Intramuscular Myxoma

Intramuscular myxoma is a benign soft-tissue tumour that arises deep within skeletal muscle, made of bland spindle and stellate cells set in an abundant mucoid (myxoid) matrix. It matters out of proportion to its rarity for two reasons: it is a classic mimic of the myxoid sarcomas on both imaging and small biopsies, and it is the soft-tissue half of Mazabraud syndrome. Getting the diagnosis right prevents a benign lesion being treated as a sarcoma.

Who. A middle-aged adult, peaking in the fifth to sixth decade, more often a woman (roughly 2:1 in large series), with a slowly enlarging, deep, painless mass. The largest classic series of 51 cases had 35 women and 16 men with a mean age of 52 years (Nielsen et al., Am J Surg Pathol 1998; DOI). A surgical series of 17 patients reported a similar profile, with a mean age of 55 years and a female predominance (Silver et al., Clin Orthop Relat Res 2002, PMID 12360026). A deep myxoid mass in a young child, or a superficial myxoid mass, should prompt you to reconsider the diagnosis.

Where. The thigh is the single most common site, and the great majority of Silver's tumours were there. In order:

• Thigh - quadriceps and adductors
• Buttock and gluteal region - second most common
• Shoulder and upper arm - frequent in the upper limb
• Calf and other large muscles - less common

The driver. Intramuscular myxoma and fibrous dysplasia share one molecular driver: an activating, post-zygotic (somatic) mutation in GNAS, most often at codon R201. GNAS, on chromosome 20q13, encodes the alpha subunit of the stimulatory G protein (Gs-alpha). The mutation locks Gs-alpha in its active state, constitutively driving adenylate cyclase and raising intracellular cyclic AMP, which in turn alters proliferation and differentiation of the affected mesenchymal cells.

The variants. p.R201C and p.R201H are the most common hotspot mutations. Rarer R201S/L/P and Q227 variants have been reported.

Why the lesions are patchy. Because the mutation occurs after fertilisation it is not inherited, and it produces a mosaic distribution. If it arises early and involves skeletal precursors, the result is fibrous dysplasia (and, when extensive with endocrine features, McCune-Albright syndrome); when it also affects intramuscular mesenchymal cells, intramuscular myxoma can develop.

Mazabraud syndrome. Fibrous dysplasia arises from a post-zygotic GNAS mutation that misregulates osteogenesis, and its combination with intramuscular myxoma defines Mazabraud syndrome (Wirth, Orthopade 2020; DOI), the clinical expression of the shared mutation in two tissues.

What Mazabraud means for the patient in front of you.

• A patient with intramuscular myxoma may have occult bone disease: image for it and ask about it
• The bone disease may be monostotic or polyostotic
• The myxomas are often multiple and tend to cluster near the most severely affected bone

How often the mutation is found. Sensitive sequencing detects GNAS mutations in a majority of intramuscular myxomas. A next-generation sequencing study combining two assays found them in 68% of sporadic myxomas (Bekers et al., Diagn Pathol 2019; DOI), and a COLD-PCR study raised detection from 29% with conventional PCR to 61% with the more sensitive method (Delaney et al., Mod Pathol 2009; DOI).

Using GNAS on a biopsy. The malignant myxoid mimics (myxofibrosarcoma, low-grade fibromyxoid sarcoma, myxoid liposarcoma) do not carry GNAS mutations, so a detectable mutation is a useful tie-breaker on a difficult small biopsy. In Delaney's study GNAS testing improved diagnostic accuracy when separating intramuscular myxoma from low-grade myxofibrosarcoma: mutations were found in two cases originally favoured as myxofibrosarcoma, and in none of a panel of low- and high-grade myxofibrosarcomas. A negative result does not exclude myxoma, since detection depends on assay sensitivity and the low cellularity of these tumours.

Macroscopic. The tumour is well circumscribed but unencapsulated, with a glistening, gelatinous, lobulated cut surface. It is soft and mucoid and may contain small cystic spaces, and despite looking discrete it blends into the surrounding muscle at its edge. Most measure 2 to 15 cm, averaging around 5 to 6 cm (5.6 cm in Nielsen's 51 cases).

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The classic pattern. The tumour is hypocellular and hypovascular: few, widely spaced, bland spindle and stellate fibroblast-like cells in abundant pale myxoid matrix, with scant, inconspicuous vessels. There is no atypia, no mitotic activity and no necrosis.

