Basal cell adenoma (BCA) of the salivary glands is a rare benign disease that accounts for 1% to 3% of salivary gland tumors [1]. BCA is histologically similar to other tumors, such as pleomorphic adenoma, Warthin’s tumor, and basal cell adenocarcinoma (BCAC). Therefore, it is challenging to differentiate these diseases through fine needle aspiration (FNA), making superficial or total parotidectomy necessary for diagnosis and treatment [2,3]. Clinically, BCA is usually a slow-growing, asymptomatic, and movable mass. In this paper, we report rare but typical cases of BCA of the parotid gland. Monomorphic basaloid epithelial cell proliferation constitutes the histopathological hallmark of BCA. Typically, BCA is described as a slow-growing mass that is seldom symptomatic and is freely movable [4]. Adults between their fifth and seventh decades of life are most commonly affected. BCA carries a favorable prognosis in comparison with other salivary gland neoplasms, especially since recurrence is not common. Here, we report cases of the parotid mass presenting as an asymptomatic swelling histopathologically identified as BCA.

BCA is a rare benign disease that arises from the basal cells of the salivary glands, accounting for 1% to 3% of salivary gland adenomas [4-6]. The condition is commonly observed in women aged over 50 years, and there is no specific known causative factor [7]. BCA usually develops in the parotid gland or minor salivary glands, but it can also occur in atypical locations such as the lips, buccal mucosa, palate, and nasal septum [8]. The tumors are slow-growing and typically asymptomatic, but tenderness, repeated nasal bleeding, or facial nerve compression due to the mass have been reported [9]. While they are typically benign, there is a small risk of recurrence if the entire tumor is not removed during surgery.

The diagnosis of BCA in the parotid gland involves a physical examination and imaging studies such as a CT scan or MRI. For tumors originating in the parotid gland, the extent of resection before surgery needs to be contemplated [10]. Membranous BCA, adenoid cystic carcinoma, pleomorphic adenoma, and BCA are all composed of basaloid cells. Therefore, cell cytology alone is not sufficient for diagnosis, even when considering differences in the ratio of cytoplasm. Some argue that core needle biopsy is helpful, but it may not be appropriate in cases involving hematoma and nerve damage. FNA is commonly used in conjunction with MRI. However, it is crucial to have a clinical suspicion, and even if the accuracy of FNA is low, the routine application of core biopsy and frozen section is not appropriate [11].

Preoperative imaging studies are also useful, as BCA shows contrast enhancement in a well-defined lesion on the early phase of enhanced CT [1]. It differs from pleomorphic adenoma, which shows contrast in the late phase with irregular margins, and from Warthin’s tumor, which occurs mainly in the caudal portion of the parotid gland in male smokers aged over 50 years [1]. As benign tumors of the parotid gland and their malignant counterparts have several overlapping features, a comprehensive interpretation based on clinical findings is necessary [12].

Histologically, BCA is composed of basaloid cells with a scanty pale cytoplasm and is characterized by the absence of a chondromyxoid stromal matrix. There are four histological patterns of BCA, namely solid, trabecular, tubular, and membranous types [13,14], with all types usually presenting a similar clinical prognosis. The recurrence rate for solid and trabecular-tubular variants is almost nonexistent. However, the membranous type is known to have a more progressive growth and higher recurrence rate, almost 25% [7,15]. The solid type is similar to the skin’s basal cell carcinoma with solid nests of basaloid cells surrounded by an outer layer of columnar/cuboidal cells. The trabecular type has basaloid cells arranged in narrow strands and cords separated by a fibrous and vascular stroma. The tubular type has basaloid cells and multiple small duct lumens lined by eosinophilic, cuboidal cells [16,17]. Immunohistochemistry has been studied for a more accurate diagnosis, and nuclear β-catenin expression is known to be common in BCA and BCAC, unlike other salivary gland tumors [18].

The differential diagnosis must include the pleomorphic adenoma, the most common benign tumor of the salivary glands, and other salivary gland tumors. BCAC, another malignant tumor sharing histologic features with the BCA, is distinguished from BCA by the histologic characteristics of invasion, mitotic activity, and neural or vascular involvement [19-21]. The transformation of BCA into BCAC is still controversial, but there is a report that BCA can transform into malignancy in up to 4.3% of cases [22]. However, most BCAs remain benign throughout their course. If malignant transformation of a BCA is suspected, additional testing and imaging may be necessary to confirm the diagnosis. BCAC, a malignant counterpart of BCA, is known to have a generally good prognosis due to low-grade malignancy. Radiotherapy may be required if the stage is ≥ T3 in patients aged over 65 years [23,24]. It has been found that BCA and BCAC have the same genetic modification, but whether BCA undergoes malignant transformation to BCAC has not been determined [25].

Recently, there has been an argument that enucleation can help reduce morbidity and improve facial aesthetics compared to parotidectomy in cases of clinically benign lumps, while still maintaining oncological safety [26]. However, for BCA of the parotid gland, parotidectomy remains the treatment of choice, and enucleation is not recommended due to higher rates of local recurrence. In cases where the tumor is located in the deep lobe, total parotidectomy is recommended as there is a high probability of malignancy [27,28].

Despite its low incidence, BCA is the third most common benign tumor of the salivary gland after pleomorphic adenoma and Warthin’s tumor. There has been recent debate regarding the oncological safety of enucleation for clinically benign lumps. However, considering the high recurrence rate of the membranous type of BCA, which can reach up to 25% even after enucleation, it is recommended to closely monitor patients and, if feasible, consider surgical removal beyond superficial parotidectomy. Additional reoperation may ultimately be necessary depending on the pathology type. Whenever possible, excision surgeries should aim for nerve sparing and include as much normal tissue as possible without damaging the capsule within the range of nerve preservation.

In these cases, parotidectomy was planned prior to surgery to exclude pleomorphic adenoma due to inconclusive results from FNA. In the first case, total parotidectomy was performed after intraoperative findings indicated invasion of the deep lobe, while in the second case, superficial parotidectomy was chosen considering the size and location of the mass. Both cases confirmed the pathology of BCA, specifically excluding the membranous type with its high recurrence rate. Furthermore, there were no permanent sequelae resulting from facial nerve injury or the surgery itself. With respective follow-up periods of 2 years and 3 months, there have been no instances of recurrence or complications. The indication for parotidectomy should be determined by considering the possibility of malignancy and the recurrence rate. It cannot be ignored if there is a diagnosis that indicates the need for parotidectomy based on FNA.