Grading and staging
Staging incorporates the degree of differentiation as well as local and distant spread, in order to estimate the prognosis of the patient. The universal TNM staging system is not commonly used for sarcomas because of their rarity to metastasize in lymph nodes. The system used most often to formally stage bone sarcomas is known as the Enneking system. It is based on the grade (G) of the tumour, the local extent of the primary tumour (T), and whether or not it has metastasized to regional lymph nodes or other organs (M).
The grade is divided into low grade (G1) and high grade (G2).
The extent of the primary tumour is classified as either intracompartmental (T1), meaning it has basically remained in place, or extracompartmental (T2), meaning it has extended into other nearby structures.
Tumours that have not spread to the lymph nodes or other organs are considered M0, while those that have spread are M1.
These factors are combined to give an overall stage (table 2).
|IIIA||G1 or G2||T1||M1|
|IIIB||G1 or G2||T2||M1|
In summary, low-grade tumours are stage I, high-grade tumours are stage II, and metastatic tumours (regardless of grade) are stage III.
Osteosarcoma can be localized or metastatic. Localized tumours are limited to the bone of origin, although local skip metastases may be apparent within the bone, indicating a worse prognosis. Radiologic evidence of metastatic tumour deposits in lung, other bones and other distant sites is found in 15% to 20% of patients at diagnosis; 85% to 90% of metastatic disease is in the lungs. The second most common site of metastasis is another bone.