With the exception of case reports and small retrospective case series, no adequate data describing HNACC demographics are available [4, 9, 10]. In the current investigation, the gender incidence distribution showed a higher number of females than males, and there was a statistically significant predominance of HNACC in females (P < 0.001). HNACC could be found at any age, and the mean age at diagnosis was 50 years. Previous research on HNACC is based on single institutional experiences, and because of the small sample size, those studies are often not sufficiently powerful to find considerable differences in survival that are related to general demographic parameters [11–13]. In the survival analysis in our study, significant differences in DSS and OS were found to be related to age range, mean age, gender, race and marital status (P < 0.001). The most important findings regarding the demographics of HNACC are that gender and mean age are independent prognostic indicators for DSS and OS and that females under fifty years of age had more favourable prognoses than males over fifty.
TNM/AJCC staging plays an essential role in the planning of tumour treatment and in prognostic evaluation. The lymph node and distant metastasis rates of HNACC are low; the lymph node metastasis rate is less than 10%, and the distant metastasis rate is approximately five percent . In this circumstance, AJCC T stage plays a crucial role in prognosis evaluation; the larger the volume of the tumour is, the worse the prognosis is. In our results, AJCC T3 + T4 stage is an adverse independent OS prognostic indicator. There is a question of whether elective neck dissection should be performed in HNACC patients. In our DDS analysis, HNACC patients did not benefit from elective neck dissection. Patients with and without elective neck dissection showed no statistically significant differences in OS or in the survival analysis of the pathological grade III + IV subgroup and the AJCC T3 + T4 stage subgroup. In view of the low frequency of metastases, routine elective treatment of the neck is not recommended. However, elective neck dissection should be considered in the treatment of large and high-grade tumours .
In previous articles, pathological grade was used as an important prognostic reference [6, 16, 17]. Consistent with earlier reports, our results indicate that pathological differentiation is the strongest prognostic indicator. Compared to well-differentiated Grade I HNACC cases, Grades II, III and IV are unfavourably associated with DSS and OS. The pathologic subtypes of ACC have been reported [3, 18]. Unfortunately, the effects of these variants on prognosis could not be established because of the limited data. HNACC was mainly found in the parotid gland, and surgical resection was the primary treatment modality. Unlike the rest of the head and neck region, because of the presence of the facial nerve, the orientation of parotid gland ACC determines the type of surgery that is performed (partial parotidectomy, superficial parotidectomy or total parotidectomy). The choice of surgical type has a huge impact on the prognosis of patients with parotid gland malignancies [19, 20]. According to the distribution of sample size and site, we divided the study population into two groups: (a) parotid gland group and (b) beyond parotid gland group. The results of survival analysis show that there was no significant difference in the survival of these two groups. The SEER system does not provide detailed information about the location of parotid gland ACC or the type of surgery performed. Thus, we were unable to perform further analysis based on surgical type. In this cohort, 839 patients received radiotherapy; the prognosis of those patients was poor compared to that of patients who were treated with surgery alone. This result should be interpreted cautiously because in the clinical treatment of parotid gland ACC, radiotherapy is often implemented when adverse factors such as positive surgical margins or poor pathologic differentiation are present .
Several important limitations of this study are acknowledged. First, this SEER-based investigation was a retrospective analysis with some inherent bias. Some crucial data such as surgical type, surgical margins, and details of neck dissection could not be obtained, restricting further analysis. A geographic bias could not be totally avoided because of the nonuniform distribution of the SEER registries. Second, retrospective SEER-based analysis depends on accurate coding and consistent data collection among numerous sites, both of which can be imprecise. Third, due to the lack of complete data on adjuvant therapy, the role of radiotherapy and chemotherapy could not be well established. Finally, the retrospective nature of the study and the incompleteness of the clinicopathological data may weaken the strength of our conclusions.
In summary, despite the limitations of the incomplete data and of the study itself, to the best of our knowledge the present study is the first investigation to use a large study population and a long follow-up time to define the clinicopathologic characteristics and identify the prognostic indicators of ACC in the head and neck region. Three earlier reports described ACC cases reported in the National Cancer Database and the SEER database [2, 5, 14]. Although there is overlap among the study samples, the conclusions are complementary and enhance each other due to the different research intentions. Gender was the only demographic independent prognostic factor for both DSS and OS. With respect to treatment, surgical treatment was the only independent favourable prognostic factor for both DSS and OS. In addition, age, pathological differentiation, and TNM/AJCC stage were associated with DSS and OS.