Ewing's sarcoma is a rare malignant neoplasm, predominantly affecting young patients of the ages five to 20 years. It involves the diaphyses of long bones and occurs less commonly in flat bones . Clinical and laboratory features include local pain, soft-tissue swelling and erythema, occasionally accompanied with fever, anemia, leukocytosis, and accelerated erythrocyte sedimentation rate . It rarely affects the feet.
Cook listed 29 cases of Ewing's sarcoma of the calcaneus in the literature since 1921 . These rare cases are usually misdiagnosed, leading to treatment delay, which is detrimental to the outcome.
According to a retrospective study concerning 235 patients with non-metastatic Ewing's sarcoma of the bone, 15 patients were identified with a skip lesion at diagnosis. However, the skip lesions were located in adjacent juxta-articular bone in only two cases .
The radiographic features of Ewing's sarcoma in our case were those of classic Ewing's sarcoma: a permeative, lytic and condensed lesion with cortical destruction, aggressive periosteal reaction, large extraosseous soft-tissue component and often sclerotic reaction [5, 6]. In spite of clinical and radiological findings, Ewing's sarcoma can be misinterpreted as osteomyelitis, cartilaginous tumor, giant cell lesion, lymphoma or osteosarcoma, and the distinction often requires extensive evaluation using varied imaging modalities .
CT can reveal a soft-tissue mass of the foot, such as permeative lytic lesions of the bone with aggressive periosteal reaction and cortical destruction, but the distinction between osseous remnants, reactive changes and tumor matrix can sometimes be challenging . Bone scintigraphy of the whole skeleton demonstrates a focus of increased uptake of technetium-99 m-methylene diphosphonate [8, 9].
T2-weighted MRI cannot adequately distinguish tumor from necrosis, and lesion boundaries are frequently overestimated because of the presence of edema and hemorrhage . The enhancement pattern after administration of contrast medium on MRI allows differentiation between a tumor and peritumoral reactive edema. Furthermore, MRI can often distinguish the large solid sarcomatous soft-tissue mass around the involved bone from edema or an osteomyelitic abscess. MRI findings can narrow the differential diagnosis, but a specific diagnosis can rarely be established. Therefore, a biopsy of the tumor with histopathological analysis is needed to confirm the diagnosis. Staging, prior to biopsy, is essential to document the local and distant spread of the tumor. In Ewing's sarcoma, the metastatic pattern may be pulmonary involvement, bone or bone marrow spreading, skip metastases, or combined metastatic disease [7, 9].
The imaging features of local spread of Ewing's sarcoma, involving small bones to adjacent bones, have not been described in the recent literature. It wasn't possible to determine the exact local extent of the tumor by means of conventional radiography and CT [5, 9].
Due to its superior contrast resolution and multiplanar capabilities, MRI is more sensitive than other imaging techniques, especially for the investigation of tumor spread to bony structures and bone marrow. MRI should always be performed in the analysis of Ewing's sarcoma since it allows accurate evaluation of the tumor extent, which is decisive for treatment .
Skip lesions in patients with otherwise non-metastatic skeletal Ewing's sarcoma may be of the same importance as the molecular detection of marrow metastases, and possibly confer a worse prognosis. Newer imaging modalities like positron emission tomography-computed tomography and careful staging work-up may indicate that skip metastases in Ewing's sarcoma are more common than previously suspected [8, 10].