After extracting the patients through the database search using the ICD codes and key words, the records of individual patients were scrutinized to exclude patients with other diseases, patients in whom the diagnosis was not based on histological evaluation at our institution, and patients who were referred solely for a second opinion. Finally, 173 patients with SGTCs, including SDC, AdCC, MEC, EMC, AcCC, or PAC at any primary site were identified. Table 1 shows the characteristics of the patients. The cohort comprised 45% women, with a median age of 64 (range 6–92) years. Patients with SDC, AdCC, MEC, EMC, AcCC, and PAC accounted for 20%, 42%, 27%, 3%, 8%, and 1% of the entire cohort, respectively. There was statistically significant difference in the gender distribution according to the histological type where male was more predominant in SDC than AdCC and MEC. The patient age also differed significantly among the histological types (Fig. 1). In regard to the age distribution as a function of the histological type, the patients with SDC and AdCC were predominantly in their 70s, whereas patients with MEC were predominantly in their 60s. Patients with AcCC distributes evenly throughout all age group without a peak. The smoking status did not differ significantly among the histological types. The percentage of patients with nodal involvement/distal metastasis differed significantly among the histological types, with SDC patients having more positive N and M factors than other histological types (Table 1).

Age distribution of the patients with SDC (a), AdCC (b), MEC (c) and AcCC (d) are depicted. The horizontal axis represents the age groups, while the vertical axis indicates the number of patients. Patients with SDC and AdCC were predominantly in their 70s, whereas patients with MEC were predominantly in their 60s. Patients with AcCC distribute evenly throughout all age group without a peak

The primary sites of the tumors in the 173 patients were the major salivary glands (n = 110), H/N exc MSG regions (n = 47), broncho-pulmonary regions (n = 10), and “others” (n = 6). The numbers of patients with each histological type in each of the major salivary glands and each of the H/N exc MSG regions are shown in Table 2. Specific information about patients classified as having tumors arising from sites included in the “others” category is presented in Table 3. Among the tumors arising from the major salivary glands, 70% (77/110) originated in the parotid gland, 24% (26/110) in the submandibular glands, and 6% (7/110) in the sublingual glands. In regard to the tumors arising from other H/N exc MSG regions, paranasal sinuses were the most frequently affected sites, followed by the nasal cavity, oral floor, and the nasopharynx (Table 2). In 6 patients, the SGTCs arose from sites included in the “others” category, including the orbit, lacrimal gland, Bartholin gland, and skin (Table 3).

The distribution of the histological type varied significantly among the primary sites. The most frequent primary sites of origin of SGTCs overall, SDC, MEC, EMC and AcCC were the major salivary glands, followed by H/N exc MSG regions. SDC was exclusively originated from major salivary glands. In contrast, the most frequent sites of origin of AdCCs were H/N exc MSG regions, especially nasal/paranasal sinus, followed by the major salivary glands (Table 1). The cohort included a single patient of PAC arising from the palate (Table 2). SGTCs originating from the broncho-pulmonary regions consisted of 6 patients with AdCC and 4 patients with MEC. Distribution of the histological types also differed significantly among the major salivary glands: 80% (28/35) of all SDC and 71% (25/35) of all MEC arose in the parotid gland, whereas 50% (12/24) of all AdCC arose in the submandibular gland. In all the cases of AcCC (n = 11) arising from the major salivary glands, the primary site was the parotid gland (Table 2).

The treatment strategies adopted for the patients overall, and those according to the primary sites and histological types are presented in Table 4. Overall, 40% (70/173) of patients were treated by surgery alone, and an additional 33% (57/173) were treated by surgery in combination with other modalities (Table 4). In regard to the treatment method according to the primary site, surgical treatment alone or combined with other modalities was undertaken in 84% (92/110) of cases of SGTCs arising from the major salivary glands, 49% (23/47) of SGTCs arising from other H/N exc MSG regions, 90% (9/10) of SGTCs arising from the broncho-pulmonary regions, and 50% (3/6) of SGTCs arising from sites classified as “others.” In regard to the treatment according to the histological type, surgical treatment alone or combined with other modalities was undertaken in 91% (32/35) of cases of SDC, 57% (41/72) of cases of AdCC, 85% (39/46) of cases of MEC, 67% (4/6) cases of EMC, and 92% (12/13) of cases of AcCC. There were statistically significant differences in the therapeutic strategies adopted according to the primary site and histological type. More specifically, patients with tumors arising from the major salivary glands were more often treated by surgery than those with tumors arising from other H/N exc MSG regions. SDC, MEC and AcCC were more often treated by surgery than AdCC (Table 4). Consequently, the overall survival (OS) rates of all the patients with SGTCs were good, with 1-, 3-, and 5-year survival rates of 96%, 83%, and 76%, respectively. At the median follow up period of 50 months, 74% of the patients were alive, and the median OS was not yet reached (Figs. 2a). The OS tended to vary according to the histological type (p = 0.0019), and patients with MEC showed a better prognosis than those with SDC (p = 0.0007) or EMC (p = 0.0048), with 5-year OS rates of 86%, 58%, and 50%; and patients with AdCC showed a better prognosis than those with EMC (p = 0.010) with 5-year OS rates of 78% and 50%, respectively; the differences were statistically significant (Fig. 2a). As for prognosis according to the primary site, there was no statistically significant differences among the five categories (Fig. 2b). In addition, there was no statistically significant differences among the three subcategories of patients with SGTCs originated from H/N exc MSG regions (Fig. 2c).

Overall survival curves are presented based on the histological type (a), and according to the primary site (b), together with overall survival curves of the three subcategories of H/N exc MSG (c). The curves for all patients are duplicated in (a) and (b). Vertical lines denote censored cases. The log-rank test was conducted to compare among all groups together