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Bone scan
Radiographs
Magnetic resonance imaging
Resection specimen - third toe of left foot
MicroscopyPlanar images show contamination in the area of the left sacrum. There is increased activity in a left toe. Spot views of the feet confirm the focally increased activity, most likely in the third or fourth toe on the left. Correlative plain films show an expansile, lytic lesion involving the proximal phalanx of the third toe. The overlying cortex appears to be intact. There is no clear matrix, obvious soft tissue mass, or periosteal reaction.
A T1-weighted MR image demonstrates nearly complete replacement of the marrow fat in the proximal phalanx. It is difficult to see if the overlying cortex is intact throughout. A fat-saturated image shows peripheral enhancement. The STIR image shows diffuse T2 hyperintensity that does extend beyond the contour of the bone.
The third toe of the left foot was resected. A cross-sectional view of the gross specimen shows a homogeneous white mass infiltrating through the bone and periosteum. Microscopy demonstrates intraosseous proliferation of spindle cells in a fascicular pattern. At higher magnification, the nuclear morphology is bland morphology. There is no necrosis and the mitotic rate is very low. At this point, the differential diagnosis inclues intraosseous myoepithelioma, intraosseous schwannoma, and myofibroma. Immunohistochemical analysis was positive for several markers (S100, Cam 5.2, EMA, and AE1/3) that confirmed the diagnosis of myoepithelioma.
Radiology Discussion:
Intraosseous spindle cell myoepithelioma is exceptionally rare; searching on this diagnosis returns no results in Medline. It is not included in the differential diagnosis because no one, when faced with a lucent bone lesion, would think of it. Narrowing the likely diagnosis of a solitary lucent bone lesion is based on six steps. Step 1 is to consider the most likely possibilities, represented by the mnemonic "FOG MACHINES":
Step 2 focuses on the patient’s age. According to Edeiken, 80% of malignant tumors can be correctly diagnosed on the basis of age alone. For a solitary lucent bone lesion, the most likely diagnoses by age include:
Step 3 is to determine how aggressive the lesion is. The continuum begins with a normal bone appearance in a non-aggressive lesion. The insidious nature is assumed to increase through the following:
Step 4 focuses on the matrix. Most lesions do not produce a matrix and thus appear radiolucent. A chondroid matrix, characterized by a "rings and arcs" appearance, is typical in enchondroma, chondrosarcoma, chondromyxoid fibroma, and other cartilage-based tumors. An osteoid matrix, typically described as "cloud-like", is a common feature of osteoma, osteoblastoma, bone island, and osteosarcoma.
Step 5 considers the periosteal reaction. Periosteal reaction varies widely and represents involvement of the outer cortical rim by the tumor. Its appearance can also be described by a continuum from non-aggressive to very aggressive. Solid periosteal reaction, the least aggressive type, can be caused by infection, benign neoplasms like osteoid osteoma and eosinophilic granuloma, hypertrophic pulmonary osteoarthropathy, or deep venous thrombosis (in the lower extremity). More aggressive reactions, including lamellated, "hair-on-end" or sunburst appearances, or Codman’s triangle, can be caused by osteomyelitis or malignant neoplasms, such as osteosarcoma, chondrosarcoma, fibrosarcoma, lymphoma, leukemia, and metastasis.
In the final step, the location within the bone is taken into account. Chondroblastoma and osteomyelitis typically arise in the epiphysis. The metaphysis is the most likely location for a primary neoplasm other than chondroblastoma, including all other entities recalled by the "FOG MACHINES" mnemonic. Similarly, all FOG MACHINES entities except chondroblastoma, giant cell tumor, and osteoblastoma may arise in the diaphysis.
By careful examination of the features of the lesion you can categorize the lesion as aggressive or non-aggressive. These features, considered within the context of the patient’s demographics and the location of the lesion, allow a reasonable differential diagnosis prior to biopsy and will serve as a guide to planning surgical management.
Richardson M. Approaches To Differential Diagnosis In Musculoskeletal Imaging. http://www.rad.washington.edu/mskbook/lucent.html
Parsons TW. Radiographic Imaging of Musculoskeletal Neoplasia. Cancer Control: Journal of the Moffitt Cancer Center http://www.moffitt.usf.edu/pubs/ccj/.
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