**2. Epidemiology**

OS itself is a rare tumor, accounting for less than 1% of all cancers diagnosed annually in the United States [4], and osteosarcomas presenting on the head and neck region are even more uncommon, with incidence approximately of 2–3 per million persons per year [6].

In addition, OSHN has specific demographic characteristics at diagnosis.

While the peak incidence of extremity osteosarcomas occurs during adolescence, HNOS generally presents at a later age, albeit with significant variability. According to different series, HNOS usually presents between the third and fourth decade of life, with a wide range. Kassier et al. [7], for example, in a meta-analysis of non-randomized studies between 1980 and 1994 with 173 patients, report a median age of presentation of 36 years, with a range from 5 to 78 years. Smith et al. [8], in a review of the US National Cancer Database (NCDB) cancer registry, with 496 patients diagnosed with HNOS between 1985 and 1996, describes a median age at presentation of 38 years, with 41% of patients aged 30–60 years, 35% younger than 30 years, and 24% older than 60 years. In this study, it is also noteworthy that the age at presentation was strikingly lower in men than in women (34 vs. 44 years respectively, p < 0.001). Finally, in a study conducted by Lee et al. [9], published in 2015, using the Surveillance, Epidemiology and End Results (SEER) cancer registry database to determine the epidemiology and prognostic factors associated with osteosarcoma of the jaw (OSJ), with 541 patients (1973–2011), reported an age at presentation with a median age of 41 years and a range of 0–91 years [9]. Furthermore, the demographic distribution of this study showed 75% where white, 17% African-American, and 8% other races, and in terms of gender as a risk factor, in these two large cohorts, the sex distribution was equal, with a 1:1 ratio [8, 9].

Regarding the clinical subsite, HNOS affects the jaw in more than 80% of cases, with the mandible usually being the most common site [10, 11]. In the mandibular region, it frequently involves the mandibular body and ramus, and in the maxilla, the upper alveolar ridge, maxillary sinus floor, or hard palate. In fact, in a study by Guadagnolo et al. [10] from MD Anderson, with 119 cases of craniofacial OS, they observed 45% mandibular OS, disease in the mandible, 40% maxilla, calvarium 5%; paranasal sinuses 2%, hard palate, 2%; mastoid, 2%; skull base, 1%; zygoma, 1%; infratemporal fossa, 1%; and cervical soft tissues 1%. However, some series report a slightly higher percentage in the skull and facial bones, as in the Smith et al. [8] analysis of the NCDB where the majority of patients (55.6%) had HNOS of the skull and facial bones, and HNOS of the mandible accounted for 38.9%. Approximately 5% of patients had HNOS tumors in the other subsites, which included the soft tissues of the head and neck, parotid gland, the nasopharynx, and the tongue [8]. Likewise, in the SEER series by Lee et al. [9], the distribution of HNOS was 55.6% in the skull or facial bones and 44.4% in the mandible [9].
