma is a benign, soft tissue tumor typically presenting in middle aged adults, often in the thigh or upper arm. On MRI, it characteristically appears as a well circumscribed lesion with high T2 and low T1 signal intensities. Histologically, it is composed of sparse, bland spindle cells in a myxoid stroma with a paucity of blood vessels. Immunohistochemical staining for markers like S100 and SOX10 is generally negative in intramuscular myxomas; while CD34 is often positive, this is not always the case.

Answer C is incorrect because, while myxoid liposarcomas can also present as well circumscribed lesions on MRI with high T2 signal, histologically they generally show lipoblasts and a more vascular pattern, including chicken wire capillaries, which are absent in this case. Answer B is incorrect because low grade fibromyxoid sarcoma has a more cellular and less myxoid appearance than intramuscular myxoma and often contains alternating areas of collagenous and myxoid stroma. It typically stains positive for MUC4, which is a helpful distinguishing marker, whereas intramuscular myxomas are MUC4 negative. Answer D is incorrect because neurofibromas are often positive for S100 and SOX10 due to their neural origin. They usually contain wavy, spindle shaped cells within a collagenous or myxoid background. Answer E is incorrect because, like neurofibromas, schwannomas typically show positive staining for S100 and SOX10. Histologically, they have cellular Antoni A areas and less cellular Antoni B areas, which are not described in this case.

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Reference: Intramuscular myxoma

A 47 year old woman presents with a slow growing mass in her left thigh. Magnetic resonance imaging demonstrates a well circumscribed, T2 hyperintense, intramuscular lesion. A biopsy of the lesion contains a paucicellular neoplasm composed of cytologically bland spindle shaped cells within a myxoid matrix with scant thin walled blood vessels, lacking perivascular condensation of the tumor cells. Which of the following genetic alterations is typically present in this tumor?

1. AMACR amplification
2. GNAS1 point mutation
3. NF1 deletion
4. NTRK1 amplification
5. TP53 point mutation

B. GNAS1 point mutation. GNAS1 activating mutations are seen in > 90% of intramuscular myxomas. Answer A is incorrect because this aberration is not seen in intramuscular myxoma and, while not specific, may be present in myxofibrosarcoma. Answer E is incorrect because this aberration is not seen in intramuscular myxoma. TP53 mutations are characteristic of many malignancies and are rarely seen in benign neoplasms. Answer D is incorrect because this aberration is not seen in intramuscular myxoma and while not specific is commonly seen in a variety of soft tissue sarcomas. Answer C is incorrect because somatic alterations in NF1 may be present in a variety of malignant neoplasms (up to 5% of cancers). Germline NF1 mutations are associated with type I neurofibromatosis and present in nerve sheath neoplasms associated with this condition.

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Reference: Intramuscular myxoma