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Myositis ossificans circumscripta (MOC) is a benign condition of non-neoplastic heterotopic bone formation in the muscle or soft tissue. Trauma plays a role in the development of MOC, thus, non-traumatic MOC is very rare. Although MOC may occur anywhere in the body, the lesions are localized predominantly in the high-risk sites of injury, such as the thigh, buttock, and elbow. MOC can easily be mistaken for osteomyelitis or a malignant tumor, specifically osteosarcoma or soft-tissue sarcoma. We report a rare case of non-traumatic myositis ossificans circumscripta of thigh which appear clinically and radiologically as a malignant neoplasm. Despite its rarity, MOC should be contemplated in the differential diagnosis of malignant tumors.
Non-traumatic MOC is a rarely reported benign heterotopic ossification characterized by the aberrant formation of bone in extraskeletal soft tissues. It is usually confined to a single muscle or muscle group, and is most common in active male within the second and third decade of life.
It usually occurs after muscle injury such as repeated microtrauma, but it can also occur without previous trauma. In a small number of cases, possible etiologies include infections, burns, neuromuscular disorders, hemophilia (factor-IX deficiency), tetanus, and drug abuse. Clinically and radiologically, it is difficult to distinguish this disease from soft tissue and bone malignancy, so a biopsy is necessary to confirm the diagnosis.
A 27-year-old woman presented with a 7 month history of insidious onset of an isolated left thigh mass, located dorsally at the junction of middle and distal third of the thigh. The mass was painful, and slowly enlarging. The patient denied any history of trauma, fever, chills or weight loss. She wasn't involved in professional or recreational sports activities. Physical examination revealed a 7 × 8 cm, soft and painful mass in the distal one-third of the left thigh, little sensitive to pressure, firmly attached to deeper tissues. There were no motor deficits or neurologic symptoms. The pain was not severe enough to disturb sleep or to hinder physical activities but worsened after muscular contraction and mobilization of neighboring joints (i.e. hip and knee). There is no stiffness of the knee with normal joint amplitudes. Laboratory tests revealed normal white blood cell count (6400/mm3), a slight increase of Erythrocyte Sedimentation Rate (31 mm/h) and C-reactive protein (63 mg/dl), alkaline phosphatase levels of 90 U/L, calcium levels of 10 mg/dl, and creatine phosphokinase levels of 200 U/L. Plain radiographs initially revealed no specific bone abnormalities and didn't show any calcifications facing the mass (Fig. 1). Radiography performed 5 months later showed an oval mass with a regular border containing dense peripheral calcifications with centripetal evolution; these ossifications were distant from adjacent bony structures associated with unilamellar periosteal reaction of the underlying bone (Fig. 2). Magnetic resonance imaging (MRI) objectified tumoral process of posterior muscular compartment of the left thigh with crown aspect (periphery with hyperintense signal on T2 and center with hypointense signal on T2 and hyperintense signal on T1) (Fig. 3A and B). This process measured 6 × 4 cm with refoulement of vessel elements (Fig. 3C). Surgical biopsy was performed in the posterior face of the left thigh (Fig. 4). Histological examination of the specimen revealed fibrous and adipose tissue altered by a mononuclear inflammatory infiltrate associated with a proliferation of spindle cells. These cells showed a regular nuclei, sometimes vesicular and nucleolus. These elements are arranged in small clusters, developing immature osteoid lined by regular osteoblastic cells. Besides this proliferation, there is bone maturation, made by anastomotic and regular cords. These microscopic features were in favor of a myositis ossificans (Fig. 5A and B).
The patient was received radiotherapy with low-dose (600 cGy) in six fractions. At the 2 months follow-up, the patient had no recurrence of pain and he had full range of motion with a clinical reduction in the size of the tumor. The surgery by resection of the mass will be programmed after the radiotherapy, when the tumor would be sufficiently mature.
Myositis ossificans (MO) is an inflammatory pseudotumor of the muscle that may be confused clinically, radiologically and histologically with a malignant soft tissue tumor. It is a pseudo-inflammatory tumor that originates from skeletal muscle and corresponds to a heterotopic, metaplastic, non malignant bone tumor.
Men and women are equally affected; no preferential anatomical site can be identified. Non-traumatic MOC is less frequent and seems to have no preference for gender. Adolescents and young adults under the age of 30 years are most commonly affected.
which may be associated with periosteal reaction in 7–10 days. Flocculent dense lesions signifying the onset of ossification arise in the mass from 11 days to 6 weeks. At 6–8 weeks, a lacy pattern of new bone with a sharp peripheral cortex is formed. From 10 weeks to 6 months, the central zone may enlarge and produce the appearance of an “egg shell” at the end stage of maturation.
Plain radiography doesn't reveal any abnormality in the early stages of MO. Radiographs repeated later may show pathognomonic ossification surrounding a clear central area; typically, these ossifications are distant from adjacent bony structures. However, the ossifications are often missed on radiographs when these are performed 2–3 weeks after MO onset.
The appearance of MO on MRI is variable and depends on the maturity and the variation of the histological pattern within the lesion. In the early stages, T2-weighted images may show an inhomogenous focal mass with high central signal intensity. As the lesion matures and the peripheral ossification becomes denser, the images show a hyperintense center surrounded by a hypointense rim corresponding to peripheral ossification.
Histologically, the proliferation is composed of mesenchymal cells secreting a myxoid matrix as well as fibroblasts exhibiting numerous mitoses, which gives it a pseudo-fibrosarcomatous appearance. This is followed by the subacute phase, which continues for about 10 days. The fibroblasts differentiate into osteoblasts and secrete an osteoid matrix at the periphery of the initial myxoid zone, giving it a pseudo-osteosarcomatous appearance. The late phase, also called the maturation phase, usually starts between the second and fifth weeks of the evolution of MO. Bone production can be observed at the periphery of the lesion. At this stage, biopsy will reveal the three characteristic zones of MO and thus allow the correct final diagnosis to be made
: a central zone characterized by the presence of an inflammatory infiltrate with macrophages, lymphocytes, polymorphic fibroblasts and angiogenesis phenomena, muscular fibers with atrophic or degenerative appearance; an intermediate zone with a more regular appearance with collagen trabecule and immature, osteoid cells; and a peripheral zone made by calcified osteoid with areas of cartilagineous metaplasia and lamellar mature bone separated from the surrounding muscle by connective tissue without inflammatory infiltrate.
The differential diagnosis can be represented by non-neoplastic pathologies (calcified fibromatosis, local infections) but especially by malignant tumors (lymphoma, osteosarcoma, rhabdomyosarcoma). The most frequently clinical misdiagnoses are tumor, especially extraskeletal osteosarcoma, which has similar clinical and pathological characteristics.
The treatment in most cases is conservative; it consists of rest, ice and anti-inflammatory drugs to relieve pain. Typically, a regression of the symptoms is seen in the course of disease (30%). A spontaneous resorption or an incomplete regression can occur. Radiation therapy can be used to reduce the size and accelerate the maturation of the lesion.
Surgical intervention is recommended when the heterotopic bone has matured. It is also recommended if it interferes with joint motion, functional limitation or if it generates a significant pain, due to the compression of nerve trunk.
Myositis ossificans circumscripta is a rare benign lesion with an excellent prognosis but may appear clinically and radiologically as a malignant neoplasm, which can make its diagnosis very difficult and sometimes delayed. A thorough knowledge of the clinical and morphological study is necessary to differentiate this lesion from a malignant soft tissue tumor.