**6. Staging**

Unlike the vast majority of cancers, in OS, the staging system must incorporate not only local and distant spread, but also the degree of differentiation, in order to estimate the prognosis of the patient.

The commonly used lymph node metastasis (TNM) staging system is not commonly used for HNOS because they are unlikely to metastasize to lymph nodes. Also, the current version has been tailored for OS long bones, so it is not entirely applicable to the head and neck region, for example, the tumor size for T1 is up to 8 cm, which in this anatomical subsites generally represents a very locally advanced tumor (**Table 1**).

The most commonly system used most often to formally stage bone sarcomas is known as the Musculo-skeletal Tumor Society (MSTS) or Enneking system [13]. It is based on the grade (G) of the tumor, the local extent of the primary tumor (T), and whether or not it has metastasized to regional lymph nodes or other organs (M). The extent of the primary tumor is classified as either intra-compartmental (T1), which refers to the tumor remaining confined to the subsite in which it originated, or extracompartmental (T2), meaning it has extended into other nearby structures. Tumors that have not spread to the lymph nodes or other organs are considered M0, while those that have spread are M1 (**Table 1**) [25].


### **Table 1.**

*AJCC 8th edition TNM staging for bone sarcomas [24].*

In summary, with this staging system, low-grade tumors are defined as stage I, regardless of extend of primary tumor, high-grade tumors as stage II, and metastatic tumors (regardless of grade) as stage III (**Table 2**).

At presentation, Lee et al. describe 18.5% of patients with stage IA disease; 0.7%, stage IB; 24.4%, stage IIA; 2.2%, stage IIB; 10.7%, stage III, stage IVA, or stage IVB (advanced disease); and 43.5%, unknown stage [9]. In Smith's et al. analysis of the NCDB, of the 487 patients with tumors that could be staged, an AJCC stage was recorded for only 56.1%, of which 90.1% of patients with locally confined (Stage I


### **Table 2.**

*AJCC 8th edition prognostic stage groups for bone stage primary tumor (T) sarcoma in the appendicular skeleton, trunk, skull, and facial bones.*

45%, stage II 38.9%, stage III 6.2%, and stage IV 9.9%). Interestingly, in this study, a difference was noted with regard to stage distribution by tumor location, with mandibular tumors being more likely to remain localized than skull/facial bones (92.7 vs. 82.8%, p = .032) and the other craniofacial bony sites showing higher rate of metastases of metastases than mandible (10.5% vs. 3.3) [8].
