Background: Salivary Adenoid Cystic Carcinoma (ADCC), also known as “Wolf in Sheep's clothing,” is associated with poor prognosis. Regardless of the vigorous treatment, ADCC has been known to recur and metastasize. Hence, identifying informative prognostic biomarkers for Salivary ADCC is of great importance to better predict tumor behavior and to guide treatment planning. Various immunohistochemical biomarkers along with other factors like the histologic grade of the tumor, site of tumor, and age have been studied to establish their correlation with the prognosis of ADCC. Aim: The aim of the systematic review was to identify various markers that have the potential to predict the prognosis in salivary ADCC. Materials and Methods: A literature search was conducted using PubMed and Scopus as database and Google Scholar as an additional source. Studies that performed immunohistochemical analysis predicting the overall survival of patients with salivary ADCC were included in this review. Studies published from 1977 to August 2018 were included. Case reports, review, letter to editors, and articles in languages other than English were excluded from the review. The following outcomes were examined: overall/disease-free survival, metastasis, and recurrence using immunohistochemistry as a prognostic marker. Results: A number of biomarkers were identified by the evaluation of 68 studies, which predicted overall survival and prognosis in terms of recurrence and metastasis. Out of these biomarkers, four markers most frequently assessed markers were Ki-67, p53, vascular endothelial growth factor (VEGF), and cyclin D1, which showed reproducible results. Many other markers showed significant results, but since only a single study was carried out with these markers, it was difficult to assess their predictability. Conclusion: The review thus identified that Ki-67, p53, VEGF could effectively predict metastasis and recurrence in salivary ADCC. More research work is, however, required to validate the accuracy of these markers for their prognostic significance. Many other markers also showed a significant correlation to prognosis; however, multiple studies are required to establish their prognostic value.

Introduction

Adenoid cystic carcinoma (ADCC) is a relatively uncommon salivary gland neoplasm and accounts for 10% of all salivary gland tumors having a predisposition for minor salivary glands.[1] Among the major salivary glands, parotid gland ADCC accounts for 2%–3% of all tumors.[2] Although ADCC appears innocuous due to its slow growth and size, it has an invasive potential.[3] The clinical course of ADCC is aggressive and has a predisposition to distant metastasis and recurrence after the removal of the primary tumor. The most common site of distant metastasis of ADCC is the lungs.[2] This tumor also has a tendency for perineural invasion.[3]

Histologically, ADCC shows ductal and myoepithelial cell differentiation and is also known as “cylindroma” due to its histologic appearance. Three patterns of ADCC have been recognized: solid, tubular, and cribriform. The cribriform type shows a typical Swiss cheese pattern due to the cyst-like spaces within the tumor. The solid pattern is found to be associated with poor outcome.[3]

Complete surgical resection followed by radiotherapy remains the treatment of choice for ADCC. However, recurrence and distant metastasis still prevail. The median survival duration following disease metastasis is about 3 years.[4] Hence, it becomes necessary to identify the various factors that predict the prognosis of the tumor and improve the survival outcome. Although factors such as histologic grade, age, site and size of the primary tumor and positive resection margins have been suggested to have an impact on the survival and prognosis of the tumor, their role has not been clearly established.[3] Hence, it becomes essential to recognize biomarkers that could predict the prognosis in terms of metastasis and recurrence.

Various immunohistochemical markers have also been studied that speculate the progressive nature of ADCC. Immunohistochemistry is often used to establish the diagnosis of ADCC. Expression of biomarkers like p53 and Ki-67 have been associated with poor prognosis of the disease. Few markers like bcl-2 have also been predicted to have a role in the pathogenesis of ADCC. However, the prognostic specificity of such markers remains unexplored.[3]

The aim of this systematic review was to compile the data that shows the expression of various immunohistochemical markers and its correlation with recurrence/metastasis along with the overall survival of the patients, thereby aiding the surgeon to render effective treatment.

Materials and Methods

Location

A comprehensive search of PubMed and Scopus databases, along with additional searches in search engines such as Google Scholar, was done.

Data selection

The following keywords were used for the selection of studies - tumor markers AND prognosis AND salivary ADCC. Synonyms of the keywords were also used for the search. Studies published from 1977 to August 2018 were included. Only studies that were published in English were selected. Additional citations identified through the reference lists of the selected studies and bibliographic linkages were included in the review. Editorials, review, case reports, study on cell lines, articles in languages other than English were excluded from the study.

