A presentation of basal cell adenoma: a case report

Abstract

Background

While statistically rare in comparison to other head and neck tumours, parotid gland swellings are often encountered in clinical practice where one of the primary goals of examination becomes distinction between benign and malignant lesions. Hallmarks of malignancy are characterized by a female preponderance, history of radiation exposure, a positive family history, and clinical features like heterogenous consistency, fixity to skin/underlying tissues and involvement of facial nerve.

Case presentation

Here we present a case of parotid swelling in a 72-year old gentleman from south India that had a curious amalgamation of both benign and malignant features.

Conclusions

While benign, the risk of malignant transformation and rare multicentric occurrence indicates a need to keep basal cell adenoma in mind in case of parotid swellings and their surgical management.

Introduction

Salivary gland tumours are uncommon, representing around 3–4% of head and neck neoplasia. Of these, parotid gland is the most frequent site in around 70% of cases [1]. While approximately 3/4th of parotid lesions are benign, 25% are malignant [2]. Benign lesions of parotid can be differentiated based on cell lines as ‘monomorphic’ and ‘pleomorphic’. Basal cell adenomas (BCA) constitute a common example of the former.

The first instance of categorizing BCA as a distinct tumour entity occurred in 1967 when Kleinsasser et al. introduced the term to characterize a salivary gland tumour composed of a uniform population of basaloid epithelial cells [3]. This description was further ratified in 1991 in 2nd edition of the WHO Histological Classification of Salivary Gland Tumours which included BCA, alongside malignant variants basal cell adenocarcinoma and canalicular adenoma as discrete entities.

The classical presentation of BCA is that of an elderly woman, between 50 and 70 years of age, who presents with a slow-growing, painless, mobile, firm lesion with epicentre in the superficial parotid lobe, rarely exceeding 3 cm. There is no skin involvement or facial palsy and surgery usually reveals a well-encapsulated, uniformly solid nodular lesion.

Here, we report a case of BCA that clinically deviated concerningly from these typical characteristics.

Case report

Patient information

A 72-year-old male farmer hailing from south India presented to our outpatient department with 3-year history of swelling in front of his left ear which was insidious and progressive. There was no history of pain or sudden increase in size or change during/after meals. There was no history suggestive of trauma, local infections, or any salivary complaints. The patient was nil comorbid but did give history of habitual beedi smoking for 20 years as well as social drinking. There was no family history of similar lesions.

Clinical findings

A diffuse non-tender left parotid swelling was identified with mixed consistency (predominantly cystic but with few hard areas) and fixity to underlying structures. A distinct hard nodule was identified at anterosuperior aspect with fixity to skin. The skin over the swelling was otherwise normal and there was no change of parameters with jaw clenching (Fig. 1). Facial nerve function was preserved, and intra-oral examination as well as neck examination was normal. Ear and nose were found normal.

Fig. 1.

A, B Clinical photograph of the left parotid swelling

Diagnostic assessment

High-resolution ultrasonography gave the impression of pleomorphic adenoma (PA), however ultrasound-guided fine needle aspiration cytology (FNAC) favoured salivary gland neoplasia of uncertain malignant potential with differential diagnoses of cellular basaloid neoplasm/myoepithelial neoplasm. Hence, contrast magnetic resonance imaging (MRI) was advised, which showed multiple lobulated altered signal intensity lesions with heterogenous enhancement suggesting a malignant lesion (Fig. 2).

Fig. 2.

Contrast magnetic resonance imaging (MRI) image coronal (white arrow) and axial sections showing lobulated altered signal intensity lesions with heterogenous enhancement

Therapeutic intervention

Given the conflicting reports and ambiguous clinical findings, the patient was taken up for left total conservative parotidectomy with selective neck dissection (supra-omohyoid). Intra-operatively, a 6 × 5 cm superficial left parotid lobe was identified and excised along with a cuff of skin to which it was found attached at the anterosuperior aspect. Along with this specimen, left level I, II and III lymph nodes were also dissected out en bloc. Facial nerve was identified with terminal branches and was carefully preserved. A 2 × 1 cm-sized deep parotid lobe was identified and excised along with one hard, fixed, intraparotid node (Fig. 3). Post-operatively, patient recovered well. The corrugated drain was removed on postoperative day five in the absence of any collection. Grade III facial palsy which developed in immediate post-operative period, recovered during the course of the hospital stay and further improved after discharge with appropriate physiotherapy. Histopathological examination of the specimen was carried out (Fig. 4). Sections from the gland showed well-circumscribed proliferation of cells in trabecular arrangement with tubular and solid patterns. The cells themselves were basaloid with scanty cytoplasm, and indistinct cell borders, round-to-oval nuclei with few nucleoli. Peripheral palisading was present, and some cystic changes were noted within the specimen. Of the 7 lymph nodes dispatched, all showed reactive hyperplasia. Based on these findings, a final diagnosis of BCA was made.

