Intramuscular Myxoma

Definition

• Generally hypocellular, hypovascular, bland pure myxoid lesion arising in skeletal muscle

Diagnostic Criteria

• Markedly hypocellular and hypovascular
  • Abundant myxoid matrix
  • Areas of increased cellularity and vascularity may be seen
    • Must not involve >20% of a given tumor
      • If >20%, designate as cellular myxoma
    • No atypia, mitoses or necrosis even in cellular areas
    • No difference in clinical behavior
• Involves skeletal muscle
• Grossly circumscribed, microscopically infiltrative
• Cytologically bland
  • Small stellate or spindled cells
  • No pleomorphism
  • No atypia, mitoses or necrosis
  • May have microcystic appearing acellular areas
• Muciphages may be present
• No lipoblasts

Richard L Kempson MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting:: September 23, 2007

Supplemental studies

Immunohistology

Genetic study

• Gs(alpha) mutation of fibrous dysplasia identified in 5/6 myxomas tested, with and without associated fibrous dysplasia

Differential Diagnosis

• Aggressive angiomyxoma
• Myxoid neurofibroma
• Low grade fibromyxoid sarcoma
• Myxoid liposarcoma
• Low grade myxofibrosarcoma
• Cellular myxoma
• Juxta-articular myxoma
• Nodular fasciitis

Intramuscular Myxoma	Aggressive Angiomyxoma
Frequently on extremities	Perineal or pelvic
Hypovascular	Prominent medium to large vessels

Myxoid Neurofibroma	Intramuscular Myxoma
May involve nerve	No nerve involvement
S100 positive	S100 negative

Low Grade Fibromyxoid Sarcoma	Intramuscular Myxoma
Alternating fibrous and myxoid patterns	Uniform myxoid pattern
Swirling, whorled pattern	No swirling, whorled pattern

Myxoid Liposarcoma	Intramuscular Myxoma
Arborizing vascular pattern	Hypovascular
Lipoblasts present, usually signet ring type	No lipoblasts
More cellular	Extremely paucicellular

Myxofibrosarcoma	Intramuscular Myxoma and Juxta-articular Myxoma
Pleomorphism present	Uniformly small bland cells
More cellular	Extremely paucicellular
Mitotic figures may be present	Mitotic figures practically never present
Arching vessels	Markedly hypovascular

Cellular Myxoma	Intramuscular Myxoma
Moderate cellularity must comprise at least 20% of lesion	Markedly hypocellular
Arching vessels may be present	Markedly hypovascular
May have areas of collagenous stroma	Uniformly myxoid stroma

Juxta-articular Myxoma	Intramuscular Myxoma
Adjacent to large joint	Intramuscular
May be more cellular	Extremely paucicellular
Lacks Gs(alpha) mutation	Gs(alpha) mutation frequent

Intramuscular Myxoma	Nodular Fasciitis
Very hypocellular	Moderately cellular
No grouping of cells	Undulating bundles of cells
No tissue culture appearance	Tissue culture appearance with tears and spaces in tissue
Rare mitotic figures	Frequent mitotic figures

Clinical

• Age 25-89 years
• Size 1.5-20 cm, rare >10 cm
• Proximal limbs and adjacent trunk
• Involves muscle, may or not be confined to it
• Rarely associated with fibrous dysplasia (Mazabraud syndrome)
  • Gs(alpha) mutation of fibrous dysplasia identified in 5/6 myxomas tested
• Seen in cases with and without associated fibrous dysplasia
• Minor radiographic bone abnormalities reported in 14/16 cases studied
• No recurrences if completely excised
• No metastases

Grading / Staging / Report

Grading and staging not applicable

The surgical pathology report should contain or address the following:

• Location
• Type of resection or biopsy
• Histologic diagnosis
• Managerial category Ia (Local excision is almost always curative; metastasis never occurs)
• Extent of tumor cell necrosis
• Size
• Depth (dermis, subcutis, below fascia, body cavity)
• Margins
  • Involved
  • Not involved
• Results of supplementary studies if performed
• Relationship to other specimens from the same patient

Lists

Soft tissue lesions that frequently are prominently myxoid

• Aggressive angiomyxoma
• Cellular myxoma
• Intramuscular myxoma
• Juxta-articular myxoma
• Low grade fibromyxoid sarcoma
• Low grade myxofibrosarcoma
• Myxoid chondrosarcoma
• Myxoid liposarcoma
• Myxolipoma
• Ossifying fibromyxoid tumor
• Myxoid nerve sheath tumor
• Superficial acral fibromyxoma
• Superficial angiomyxoma

Bibliography

• Kempson RL, Fletcher CDM, Evans HL, Henrickson MR, Sibley RS. Tumors of the Soft Tissues, Atlas of Tumor Pathology, AFIP Third Series, Fascicle 30, 2001
• Fletcher CDM, Unni KK, Mertens F. Pathology and Genetics of Tumours of Soft Tissue and Bone, World Health Organization Classification of Tumours 2002
• Weiss SW, Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors, 4th edition, 2001
• van Roggen JF, McMenamin ME, Fletcher CD. Cellular myxoma of soft tissue: a clinicopathological study of 38 cases confirming indolent clinical behaviour. Histopathology 2001 Sep;39(3):287-97
• Okamoto S, Hisaoka M, Ushijima M, Nakahara S, Toyoshima S, Hashimoto H. Activating Gs(alpha) mutation in intramuscular myxomas with and without fibrous dysplasia of bone. Virchows Arch 2000 Aug;437(2):133-7
• Allen PW. Myxoma is not a single entity: a review of the concept of myxoma. Ann Diagn Pathol 2000 Apr;4(2):99-123
• Nielsen GP, O'Connell JX, Rosenberg AE. Intramuscular myxoma: a clinicopathologic study of 51 cases with emphasis on hypercellular and hypervascular variants. Am J Surg Pathol 1998 Oct;22(10):1222-7
• Szendroi M, Rahoty P, Antal I, Kiss J. Fibrous dysplasia associated with intramuscular myxoma (Mazabraud's syndrome): a long-term follow-up of three cases. J Cancer Res Clin Oncol 1998;124(7):401-6
• Hashimoto H, Tsuneyoshi M, Daimaru Y, Enjoji M, Shinohara N. Intramuscular myxoma. A clinicopathologic, immunohistochemical, and electron microscopic study. Cancer 1986 Aug 1;58(3):740-7
• Miettinen M, Hockerstedt K, Reitamo J, Totterman S. Intramuscular myxoma--a clinicopathological study of twenty-three cases. Am J Clin Pathol 1985 Sep;84(3):265-72