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The cellular variant. Some tumours contain focal areas of increased cellularity and vascularity, with more collagen, and these can occupy a large part of the tumour. They are the main reason myxoma is misdiagnosed as sarcoma, because hypercellular and hypervascular foci can be misread as myxofibrosarcoma or low-grade fibromyxoid sarcoma. In Nielsen's series 38 of 51 tumours contained hypercellular regions occupying 10 to 80% of the lesion, yet none showed cytologic atypia, mitotic activity or necrosis; recognising these foci is essential to avoid an erroneous diagnosis of sarcoma (Nielsen et al., Am J Surg Pathol 1998; DOI).

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Immunohistochemistry. There is no single specific stain, and immunohistochemistry is used mainly to exclude mimics. Beyond the markers below, the lesional cells are negative for desmin and CD34.

There is no formal grading system, because intramuscular myxoma is benign. The useful classification is contextual - solitary or multiple, sporadic or Mazabraud-associated - and, on histology, classic or cellular variant. The exam-critical task is the differential diagnosis against the myxoid sarcomas.

Benign lesions to distinguish.

The myxoid sarcomas, which must be excluded.

Site and association. Juxta-articular myxoma arises in the soft tissue immediately around a large joint, most often the knee, and less commonly the shoulder, elbow, hip or ankle. Unlike intramuscular myxoma it is strongly associated with internal joint derangement or degenerative joint disease and is thought to be related to ganglion/meniscal-cyst-type mechanisms. It therefore often looks cystic, and communicates with, or lies adjacent to, the joint.

Different molecular biology. Intramuscular myxoma is GNAS-driven, whereas juxta-articular myxoma is generally GNAS-negative and is not part of the Mazabraud/fibrous-dysplasia spectrum, a useful discriminator when the two are considered together.

Local recurrence is the key clinical difference. Both are benign and neither metastasises, but juxta-articular myxoma has a substantially higher local recurrence rate, roughly a third of cases, against the near-zero recurrence of intramuscular myxoma. Its cystic, para-articular, infiltrative-edged nature makes complete excision harder, so it needs more careful, complete local excision and follow-up.

Histology overlaps. Bland cells in myxoid stroma are common to both, but ganglion-like cystic spaces and a juxta-articular location point to the diagnosis. It must still be separated from the myxoid sarcomas on the same principles.

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Deep angiomyxoma is the myxoid tumour most likely to be confused clinically with a deep myxoma, yet it behaves very differently, which makes it a high-yield contrast.

A different patient and site. It is a rare myxoid tumour of the pelviperineal and inguinoscrotal soft tissues, overwhelmingly in adult women, though it occurs in men. It typically presents as a slowly growing, deep, painless mass that can be mistaken for a Bartholin cyst, hernia or abscess.

"Aggressive" means locally infiltrative, not malignant. It does not (or very rarely) metastasises, but it is deeply infiltrative, with finger-like extensions into the pelvic floor and perineum. That is why it has a high local recurrence rate, of the order of a third or more, and can be difficult to excise completely.

Distinct, hormone-responsive biology. It is characteristically oestrogen- and progesterone-receptor positive and harbours HMGA2 (chromosome 12q) rearrangements, unlike GNAS-driven intramuscular myxoma. Its hormone sensitivity means GnRH agonists (e.g. leuprolide) can shrink the tumour; they are used as an adjunct or to downsize before surgery, a treatment lever with no equivalent in intramuscular myxoma.

Imaging and management. MRI shows a T2-hyperintense mass, often with a characteristic "swirled" or layered internal pattern and infiltrative margins. Management is complete (often wide) local excision, deliberately more aggressive than the marginal excision that cures an intramuscular myxoma, with hormonal therapy in selected cases and long-term surveillance for recurrence.

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History. Most patients present with a slowly growing, deep, painless mass, often in the thigh, noticed for months to years. Pain or aching occurs in a minority and may reflect size or local pressure rather than aggressive behaviour, and there are no constitutional ("B") symptoms. The mass is usually single, and multiple masses suggest Mazabraud syndrome. Ask specifically about bone pain, fractures and deformity.

Examination. Because the lesion is intramuscular it is a deep, firm, smooth mass within a muscle belly, non-tender in most patients. It is fixed to muscle, so it is mobile only in the plane perpendicular to the muscle fibres: across them, not along them. There is no overlying skin change and no regional lymphadenopathy, which is reassuring but not diagnostic.

