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Presented by Edward McCarthy, M.D. and prepared by Angelique W. Levi, M.D.
Case 6: A 14 year old girl had a painful upper arm for a few weeks.
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Week 29: Case 6
A 14 year old girl had a painful upper arm for a few weeks. Radiographically there was a symmetrical lytic lesion in the metaphysis of the proximal humerus. An MRI demonstrated that this lesion was filled with fluid./images/EMCcase6xray.jpg
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Answer: Unicameral bone cyst
Histology: Histologically, the lining of a unicameral bone cyst shows spindle cells, a few multinucleated giant cells, and irregular seams of reactive osteoid.
Discussion: Unicameral bone cysts are developmental disorders that occur almost exclusively in the proximal humerus or proximal femur. They always involve the metaphyseal portion of bone. However, as the patient ages, the lesion tends to migrate away from the epiphyseal plate. They are filled with fluid which can be demonstrated on an MRI. The fluid cavity is generally a single chamber. Tissue removed at surgery is a simple lining. Generally the small amount of tissue removed by curettage of the lining does not correlate with the seemingly large size on a plain film. This is because most of the lesion is fluid.
By contrast, fibrous dysplasia is a solid lesion. However, it may produce a very similar radiographic appearance. These lesions can be distinguished using an MRI. Fibrous dysplasia does not show a fluid filled cavity. Histologically, fibrous dysplasia consists of sheets of spindle cells with interlacing strands of osteoid.
Aneurysmal bone cyst is also a cystic lesion. However, the cyst is multiloculated, therefore, an MRI shows multiple cystic cavities with a blood serum line indicating that the cavities are filled with blood rather than serous fluid as in unicameral bone cyst. Histologically, aneurysmal bone cyst shows abundant multinucleated giant cells in a fibrous stroma with reactive bone. The key histologic feature, absent in unicameral bone cysts, is multiple blood filled lakes.
Non-ossifying fibroma is also metaphyseal lesion. However, radiographically this lesion is not symmetrical in the center of the metaphyseal zone. Rather, it is asymmetrical and centered on one cortex. Also, non-ossifying fibroma is not fluid filled. This indicates the importance of the MRI to the preoperative diagnosis of most bone tumors. Histologically, non-ossifying fibroma shows sheets of spindle cells in a storiform pattern admixed with multinucleated giant cells.
Incorrect
Answer: Unicameral bone cyst
Histology: Histologically, the lining of a unicameral bone cyst shows spindle cells, a few multinucleated giant cells, and irregular seams of reactive osteoid.
Discussion: Unicameral bone cysts are developmental disorders that occur almost exclusively in the proximal humerus or proximal femur. They always involve the metaphyseal portion of bone. However, as the patient ages, the lesion tends to migrate away from the epiphyseal plate. They are filled with fluid which can be demonstrated on an MRI. The fluid cavity is generally a single chamber. Tissue removed at surgery is a simple lining. Generally the small amount of tissue removed by curettage of the lining does not correlate with the seemingly large size on a plain film. This is because most of the lesion is fluid.
By contrast, fibrous dysplasia is a solid lesion. However, it may produce a very similar radiographic appearance. These lesions can be distinguished using an MRI. Fibrous dysplasia does not show a fluid filled cavity. Histologically, fibrous dysplasia consists of sheets of spindle cells with interlacing strands of osteoid.
Aneurysmal bone cyst is also a cystic lesion. However, the cyst is multiloculated, therefore, an MRI shows multiple cystic cavities with a blood serum line indicating that the cavities are filled with blood rather than serous fluid as in unicameral bone cyst. Histologically, aneurysmal bone cyst shows abundant multinucleated giant cells in a fibrous stroma with reactive bone. The key histologic feature, absent in unicameral bone cysts, is multiple blood filled lakes.
Non-ossifying fibroma is also metaphyseal lesion. However, radiographically this lesion is not symmetrical in the center of the metaphyseal zone. Rather, it is asymmetrical and centered on one cortex. Also, non-ossifying fibroma is not fluid filled. This indicates the importance of the MRI to the preoperative diagnosis of most bone tumors. Histologically, non-ossifying fibroma shows sheets of spindle cells in a storiform pattern admixed with multinucleated giant cells.
