Ewing sarcoma

Growth patterns and typical localization

This neoplasm originates from the central parts of the bone, having a typical medullary origin (medullary in origin). Unlike many other bone sarcomas, it prefers to locate in the diaphyseal or metadiaphyseal region of long tubular bones.

Looking at the frequency of involvement of various skeletal segments, the picture is as follows:

• The femur and humerus (femur and humerus) are most commonly affected, in 31% of cases.
• The distal parts of the appendicular skeleton (distal appendicular skeleton), which include the bones of the hands and feet, are involved in 27% of cases.
• The pelvic bones (pelvic bones), among which the ilium is the most frequent, are affected in 21% of cases. Classical involvement of flat bones is often accompanied by a massive extension of tumor masses beyond the bone contour.
• The ribs (ribs) are affected relatively rarely—in less than 8% of cases.

A separate entity is the Askin tumor (Askin tumour). This is a rare variant of primitive neuroectodermal tumor (PNET) of the chest wall, which belongs to the same family and is diagnosed predominantly in children and young adults.

Radiological presentation

On standard radiographs, the tumor demonstrates pronounced features of aggressiveness. The main pattern is a permeative destructive lesion (permeative destructive lesion) with a wide zone of transition (wide zone of transition). It is important to remember that in the earliest stages, at the patient's first presentation, these destructive changes may be barely perceptible, requiring high diagnostic vigilance.

Of particular note are the periosteal reactions (periosteal reactions) that accompany the destruction of the cortical layer:

1. Spiculated type / "hair-on-end" (vertical 'hair-on-end'). The most classic sign, detected in half of patients. A similar picture is often found in osteosarcoma.
2. Interrupted type with Codman triangle formation (interrupted / Codman angle). Observed at the edges of the tumor focus in approximately 27% of cases.
3. Multilamellar type / "onion skin" (multilamellar / 'onion skin'). Contrary to popular stereotype, this is uncommon in clinical practice.

There is also a rare periosteal growth variant, in which radiographs show a saucer-shaped depression (saucerisation) of the outer cortical layer. In this case, the intraosseous component is small, and the tumor grows eccentrically, forming significant soft-tissue masses. Overall, the soft-tissue component (soft-tissue extension) is clearly visualized in 80% of Ewing sarcoma cases.

Imaging nuances in flat bones

When localized in flat bones and the spine, the radiological picture can change dramatically. The tumor often acquires a mixed or predominantly sclerotic appearance (sclerotic appearance). At first glance, this strongly resembles osteosarcoma.

However, the nature of this sclerosis is different: it arises solely from reactive bone sclerosis combined with the superimposition of a spiculated periosteal reaction when viewed "end-on" (end-on). The production of malignant osteoid is not characteristic of Ewing sarcoma—the tumor is not bone-forming.

When such destructive processes are identified, a strict differential diagnosis must be performed. On radiographs, the pathology should be distinguished from acute osteomyelitis (osteomyelitis), Langerhans cell histiocytosis (Langerhans cell histiocytosis), and stress fractures (stress fracture).

MRI and staging algorithm

Magnetic resonance imaging is the primary method for local staging (local staging). It allows for extremely precise determination of the boundaries of medullary infiltration and the true dimensions of the extraosseous soft-tissue component.

Signal intensity (relative to skeletal muscle) is distributed as follows:

• On T1-weighted images (T1 weighted) — intermediate intensity.
• On T2-weighted (T2 weighted) and STIR sequences — intermediate or high signal.

Ewing sarcoma metastasizes readily. During initial imaging evaluation, nearly 12% of patients are found to have bone marrow skip metastases (skip metastases). Distant metastases to the lungs and other skeletal sites are detected in less than 20% of cases.

To search for bone metastases and monitor the tumor's response to adjuvant chemotherapy, FDG-PET (FDG-PET) and whole-body MRI (whole-body MRI) are optimally used. These modalities have significantly higher sensitivity compared to conventional bone scintigraphy. Assessment of metastatic lung involvement, as in osteosarcoma, is performed using computed tomography (CT).

Features of bone metastases in children

Although Ewing sarcoma is a primary tumor, it is important to remember the specifics of metastatic bone disease (bone metastases) in pediatrics. In children, they are less common than in adults and almost never present as the first symptom of a hidden oncological process (by the time bones are involved, the primary focus is already known).

Bones are most frequently metastasized by:

• Neuroblastoma (neuroblastoma);
• Leukemia (leukaemia);
• Lymphoma (lymphoma);
• Rhabdomyosarcoma (rhabdomyosarcoma);
• Medulloblastoma (medulloblastoma);
• Ewing sarcoma family of tumors (ESFT).

On radiographs, such metastases most commonly appear as lytic destruction lesions (lytic appearances) with associated periosteal reaction. Sclerotic foci can occur in medulloblastoma and leukemia. A specific radiological sign of leukemia is also metaphyseal radiolucent lines (radiolucent lines), which can also be observed in rare cases in neuroblastoma.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Imaging features of osteosarcoma and its key differences from Ewing sarcoma
• Differential diagnosis of destructive bone lesions: histiocytosis and osteomyelitis
• Metastatic bone lesions in leukemia and neuroblastoma
• The role of PET-CT in evaluating the response of bone sarcomas to chemotherapy
• Rare malignant bone tumors in children: chondrosarcoma and lymphoma

Other topics in the "Musculoskeletal System" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.