Following clinical examination, imaging of the tumor, evaluation of possible metastatic disease (by chest CT and bone scan), and determination of the tumor type and grade of the lesion by the pathologist, it is appropriate to determine the stage of the lesion.
Staging describes the anatomic extent of the lesion, as well as the "degree of malignancy" and potential for development of metastatic disease. The stage of the lesion allows the clinician to prescribe treatment tailored to the patient's particular lesion while also giving the patient prognostic information regarding potential for cure and local control. Most aspects of the patient's definitive management will be based on staging.
Staging bone tumors is based on three characteristics of the tumor:
- The histological grade
- The local extent of the lesion
- The presence of metastases
A straightforward staging system was introduced by Enneking and defined as the Surgical Staging System (SSS) by the Musculoskeletal Tumor Society.
The histological grade provides the best estimate of the likelihood of metastases occurring from the tumor. Tumors may be characterized as:
- Grade 0, or benign: No risk of metastatic disease
- Grade I: Low risk of metastatic disease
- Grade II: High risk of metastatic disease
This analysis is based on both cytological features of cells (nuclear atypia, mitotic activity) and the tumor type (for example, Ewing's sarcoma implies that it is a high-grade sarcoma).
The local extent of the tumor can be described by whether it has extended beyond the compartment of origin. Most bone tumors originate in the bone and expand through the cortex with tumour progression. The lesion is described as
- Intra-compartmental if it is confined within the limits of the periosteum
- Extracompartmental if there is tumor extending beyond periosteum
On the other hand, some lesions arising on the surface of the bone (parosteal osteosarcoma, for example) are considered extracompartmental if the lesion extends within the bone. Intra-compartmental tumors are designated as "A" lesions, and extra-compartmental lesions are designated as "B" lesions.
The final issue of staging relates to the presence of metastatic disease. All patients with potential malignant bone tumors should undergo systemic staging with both bone scan and chest CT. Patients with recognized metastatic disease are characterized as Stage III.
Summarizing these three descriptive factors, benign lesions are Stage 0, low grade malignant lesions are Stage IA or IB, and high grade malignancy Stage IIA or Stage IIB. Patients with metastatic disease are Stage III.
These staging characteristics are not only useful for describing the extent of disease, but also can be used in directing therapy:
- Stage 0 lesions, for example, are usually treated with simple removal of the lesion and reconstruction of the bone.
- Stage I lesions should undergo complete resection of the lesion to avoid local relapse and possible late metastasis (although as mentioned above, this paradigm is often violated for giant cell tumor).
- Stage II lesions frequently are treated with both surgery and chemotherapy in order to decrease the risk of metastatic disease.
- Protocols for Stage III tumours often include chemotherapy and resection of metastatic disease by thoracotomy.
Staging permits the oncologist to explain the potential risks and outcomes of the tumor diagnosis to the patient and to develop a treatment plan appropriate to the tumor biology. Understanding the staging of musculoskeletal tumors, therefore, is critical to the diagnostic process.