The management of a confirmed intramuscular myxoma is reassuringly simple, but the path to that confirmation must respect sarcoma principles. Once the diagnosis is secure, treatment is conservative surgery; there is no role for chemotherapy or radiotherapy for the myxoma itself.

Marginal excision. Marginal excision is the treatment of choice and is curative; wide margins are unnecessary for a benign tumour. Function-preserving surgery is appropriate, with no need to sacrifice nerve, vessel or critical muscle to chase clearance. Recurrence even after incomplete resection is exceptional and surgical excision is virtually always curative (Nielsen et al., Am J Surg Pathol 1998; DOI), and a 17-patient surgical series treated all tumours by marginal excision with no recurrences over a mean 7-year follow-up (Silver et al., Clin Orthop Relat Res 2002, PMID 12360026).

Observation. For a small, biopsy-proven, asymptomatic myxoma in a patient unfit for or declining surgery, observation is reasonable given the benign natural history. Surgery is offered for symptoms, enlargement, functional limitation or diagnostic uncertainty. Observation is acceptable only after a confident benign diagnosis, never for an unbiopsied deep mass.

The errors lie at the two extremes. The commonest real-world errors are over-treating a benign myxoma as a sarcoma, with wide resection or adjuvant therapy, and under-investigating a deep mass that turns out to be a sarcoma by enucleating it before a tissue diagnosis.

The Mazabraud patient. When intramuscular myxoma occurs with fibrous dysplasia, the myxoma is managed as above, by marginal excision when indicated, and the bone disease on its own merits:

• Bisphosphonates for pain
• Prophylactic fixation or correction of deformity for structural bone
• Surveillance, because the dysplastic bone, not the myxoma, drives most of the morbidity (deformity, fracture, pain)

There is no convincing evidence that the myxomas of Mazabraud syndrome are more aggressive than sporadic myxomas, although they are more often multiple.

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The prognosis is excellent. Intramuscular myxoma does not metastasise, and local recurrence after marginal excision is rare to non-existent in the major series: in Nielsen's 51 cases no tumour recurred or metastasised on follow-up. The principal complications relate to diagnostic error and to the associated conditions, rather than to the tumour itself.

Global Epidemiology

Intramuscular myxoma is a rare benign soft-tissue tumour. Pooled clinicopathologic and surgical series consistently describe a middle-aged patient (mean age in the mid-50s to early 60s), a female predominance of roughly 2:1, and a strong predilection for the thigh and other large proximal muscles. Across the major series tumours average around 5 to 7 cm and behave benignly, with no metastases and exceptional recurrence after marginal excision (Nielsen et al., Am J Surg Pathol 1998; DOI; Silver et al., Clin Orthop Relat Res 2002, PMID 12360026).

Principles That Cross All Jurisdictions

Soft-tissue mass guidance worldwide (for example BSG/BOA and ESMO-EURACAN soft-tissue tumour pathways, and equivalent sarcoma-unit referral standards) converges on the same principles relevant to intramuscular myxoma, even though no guideline is specific to this benign tumour.

Classification Context

The current WHO Classification of Soft Tissue and Bone Tumours lists intramuscular myxoma among benign tumours of uncertain differentiation (the myxoma group, alongside juxta-articular myxoma), reflecting both its benign behaviour and its GNAS-driven biology shared with fibrous dysplasia. Because it is benign and does not metastasise, it is not staged with the Enneking or AJCC soft-tissue sarcoma systems; those staging frameworks are reserved for the malignant myxoid tumours in its differential. This distinction - a benign WHO category and no malignant staging - is itself an exam point.

Practice Variation

Across health systems the workflow is essentially uniform: MRI, planned core biopsy, specialist review of any indeterminate myxoid lesion, and marginal excision for confirmed myxoma. Differences are largely in access to molecular testing (GNAS sequencing is more readily available in tertiary and reference laboratories) rather than in principles. Internationally, suspected deep soft-tissue masses are referred to specialist sarcoma services, with MRI and image-guided biopsy arranged through or in conjunction with the sarcoma multidisciplinary team; GNAS and other molecular studies are available through specialist anatomical pathology and molecular laboratories, typically at reference centres. An early case report contributed to recognition of the soft-tissue-plus-bone association that defines Mazabraud syndrome (Gianoutsos et al., Aust N Z J Surg 1990; DOI).