Data extraction and synthesis

A standard pilot form in excel sheet was initially used. Data extraction was done for one article and this form was reviewed by an expert and finalized. This was followed by data extraction for all the articles.

Results

The objective of the study was to evaluate the efficiency of various biomarkers in predicting the prognosis of salivary ADCC. Through the initial literature search, 326 articles were retrieved, of which 59 articles were excluded due to overlapping of data. By screening of titles, 158 articles were selected. The abstracts and full texts of these 158 articles were further screened for relevance and 58 articles were excluded. Twenty-seven case reports and reviews were ruled out. Of the remaining 73 articles, 5 articles were excluded due to data in other languages. Hence, a total of 68 articles were selected for data extraction [Figure 1]. A number of immunohistochemical markers showed prognostic significance in salivary gland ADCC. Ki-67, p53, vascular endothelial growth factor (VEGF), and cyclin D1 are the makers that were most frequently assessed, which also showed reproducible results.{Figure 1}

Study characteristics are provided in [Table 1].{Table 1}

Discussion

The combination of keywords tumor markers, prognosis, metastasis, patient survival, and salivary ADCC used for literature search in the databases showed 326 articles in the initial search, of which 68 articles fitted our criteria of immunohistochemical markers showing prognostic significance in salivary ADCC.

The primary objective of this systematic review was to identify the IHC markers, which could predict the prognosis of salivary ADCC in terms of recurrence, metastasis, and overall survival.

Many studies evaluated IHC markers such as Myb,[5] SMR3A,[6] c-MET,[7] LC3,[8] ALDH1,[9] CEACAM1,[10] PIN1,[11],[12] bcl-2,[13],[14] and HIF-1a[15] for their prognostic significance but no substantial correlation was found between the expression of these markers and prognosis. A study by Shintani et al.[16] showed that tenascin, an extracellular matrix protein, plays a role in the invasion of ADCC, but its relation with distant metastasis and recurrence was not demonstrated.

Few studies demonstrated the role of markers such as VEGF,[17] MCM3,[18] CD166,[19] LC3, and LAMP[20] as diagnostic markers for malignant tumors, thereby differentiating them from benign tumors. This highlighted their role in the progression of the tumor. However, their role as prognostic indicators was not established in these studies.

Other markers like MACC1,[21] EphA2/ephrinA1,[22] and NCAM[23] did not show any correlation with prognosis of tumor but were closely related to other clinicopathologic characteristics like perineural invasion. Similarly, Xia et al.,[24] showed decreased immunohistochemical expression of EDNRB to have a significant correlation with the growth of tumor, but no correlation was found between its expression and other characteristics like metastasis.

Dos Santos et al.[25] in a study showed that the transformed areas, in high-grade transformation of ADCC, had an increased expression of adipophilin as compared to the conventional areas, thereby suggesting the role of lipid droplets in proliferation and progression of ADCC. However, no data was available for its role in other factors of prognosis.

Further, many markers like NPM1,[26] BNIP3,[27] HIF-1a,[27] USP22,[28] FAK,[29] H3K9me3,[30] podoplanin,[31],[32] L1,[10] EN1,[33] Ezrin,[34] Cyr61,[35] EMMPRIN,[36] EZH2,[37] NF-KB,[38] iNOS,[38] galactin-3,[39] PCNA,[40] topo II,[41] MAGE-A,[42] TARP,[43] CK14,[44] CD133,[45] TACSTD2,[46] NM23[47] and LAT1[48] were shown to have a correlation with prognosis in terms of metastasis and patient survival. Since only a single study was done, it is difficult to evaluate the relevance of these markers as prognostic indicators.

Bazarsa et al.[5] showed that the low expression of Ataxia- Telangiectasia-Mutated protein, a cell cycle regulator, is related to a poor survival rate. Similar results were shown by Yi et al.[49] and Zhao et al.[50] in their studies, where reduced expression of E-cadherin and N-cadherin correlated with the metastatic progression of SACC. Decreased expression of PTEN,[29] H3K9Ac,[30] MT,[51] PDCD4,[52] beclin 1,[8] GRP78,[8],[53] Numb,[54] p27,[55] and B-catenin,[11],[12] p-Akt,[56] p-mTOR[56] and maspin[57],[58] was also related to shorter overall survival; nevertheless, not many studies were conducted for the same to predict their prognostic significance.