Fig. 3.

A, B Intraoperative photograph of the tumour and facial nerve branches

Fig. 4.

A Gross morphology of the excised specimen B Cut surface of the excised specimen C Histopathological image of the tumour

Follow-up and outcomes

Now, almost a year after surgery, patient remains healthy with no recurrence of swelling or facial complaints.

Discussion

As per the WHO Classification of Head and Neck Tumours, 4th Edition (2017) [4], salivary gland tumours were broadly classified into malignant tumours, benign tumours and mesenchymal tumours specific to salivary glands, including both benign soft tissue lesions and hematolymphoid tumours. This classification was largely adopted unchanged in the 5th edition (2022), except for few modifications viz., inclusion of hyperplastic conditions of the salivary glands [5] under the non-neoplastic epithelial lesions category, modifications in the classification of neuroendocrine tumours based on differentiation [6], restricting lymphoproliferative conditions to only those commonly seen in salivary glands, grouping exophytic and inverted ductal papillomas, exclusion of the term ‘low-grade’ especially for pleomorphic adenomas, introduction of grading systems for mucoepidermoid and adenoid cystic carcinomas and new entities like secretory carcinoma [5].

Given the clinical findings of our case, a few differentials were kept in mind at initial presentation (Table 1).

Table 1.

Commonly encountered conditions of each category

Malignant tumours	Benign tumours	Non-neoplastic epithelial lesions	Mesenchymal lesions
Mucoepidermoid Carcinoma	Pleomorphic Adenoma	Sclerosing polycystic Adenosis	Hematolymphoid Tumours
Adenoid Cystic Carcinoma	Myoepithelioma	Nodular Oncocytic Hyperplasia	Extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT-Lymphoma)
Acinic Cell Carcinoma	Basal Cell Adenoma	Intercalated Duct Hyperplasia	Benign Soft Tissue Lesions
Basal Cell Carcinoma	Warthin's Tumour	Lymphoepithelial Sialadenitis	Hemangioma
Intraductal Carcinoma	Ductal Papillomas		Lipoma/sarcolipoma
Carcinoma ex-pleomorphic Carcinoma	Cystadenoma		Nodular Fascitis
Clear Cell Carcinoma	Sialadenoma
Secretory carcinoma
Squamous cell carcinoma

Mucoepidermoid tumour

Mucoepidermoid carcinoma stands as the most prevalent malignant tumour affecting salivary glands, typically impacting minor salivary glands and parotid gland. It constitutes roughly 50% of all malignancies in parotid gland, with an incidence rate of 2.3 per 1,000,000 individuals. Notably, mucoepidermoid carcinoma has female preponderance within the age range of fourth to sixth decades of life, with a positive correlation to ionizing radiation exposure and family history [7]. Clinically, they manifest as slow-growing painless masses, usually nodular on palpation, mimicking benign lesions like pleomorphic adenomas but with eventual fixity to underlying structures. In case of higher-grade, they are found to exhibit more rapid expansion with early invasion of adjacent tissues, though skin involvement and facial palsy are still typically indicators of late-stage disease [8].

Pleomorphic adenoma

These lesions, also called benign ‘mixed’ tumours due to their mixed epithelial and connective tissue origin, are the most commonly occurring salivary gland tumours, contributing to two-thirds of all salivary gland neoplasms [9]. Primarily, pleomorphic adenomas occur in parotid glands (85%)—typically in the superficial lobe, with a smaller proportion found in minor salivary glands (10%) and submandibular glands (5%). Clinically these also show a female predilection, presenting as unilateral, slow-growing swellings in women between 30 to 50 years of age, that are painless and mobile with heterogenous consistency; smooth, lobulated surface, and positive window sign on examination. Facial nerve is uninvolved.

