Myofibroma is one of the most common fibrous tumors occurring in infancy and childhood. However, it is a relatively rare neoplasm among all tumors.[1–4] It arises either as solitary or multicentric lesions. Multicentric lesions frequently occur in infantile myofibromatosis, whereas solitary lesions tend to occur in adults usually involving superficial soft tissues.[5–7] Solitary lesions of bones are very rare. Particularly, spine myofibroma in an adult is exceptionally rare. It occurs most commonly in boys. Craniofacial bones are involved most frequently, followed by upper extremities and trunk. They are clinically benign. However, they can be infiltrative either radiologically or histologically. To the best of our knowledge, only 3 cases affecting lumbar vertebrae have been reported in the English language literature. Of them, 2 cases were neonate and infant cases of multicentric infantile myofibromatosis.[8,9] Only 1 case was an adult case of solitary myofibroma affecting the L1 vertebra. In this report, we describe an extremely rare case of solitary myofibroma affecting the L5 vertebra of an 18-year-old man. Follow-up postoperative computed tomography (CT) or magnetic resonance images (MRI) are also shown.
2 Case report
An 18-year-old man was admitted for spontaneously increasing lower back pain for 2 months. The patient had no history of trauma. On physical examination, there was no tenderness or neurological compromise. Straight leg raising test was negative. Deep tendon reflex showed nonspecific finding for both legs. No pathologic reflexes were found.
Conventional radiographs of the lumbar spine demonstrated an expansile osteolytic lesion with thinned cortex and marginal sclerosis in the right pedicle and transverse process of L5 extending to the right posterolateral portion of the vertebral body (Fig. 1A and B). CT revealed a purely osteolytic expansile bony destructive lesion in the right pedicle and transverse process of L5 extending to the right posterolateral portion of the vertebral body. The cortical bone was abnormally thinned and partially disappeared at the anterior, superior, and inferior margins of the lesion. The lesion had a sclerotic rim in the pedicle and the right posterolateral portion of the body (Fig. 2A and B). A sharply marginated osteolytic line from the lesion was extended to the articular surface of the right superior articular process of L5, suggestive of pathologic fracture involving the facet joint (Fig. 2B). Extension of the mass into adjacent vertebrae (L4, sacrum) was not observed. Enhanced CT was unavailable. MRI demonstrated an approximately 3.7 × 3.1 × 2.3 cm sized mass with infiltration to the posterosuperior aspect of the right L5-S1 neural foramen through the inferior margin of the mass. Right L5 nerve root was displaced slightly downwards due to compression by the mass (Fig. 3A). The mass showed a homogeneously isointense signal with adjacent muscle on T1-weighted images (repetition time [TR] = 517 ms, echo time [TE] = 14 ms) (Fig. 3B) and an inhomogeneous slightly hyperintense signal on T2-weighted images (TR = 4317 ms, TE = 109 ms) (Fig. 3C). The mass reveals homogeneous avid enhancement on fat-suppressed T1-weighted images with gadolinium (TR = 500 ms, TE = 7 ms) without evidence of internal necrosis or hemorrhagic foci (Fig. 3D–F). Tc99m-MDP bone scan did not demonstrate any abnormal radiotracer uptake. In the differential diagnosis of the mass, we included solid variant of aneurysmal bone cyst, osteoblastoma, and solitary myofibroma. In addition, fibrous dysplasia, histiocytosis X, desmoplastic fibroma, metastasis, and myeloma could be contemplated in differential diagnosis of the mass.
Because the mass was an easily accessible lesion, needle biopsy was performed for accurate diagnosis and staging before surgery. Pathologically, bland spindle-shaped cells with tapering nuclei in collagenous stroma were found. More cellular areas with bundles and whorls of spindle cells showed palely eosinophilic cytoplasm associated with thin-walled branching hemangiopericytoma-like vessels. There were multifocal areas associated with pseudochondroid stromal hyalinization. The mass was diagnosed as a myofibroma. Because the mass had well-defined borders with cortical thinning based on radiographic characteristics of the tumor, it was stage 2 (benign active lesion) of Enneking surgical staging system. Afterward we performed marginal excision of the mass and posterior instrumentation of L4 to S1 with left-sided posterolateral fusion of L4–5 was followed because of increasing lower back pain in the patient, right facet joint involvement of L4-5 by pathologic fracture and infiltration to right L5-S1 neural foramen. Adjuvant therapy such as electrocautery, burring, and phenol application was not used.
Grossly, the tumor was pinkish-white colored and solid. Microscopically, the tumor showed biphasic pattern. Cytologically, bland spindle-shaped cells with tapering nuclei in collagenous stroma were found. More cellular areas with bundles and whorls of spindle cells showed palely eosinophilic cytoplasm associated with thin-walled branching hemangiopericytoma-like vessels. There were multifocal areas associated with pseudochondroid stromal hyalinization characteristic of myofibroma (Fig. 4A–C). Immunohistochemical staining showed positive reactions to smooth muscle actin (Fig. 4D) but negative reaction to CD34, S100 protein, and epithelial membrane antigen (EMA).
On the basis of radiological and pathological findings, we concluded that it was a solitary myofibroma originating in the right pedicle, the transverse process, and the right posterolateral body of L5.
