Diagnosing Malignant Bone Tumors In The Lower Extremity
Many lesions of bone that arise with relative frequency in the feet are seen less commonly in alternate locations. Similarly, there are a host of trends with regard to the biology of bone tumors that are unique to the distal extremities, particularly the feet. With that said, let’s review the most common malignant tumors of the bones of the feet and key clinical features that can aid in arriving at the most appropriate diagnosis and subsequent treatment.
While it is far from the most common malignant tumor of the skeletal system at large, Ewing’s sarcoma/primitive neuroectodermal tumor (PNET) has been documented as the most common non-hematopoietic primary malignancy of the bones of the feet in series’ assembled at two major cancer centers and elsewhere.1,2,3 In the Memorial Sloan Kettering series, which summarized all tumors of the bones of the feet over a 15-year time span, this highly aggressive neoplasm accounted for 11 percent of all bone tumors and 25 percent of all those that were malignant.1 At the Mayo clinic, Dahlin and Unni noted Ewing’s sarcoma accounted for 28 percent of all malignant bone tumors involving the bones of the feet.2
Although Ewing’s sarcoma and primitive neuroectodermal tumor are considered variations of the same disease, Ewing’s sarcoma is commonly taken to connote tumors that lack neuroectodermal differentiation by light microscopy, electron microscopy and immunohistochemistry. The term “primitive neuroectodermal tumor” is applied when one or more of these techniques demonstrates such differentiation. Poorly differentiated tumors typically require a combination of light microscopic and genetic analysis to be diagnosed as Ewing’s sarcoma. Ewing’s sarcoma and PNET have a chromosomal translocation t(11; 22) (q24; q12) in common in 85 to 90 percent of cases.4,5 The remaining 10 to 15 percent of these malignant neoplasms are believed to have alternate mutations involving the Ewing’s sarcoma gene on chromosome 22.6
Ewing’s sarcoma may affect any bone of the foot including the phalanx although the calcaneus appears to be the most commonly involved.1,2,7-11 This is of profound clinical significance as calcaneal lesions impart a worse prognosis than those arising elsewhere in the foot.1,7 Researchers have shown that males are affected more commonly than females, both when this tumor is primary to the feet and when it is seen elsewhere in the skeletal system.1,2,8 Ewing’s sarcoma predominantly affects people in their second decade of life.1,7,8 People over the age of 30 are only rarely affected. For reasons that are unclear, African-American individuals are less commonly affected by Ewing’s sarcoma than Caucasians.
As is the case with many bone tumors, the radiologic features of Ewing’s sarcoma in the feet contrast somewhat with the radiologic features of the sarcoma when it affects the long bones.12 When Ewing’s sarcoma is present in the long bones of the appendicular skeleton, one would note a diaphyseal-based tumor with admixed lytic and sclerotic features, and characteristic “onion skinning.” However, when Ewing’s sarcoma is present in the feet, onion skinning is an uncommon finding. While plain film radiographs may demonstrate a host of non-specific features that are suggestive of malignancy, they may not be particularly indicative of Ewing’s sarcoma. Reinus, et. al., noted transcortical permeation in 75 percent of cases, diffuse sclerosis in 42 percent of cases, pure lysis in 25 percent of cases, bone expansion in 33 percent of cases and cystic degeneration in 25 percent of cases of Ewing’s sarcoma involving the foot.