Polyostotic fibrous dysplasia with intramuscular myxoma in a woman with Mazabraud’s syndrome

INTRODUCTION

Fibrous dysplasia (FD) is an uncommon, nonhereditary, skeletal developmental disorder in which abnormal differentiation of osteoblasts leads to distortion of normal marrow and cancellous bone and replacement by immature bone and fibrous stroma. The exact cause is unknown; however, recent studies have linked it to postzygomatic somatic mutation in guanine nucleotide-binding protein, alpha stimulating 1 gene located at chromosome 20q13.2. It can be monostotic, polyostotic, or craniofacial. It can affect all age groups and has no specific gender predilection.¹ FD can occur as a component of McCune-Albright syndrome or Mazabraud syndrome. In McCune-Albright syndrome, FD is associated with hyperfunction of endocrine glands and overproduction of melanin, leading to café au lait cutaneous macules.² In Mazabraud syndrome, FD is associated with intramuscular myxomas. FD is diagnosed mainly based on clinical and typical radiological features. If the imaging features are characteristic of FD, the lesions don’t require histology.³ Biopsy is occasionally necessary to establish the diagnosis. Here polyostotic FD in a 21-year-old female who presented with left thigh pain is being reported.

CASE REPORT

A 21-year-old female presented with a complaint of pain in the left thigh radiating to the left hip region for 4 months. The patient was apparently asymptomatic 4 months back, then developed pain in the left thigh, which was insidious in onset, gradually progressive in nature, and radiating to the left hip region. Pain was aggravated by walking and running and relieved on taking rest. Patient has no comorbidities/fever/weight loss. No recent history of trauma.

On examination, there was no obvious swelling/tenderness and the range of movements of the left hip was normal.

Figure 1a-g shows anteroposterior dimension (AP), lateral radiographs, and computed tomography (CT) of the left lower limb were done, which showed mild coxa varus angulation of the proximal femur with multifocal geographical areas of large expansile intramedullary lytic lesion with ground glass matrix and soap bubble-like trabeculations in the left femoral neck and proximal femoral diaphysis. The lesions are associated with moderate cortical thinning with endosteal scalloping. No pathological fractures/periosteal reaction/matrix mineralization/soft tissue mass. The epiphysis is spared. Similar lesions were present in the left tibial diaphysis. Adjacent hip and knee joints appear normal.

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Figure 1:

(a–c): The radiographic appearance of fibrous dysplasia (FD) in the left femur and tibia. A classic lucent lesion surrounded by a layer of sclerotic reactive bone (the rind sign—represented by the arrow). (d–g): CT in fibrous dysplasia (FD). (d) Multiple ground glass density lesions in the left femur and tibia. (e) Characteristic lucent ground glass appearance with shepherd’s crook deformity. (f and g) Classic lucent lesion surrounded by a layer of sclerotic reactive bone—Rind Sign. (h–l): MRI in fibrous dysplasia (FD). (h–j) MRI typically shows sharply demarcated lesions (arrows) with low signal intensity on T1-weighted images (h), T2-WI (i), and hyperintense on PDFS images (j). (k and l) Lobulated T2/PDFS hyperintense lesion (myxoma) in the capsular portion of the semimembranosus muscle (arrow).

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Magnetic resonance imaging (MRI) of both lower limbs was done from the hip joints till the ankle joints, which showed diffuse intramedullary abnormality, which is intermediate to low on T1W and heterogeneously hyperintense on T2-weighted (T2W)/Short-tau Inversion Recovery (STIR) images with mild marrow expansion, causing moderate to severe cortical thinning and endosteal irregularity in the entire metadiaphysis of the left femur and tibia [Figure 1h-l]. There was no obvious cortical discontinuity. Growth plates appear normal. The transition to normal marrow was noted in the region of growth plates. In addition, multiple small T1W hypointense and T2W/STIR hyperintense areas were noted in the subcortical, intramedullary distal femoral, and proximal femoral epimetaphyseal regions—likely representing myxomatous/chondroid lesions. Mild varus deformity of the proximal femur noted (neck shaft angle −122 degrees). Figures 1k-l shows small lobulated T2/Proton Density Fat-Supressed (PDFS) hyperintense lesion measuring 22 × 10 × 15 mm (AP × TR × CC) noted in the capsular portion of the semimembranosus muscle.

