The pathological types of parotid carcinoma are various. Adenocarcinoma NOS and adenoid cystic carcinoma are not uncommon in parotid carcinoma. In the SEER database, adenocarcinoma of NOS, adenoid cystic carcinoma, and other rare types of adenocarcinoma were classified as adenocarcinoma. In this study, we analyzed the efficacy of radiotherapy in postoperative patients with adenocarcinoma. Postoperative radiotherapy can significantly improve the prognosis of patients in the total cohort, and further analysis based on patient prognostic risk model (ATNG) model revealed that the specific crowd benefiting from radiotherapy were patients with high prognostic risk. Our findings suggest the potential efficacy of RT may be heterogeneous when certain prognostic risk factors (ie, advanced T stage, node involvement, old age, and high nuclear grade) are present.
PANOS and adenoid cystic carcinoma are two common types of aggressive adenocarcinoma of the parotid gland. PNOS was an invasive and high-risk pathological type of parotid cancers, with a propensity for nodal involvement and low survival rates[4, 18–21]. Total or radical parotidectomy with routine selection neck dissection was an appropriate type of surgical procedure. For patients with advanced stages, the combination of surgery and radiotherapy significantly improved the OS compared with surgery alone. Adenoid cystic carcinoma (ACC) is also a special invasive adenocarcinoma with a low survival rate for frequent local recurrence and high rates of distant metastasis[22, 23], the heterogeneity of ACC increases the difficulties in diagnosis and resulted in variable clinical outcomes[7]. Although the comprehensive therapy has achieved superior results in many studies, and postradiotherapy was recommended as 2B evidence by NCCN guidelines, and the optimum treatment strategy for ACC has not yet been finally determined. However, studies specifically on treatment decisions are scarce limited to case series, and studies on other pathological types of parotid adenocarcinoma are even rarer. The radiosensitivity of these tumors has been a hot topic of research, but the prognostic value of RT remains controversial[24, 25].
Histological grade, tumor stage, node involvement, perineural invasion, and surgical margin were reported as the possible determinants of local control and long-term survival of parotid adenocarcinoma in previous studies. In our study, the conclusions of previous studies were further confirmed, tumor stage, node involvement, and histological grade were independent risk factors for CSS and OS, older age was also proved to be an independent indicator of the poor OS. The benefit of postoperative radiotherapy (PORT) for parotid was mainly proved by previous retrospective studies. PORT was an important adjuvant setting reserved for parotid cancers with adverse prognostic factors, such as advanced stage, nodal metastases, high grade, close or positive surgical margins, extracapsular spread (ECS) and perineural invasion[26, 27]. Otherwise, some studies suggested all patients with ACCa received PORT[28], and recommended PORT as 2B evidence in NCCN guidelines. It's critical to achieve maximal local control with the aid of the adjuvant setting. However, there is no consensus on whether the improvement of local control derived from RT would ultimately improve the survival, and the adverse effect of RT also aroused the concern of clinicians. Therefore, screening the specific population benefit from PORT is necessary to avoid overtreatment. Our study showed an improvement of OS for RT group compared with patients treated with surgery alone. Further analysis according to risk stratification showed survival improvement for patients with high prognostic risk parotid adenocarcinoma treated with adjuvant radiotherapy, and the patients with low and medium prognostic risk received no benefit from RT. These conclusions need further confirmation from other studies.
The prognostic value of biomarkers for parotid cancers has been investigated by several researchers, these biomarkers can not only predict the survival of parotid cancers but also exert influence on treatment decisions[29, 30]. What’s more, these biomarkers might be attractive therapeutic targets in patients with parotid cancers, and the prognosis associated biomarkers will further enrich the clinical prognostic model, whereas our model needs further validation to confirm how much additional prognostic information could be derived from its use.
There are several limitations to our study. Limited to the data available in the SEER database, the information such as surgical margin, immunotherapy, and patient comorbidities cannot be included in our prognostic model, which might influence the results, thereby our results should be interpreted prudently. The efficacy of RT will be overestimated if the surgical margin is close or positive. In contrast, the efficacy might be reduced when patients are suffering comorbidities. The postoperative patients with close or positive surgical margin were recommended RT and the patients’ comorbidities might reduce the RT rate, our prognostic model might be more straightforward for high-risk patients in whom the RT benefit is clear.
This is the first study to investigate the survival benefit of RT after surgery for parotid adenocarcinoma based on individualized patient risk factors. Our results suggest that further studies were needed to validate this prognosis model, and reduced the rate of overdiagnosis and overtreatment of parotid adenocarcinoma. In conclusion, our study constructs and validates a prognostic model for parotid adenocarcinoma, which can predict not only the survival but also the right patients benefit from RT after surgery. our findings need to be further confirmed by other large population-based studies or prospective studies.