**5. Staging**

Tumors are proliferations of atypical, autonomous, irreversible cell clones with a tendency to lose cell differentiation. Tumors classified as benign are those that do not present cellular atypia, grow pushing the adjacent tissues, demonstrate the histological aspect of low aggressiveness, low tendency to local recurrence, and low tendency to spread (production of metastases). On the other hand, neoplasms considered malignant have a variable degree of cellular atypia, grow infiltrating adjacent tissues, demonstrate a more aggressive histological aspect, a high tendency to local recurrence, and a high tendency to spread.

The Enneking classification [43] (**Table 3**), for bone tumors has a first structure for the evaluation of benign tumors, a second structure defined for the evaluation of malignant tumors, and both in order to present an evolutionary degree of the lesions according to the increase in the stage in the classification.

The classification of the American Joint Committee on Cancer [44] (**Table 4**), defines only primary malignant bone neoplasms (except for primary bone lymphoma and multiple myeloma). This classification evaluates factors, such as histological grade, presence of regional metastases (lymph nodes), or distance (pulmonary and non-pulmonary), in addition to the size of the lesion. Related to the tumor size, it is observed that Ewing's sarcomas ≤8 cm have a better prognosis than those >8 cm, as well as in osteosarcoma that lesions ≤8 cm have a better prognosis in relation to osteosarcomas >8 cm greater in size.

Another staging related to primary bone sarcomas is in relation to the histological response to neoadjuvant chemotherapy. The surgical specimen is evaluated to analyze the degree of tumor necrosis and in patients with osteosarcoma and Ewing's sarcoma who present a degree of necrosis ≥90% are classified as good responders and who generally have good survival. This type of analysis was developed by Huvos [45, 46] (**Table 5**), initially for patients with osteosarcoma, however, it demonstrates to be applicable to Ewing's sarcoma [47].


### **Table 3.**

*Enneking staging system for bone neoplams.*


### **Table 4.**

*AJCC staging system – 8th edition.*


### **Table 5.**

*Huvos's histologic grading of chemotherapy effect.*