Considering the imaging findings, polyostotic FD with myxomatous/chondroid lesions involving the left femur and left tibia was diagnosed. After radiological investigations, the patient underwent curettage of the lesion in the left proximal femur, and the sample was sent for biopsy, which was suggestive of fibrocartilaginous dysplasia. The histopathologic examination of the soft tissue mass confirmed cores of skeletal muscle interfacing with an ill-defined myxocollagenous lesion, consistent with intramuscular myxoma. The intramuscular myxoma in this case was biopsy proven; however, biopsy is not critical because the lesions are characteristic in this setting and there is no potential for malignant degeneration. The follow-ups were carried out for 15 months from the diagnosis. The patient is currently asymptomatic, and there was no significant increase in the lesion.

DISCUSSION

FD is a nonhereditary disorder that is linked to being caused by postzygomatic somatic mutation of the α-subunit of the Gs stimulatory protein (GNAS mutation), resulting in inappropriate overproduction of cyclic adenosine monophosphate.⁴ This leads to distortion of normal marrow and cancellous bone and replacement by immature bone and fibrous stroma. It can be localized to a single bone (monostotic FD) or involve multiple bones (polyostotic). Mazabraud syndrome is a rare condition that involves a combination of FD and intramuscular myxomas. It is most commonly associated with polyostotic FD, where multiple bones are affected. In this syndrome, intramuscular myxomas typically develop near the areas of abnormal bone and often occur within the same anatomical region.

FD usually presents as slow-growing painless mass. Patients can present with pain due to pathological fractures or due to compression or displacement of adjacent structures by expanding lesion.^1,5 A typical FD lesion in the axial skeleton appears as a radiolucent area with a ground-glass matrix. This appearance is usually smooth and homogeneous, and the lesion is not centrally located within the medullary bone. Lesions can vary in size from a small, focal abnormality to a large lesion, perhaps involving most or all of a long bone. The lesions usually cause cortical thinning due to enlarged fibro-osseous masses within the bone. Periosteal reaction is typically absent unless a pathological fracture is present. While endosteal scalloping can occur, the outer cortical contour remains smooth. The lesion may undergo expansile remodeling due to the growth of fibro-osseous tissue. A characteristic feature known as the “rind sign” is a thick layer of sclerotic bone surrounding the lesion. CT helps to indicate the extent of bone involvement and demonstrates the mild osseous expansion associated with the characteristic ground glass appearance.⁶ MRI of FDs shows homogenous low signal intensity on T1-weighted images, whereas the signal may be mixed or high on T2-weighted images. The sclerotic rim is observed as a band of low signal intensity on both T1- and T2-weighted sequences.⁷ Myxomas demonstrate a sharply defined contour, appear hypodense on CT, show low signal intensity on T1-weighted images, and show high signal intensity on T2-weighted images and STIR images. Several benign lesions, including myxolipoma, myxoid neurofibroma, and myxochondroma, can exhibit areas resembling intramuscular myxoma. Additionally, certain malignant tumors with rich myxoid components should be considered in the differential diagnosis of myxoma. These include myxoid liposarcoma, myxoid malignant fibrous histiocytoma, low-grade fibromyxoid sarcoma, extraskeletal myxoid chondrosarcoma, and botryoid-type rhabdomyosarcoma.⁷ The treatment of Mazabraud’s syndrome is dependent on the extent of the lesions. Because the myxomas are benign, conservative management is indicated. However, myxomas should be surgically removed if they cause pain or pressure symptoms. Simple local excision is sufficient in most cases. A histopathological analysis of the lesions should always be carried out to rule out differential diagnoses such as primary malignancy, mainly low-grade myxofibrosarcomas, or metastatic tumors. FD progresses slowly and generally has a good prognosis if the condition is localized. Treatment is usually conservative, focusing on preventing skeletal deformities and fractures.⁷ Bisphosphonates may help relieve pain and potentially assist in normalizing bone lesions.⁸ Surgical intervention is recommended for severe, progressive disease with persistent pain, skeletal deformity, or a risk of fracture or pathological fracture.⁹ Although Mazabraud’s syndrome is benign, postoperative follow-up is important to identify any other myxomas that may not yet be clinically apparent or to monitor for local recurrences after incomplete excision. While malignant transformation of a myxoma has not been reported, there have been six cases of fibrous dysplastic lesions transforming into osteogenic sarcoma in patients with Mazabraud’s syndrome, supporting the necessity of clinical follow-up.

CONCLUSION

Understanding Mazabraud’s syndrome and the imaging characteristics of intramuscular myxoma is crucial to prevent unnecessary biopsies of bone and soft tissue lesions. The distinctive characteristics of this disorder enable differentiation from soft tissue malignancies like sarcoma.