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Presented by Edward McCarthy, M.D. and prepared by Anil Parwani, M.D.,Ph.D
Case 3: A 35 year old woman had a swelling around her knee for the past year.
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1. Question
Week 111: Case 3
A 35 year old woman had a swelling around her knee for the past year. A radiograph demonstrated an irregular bony mass protruding from the posterior aspect of her femur./images/102102case3fig1.jpg
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/images/102102case3fig4.jpgCorrect
Answer: Parosteal osteosarcoma
Histology: The lesion is a parosteal osteosarcoma because it is entirely confined to the surface of this distal femur. There is no intramedullary involvement. In addition, the lesion shows a mature lamellar bone and a fibrous stromal with only minimal cellularity and atypia. This 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.
Discussion: 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 stromal 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 stromal. 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: The lesion is a parosteal osteosarcoma because it is entirely confined to the surface of this distal femur. There is no intramedullary involvement. In addition, the lesion shows a mature lamellar bone and a fibrous stromal with only minimal cellularity and atypia. This 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.
Discussion: 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 stromal 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 stromal. In addition, the radiographic pattern is that of an irregular ill-defined neoplasm rather than the well-defined zonal pattern of heterotopic ossification.