The patient has been doing well. Follow-up CT of the lumbar spine was taken at 1 year after the operation. Follow-up CT demonstrated that the size of the osteolytic lesion was decreased with sclerotic change at the anteromedial margin of the lesion because of bony remodeling (Fig. 5A and B). There was no evidence of tumor recurrence. Hardware removal was performed after the follow-up CT at 1 year postoperatively. Several conventional radiographs of the lumbar spine were taken for a period of 3 years from the initial operation. These conventional radiographs at 3 years postoperatively showed much decreased size with obvious sclerotic change of the lesion without tumor recurrence (Fig. 5C). Follow-up MRI at 4 years postoperatively revealed complete resolution of the lesion replaced by normal fatty marrow (Fig. 5D–F).
Myofibroma or myofibromatosis is known as congenital generalized fibromatosis or infantile myofibromatosis. It is the most common fibrous tumor of infancy, typically affecting neonates and children younger than 2 year of age, although it can affect older children less frequently and adults occasionally. Myofibromatosis usually involves the superficial layer of the dermis or subcutis. Deeper-seated lesions such as muscle and aponeuroses, or intraosseous lesions may also occur. Solitary involvement of bone is very rare. It usually involves craniofacial bones. Konishi et al searched PubMed and found 44 reported cases showing solitary involvement of bone. Only 8 of these cases affected extracraniofacial bones such as femur, tibia, ulna, clavicle, sacrum, and axis.[10,14–21] To the best of our knowledge, only 3 cases affecting lumbar vertebrae have been reported in the English language literature. Of them, 2 cases were neonate and infant cases of multicentric infantile myofibromatosis.[8,9] Only 1 case was an adult case of solitary myofibroma affecting the L1 vertebra. No young adult case of solitary myofibroma affecting the L5 vertebra has been reported.
Radiographs and CT of solitary myofibroma of bone including the L1 vertebra usually demonstrate a well-marginated purely osteolytic mass with a sclerotic rim.[10,15,22] Expansile mass may also be accompanied by pathological fracture.[10,14,15,18,19] MRI usually shows a hypo- to isointense signal with adjacent muscle on T1-weighted images and a homogeneous or inhomogeneous hyperintense signal on T2-weighted images with marked enhancement on postcontrast T1-weighted images.[10,22,23] Sometimes, the mass has nonenhanced central portion, called a “target sign”, which corresponds to the presence of central necrosis on pathological examination.[10,23]
As observed in previously reported cases, our case showed a well-demarcated, expansile, and osteolytic mass affecting the L5 vertebra with a sclerotic rim on radiographs and CT. Cortical thinning with partial loss due to cortical expansion was demonstrated with pathologic fracture. On MRI, the mass showed a homogeneously isointense signal on T1-weighted images and an inhomogeneous slightly hyperintense signal on T2-weighted images. The mass revealed homogeneous avid enhancement with gadolinium. A “target sign” presenting central necrosis was not found. These findings were compatible with those of myofibroma. However, these findings were nonspecific. Therefore, solid variant of aneurysmal bone cyst and osteoblastoma appeared as well-demarcated, expansile, osteolytic lesions with a sclerotic rim were included in differential diagnoses of our case. Solid variant of aneurysmal bone cyst is seen homogeneously hypointense signal on T1-weighted images and inhomogeneously hypointense signal with scattered hyperintense signal areas on T2-weighted images with possible fluid-fluid levels, which is highly suggestive of the diagnosis of aneurysmal bone cyst. On postcontrast T1-weighted images, solid variant of aneurysmal bone cyst shows more homogeneous enhancement throughout the lesion than conventional aneurysmal bone cyst. Osteoblastoma has expansile osteolytic lesion at more than 1.5 cm in size with little evidence of nidus calcification. It may have multiple foci of matrix mineralization or extensive sclerosis on CT sometimes. MRI of osteoblastoma demonstrates hypointense signal on T1-weighted images, mixed signal intensity on T2-weighted images due to variable matrix mineralization, and well enhancement on T1-weighted images with gadolinium. In addition, infection, fibrous dysplasia, histiocytosis X, desmoplastic fibroma, metastasis, and myeloma could be included in differential diagnosis of myofibroma.
Histologically, the differential diagnosis for the present case included solitary fibrous tumor, fibromatosis, nodular fasciitis, and neurofibroma. The features supporting the diagnosis of myofibroma include well-defined contour, nodular biphasic pattern, characteristic pseudochondroid stromal hyalinization, and smooth muscle actin immunoreactivity.
The prognosis is generally good if the tumor does not have visceral involvement with cardiopulmonary failure, diffuse infiltration, or obstruction of the gastrointestinal tract. Complete spontaneous regression is frequently observed.[12,27,28] Surgical excision of solitary bone lesions is usually curative. Spontaneous regression may occur in solitary bone lesions as well as soft tissue lesions. Because spontaneous regression can be expected, the first choice of management is close observation. However, in our case, we performed marginal excision of the mass and posterior instrumentation of L4 to S1 with left-sided posterolateral fusion of L4-5 because it had right facet joint involvement of L4-5 by pathologic fracture, resulting in lower back pain in patient, and infiltration to right L5-S1 neural foramen. On follow-up CT of postoperative 1 year and serial follow-up radiographs of postoperative 3 years, the osteolytic lesion showed gradually decreased size and sclerotic change which might have occurred due to bony remodeling without tumor recurrence, indicating benign lesion. Follow-up MRI of postoperative 4 years revealed complete resolution of the lesion replaced by normal fatty marrow.
In conclusion, although myofibroma usually involves the dermis or subcutis in neonates and children younger than 2 year of age, it may manifest as a benign-looking expansile osteolytic lesion in the spine with inhomogeneous slightly hyperintense signal on T2-weighted image and homogeneous avid enhancement, as in this case. Thus, it should be included in the differential diagnoses of spine bone tumors. It can be resolved completely.
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