Few studies showed that high expression of markers like c-kit,[59],[60] EGFR,[59],[60],[61],[62] and c-erbB[40],[63] was related to unfavorable prognosis. However, other studies that evaluated the same markers did not demonstrate any correlation of these markers to prognosis. Bmi-1,[49],[64] snail,[49],[50] slug,[49],[65] ILK,[29],[50] and p63[57],[66] markers were evaluated in two studies, each of which demonstrated their significant correlation with poor prognostic factors.

Other markers such as Ki-67, p53, VEGF, and cyclin D1, were repeatedly evaluated and showed reproducible results.

Analysis of Ki-67 as a prognostic marker

The expression of cell proliferation marker, Ki-67, was evaluated in 14 studies, of which 6 studies did not show any direct correlation with the prognosis of the tumor.[18],[28],[37],[67],[68],[69] Wang et al.,[34] in 2011, showed that high expression of Ki-67 was significantly related to poor prognosis of ADCC (P < 0.037). Similar results were seen in studies conducted by Hirabayashi.[41] and Nordgård et al.[70] where statistically significant association was present between the expression of Ki-67 and short-term prognosis for patients with ACC. Kaira et al.[53] and Tang et al.[35] showed that high expression of Ki-67 was related to low 5-year overall survival rate (38.9% and 22%, respectively). Lin et al.[71] demonstrated that along with high expression of Ki-67, other factors like old age (>60), advanced tumor stage, and higher histologic grading of the tumor also contribute as predictors of poor prognosis of ADCC. Studies by Yang et al.[36] and Kaira et al.[48] demonstrated Ki-67 nuclear immunopositivity in ADCC and its higher expression in the solid histotype, and both factors were shown to have a poor prognostic effect on the overall survival.

Analysis of p53 as a prognostic marker

The role of cell cycle regulatory protein, p53, in predicting the prognosis of ADCC was evaluated in 6 studies. Out of these six studies, two studies by Kiyoshima et al.[68] and Nagler et al.[14] did not show any correlation between the expression of p53 and prognosis. A study by Bazarsad et al.[5] showed that, though a positive expression of p53 was correlated with poor survival rate, it was not statistically significant. Jia et al.,[13] in a study on prognosis of apoptosis-associated markers in ADCC, showed that the positive expression of p53 and short-term survival of patients in ADCC had a statistically significant correlation along with a solid histologic pattern. Preisegger et al.[72] studied the immunohistochemical analysis along with mutation analysis of p53 and showed that p53 could be an independent marker for poor prognosis. Similar results were shown by Kaira et al.[48] in a study, where p53 expression was significantly associated with poor prognosis.

Analysis of vascular endothelial growth factor as a prognostic marker

VEGF promotes angiogenesis, hence plays a role in tumor growth and metastasis. Three studies evaluated the prognostic significance of VEGF in ADCC Lee et al.[59] found that there was no relationship between VEGF expression and survival rate, metastasis, or recurrence in ADCC. Inconsistent with the above-mentioned study, Yang et al.[36] and Zhang et al.[38] attributed VEGF expression to be one of the factors responsible for poor prognosis along with other factors like the clinical stage, histotype, vascular invasion, perineural invasion, metastasis, and recurrence.

Cyclin D1 as a prognostic marker

Cyclin D1, a cell cycle regulator protein, was evaluated in 3 studies for its prognostic significance. All these studies produced consistent results. Schneider et al.[11] and Zhou et al.[12] in their studies investigated the prognostic significance of cyclin D1, PIN1, and β-catenin in ADCC, and no significant association was seen between the expression of cyclin D1 and patient outcome. A study by Lin et al.[71] showed similar results where the expression of cyclin D1 was not correlated with prognosis.

Conclusion

Evaluation of prognostic factors for ADCC is important as recurrence, metastasis and a short disease-free duration is a common finding after resection. The prognostic factors evaluated in our study were recurrence, metastasis, and overall survival. The study showed that many researches correlated prognosis with the overall survival and not the disease-free survival; as a result, recurrence and presence of any residual tumor could have been missed. Amongst the various prognostic markers, the histologic tissue section represents the grade of the tumor more accurately, and immunohistochemistry is the best means known to assess the tissue and its contents. Although various IHC markers have been studied to predict the prognosis of ADCC, a few markers were used repeatedly for validation of their prognostic predictability. These markers were p53, Ki-67, VEGF, and cyclin D1. However, the results obtained were not homogenous and no conclusive data could be arrived upon. Other markers that displayed an impact on outcome need additional assessment since only a single study was done. Further, comprehensive researches are therefore required in this direction to enhance the prognostic assessment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.