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Presented by Edward McCarthy, M.D. and prepared by Jon Davison, M.D.
Case 6: A 35 year old woman had a swelling around her shoulder for the past year.
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Week 208: Case 6
A 35 year old woman had a swelling around her shoulder for the past year. A radiograph demonstrated an irregular bony mass protruding from the lateral aspect of her humerus.images/JMD_1-17-05_SPWC/Case_6/1.jpg
images/JMD_1-17-05_SPWC/Case_6/2.jpg
images/JMD_1-17-05_SPWC/Case_6/3.jpgCorrect
Answer: Parosteal osteosarcoma
Histology: none provided
Discussion: The lesion is a parosteal osteosarcoma because it is entirely confined to the surface of the distal humerus. There is no intramedullary involvement. In addition, the lesion shows mature lamellar bone and a fibrous stroma with only minimal increased cellularity and atypia. The pattern is that of a low-grade fibroblastic osteosarcoma. Low-grade fibroblastic osteosarcomas on the surface of the bone are, by definition, parosteal osteosarcomas. The atypia and mitotic activity is minimal. The lesion is not a conventional osteosarcoma because atypia and mitotic activities are minimal. This is a low grade (grade 1) lesion. In addition, this lesion does not involve the medullary canal as seen in conventional osteosarcomas. The lesion is not an osteochondroma because it does not have a cartilage cap and the cellularity of the stroma indicates a low grade neoplasm rather than a developmental process as seen in osteochondromas. The lesion is not heterotopic ossification because a zonal pattern is lacking and there is a mild atypia in the fibroblastic stroma. In addition, the radiographic pattern is that of an irregular, ill-defined neoplasm rather than the well-defined zonal pattern of heterotopic ossification.
Incorrect
Answer: Parosteal osteosarcoma
Histology: none provided
Discussion: The lesion is a parosteal osteosarcoma because it is entirely confined to the surface of the distal humerus. There is no intramedullary involvement. In addition, the lesion shows mature lamellar bone and a fibrous stroma with only minimal increased cellularity and atypia. The pattern is that of a low-grade fibroblastic osteosarcoma. Low-grade fibroblastic osteosarcomas on the surface of the bone are, by definition, parosteal osteosarcomas. The atypia and mitotic activity is minimal. The lesion is not a conventional osteosarcoma because atypia and mitotic activities are minimal. This is a low grade (grade 1) lesion. In addition, this lesion does not involve the medullary canal as seen in conventional osteosarcomas. The lesion is not an osteochondroma because it does not have a cartilage cap and the cellularity of the stroma indicates a low grade neoplasm rather than a developmental process as seen in osteochondromas. The lesion is not heterotopic ossification because a zonal pattern is lacking and there is a mild atypia in the fibroblastic stroma. In addition, the radiographic pattern is that of an irregular, ill-defined neoplasm rather than the well-defined zonal pattern of heterotopic ossification.