Squamous cell carcinoma

These are uncommon tumours, accounting for less than 1% of salivary gland tumours, with 80% incidence confined to parotids. They are aggressive cancers which are twice as common in males than females, occurring in sixth and seventh decades, typically in patients with history of radiation therapy. Presentation is one of a rapidly enlarging mass, painful in about 30% [10], fixed to underlying structures, and often associated with cervical lymphadenopathy and early facial nerve involvement [11]. Skin involvement also occurs early with many cases presenting in advanced stage with ulceration.

Adenolymphoma aka Warthin’s tumour

If two-thirds of benign parotid tumours are pleomorphic adenomas, the remaining one-third are almost all Warthin’s tumours, which constitute the most frequently encountered monomorphic adenoma of major salivary glands [12]. It primarily affects older men, with the peak incidence occurring during sixth and seventh decades of life. A positive correlation with smoking history is usually seen, however, most studies remain conflicted in attributing aetiology to tobacco exposure, with many arguing for salivary duct inclusions during embryonic development as the causative factor [13]. Clinically they are slow-growing and painless, usually unilateral in involvement and characteristically confined to lower pole of superficial lobes, soft in consistency with some cystic areas on palpation. Multifocal and multicentric cases have also been reported [12]. Facial nerve is uninvolved, though patients may experience facial pain, and in rare instances, tumours linked with inflammation and fibrosis may lead to facial nerve palsy culminating in a mistaken diagnosis of malignancy [14].

Other rarer conditions like BCA, acinic cell carcinoma, and adenoid cystic carcinoma were also kept in mind, though like with the above, while some expected findings corresponded with those in our patient, significant differences were also encountered. This led to the planning of a therapeutic + diagnostic surgery in the form of a total parotidectomy with neck dissection.

Histopathological evaluation helped establish the diagnosis of BCA. Basal cell adenomas are the third most commonly occurring benign parotid tumours, after pleomorphic adenomas and adenolymphomas, and hence, constitute the second most common monomorphic type. They represent 1–3% of benign salivary epithelial tumours [15]. More than 70% are found in the major salivary glands, followed by minor salivary glands—which mostly present as swellings of upper lip, and submandibular gland. Other sites include buccal mucosa, nasal septum and lower lip [16, 17]. Epidemiologically, they are typically seen in females in fifth to seventh decades [18]. However, few reports detail similar incidence rates in both genders especially in the membranous variant. This is most notable in cases of Brooke-Spiegler Syndrome which presents as extracutaneous tumours, parotid being one of the involved sites. These patients can be of any ethnicity with equal male and female preponderance [19].

Aetiology of BCA is much debated with various authors describing it as arising from pre-existing benign parotid lesions based on histopathological similarities. Several such possible precursor lesions have been identified, including intercalated duct lesions [20], especially for tubular BCAs; various myoepithelial lesions, or tubular and non-tubular BCAs forming a spectrum where the latter arise from the former. Some reports also describe congenital lesions occurring from embryonic glandular elements [21]. Rarely, the origin has also been ascribed to pleomorphic adenoma—the epithelial tumour cells found in pleomorphic adenoma can transform into BCA through specific differentiation processes. Within the epithelium of PA, basal cells exhibit reserve cell properties, which, via epithelial-mesenchymal transdifferentiation, give rise to the primary basaloid cell component seen in BCA [22].

Clinically, BCA of parotid presents as slow-growing, firm, mobile, painless masses rarely exceeding 3 cm in size [18, 23]. There is usually no skin involvement and facial nerve is spared. On table, they are usually seen superficially within the glandular body and appear as brownish, oval or round, encapsulated nodules [24]; however, the membranous subtype is often nonencapsulated, multicentric and multilobular. On cut sectioning, the lesions are usually uniformly solid without necrosis, though few cases with cystic lesions have been reported [25].

FNAC remains a commonly employed diagnostic modality, however, cytologically, basal cell adenocarcinoma is indistinguishable from BCA [26]. On occasion, FNAC may even only reveal benign lesions with inflammatory and cystic characteristics, leading to the possibility of other benign cysts as a cytological differential. Hence, histopathology is considered mandatory for accurate diagnosis and subtyping.

Histologically BCA is characterized by the presence of uniform and regular basaloid cells, which give this entity its name. A basement membrane-like structure surrounds these tumoral nests, separating them from the surrounding connective tissue [27]. These basaloid cells can be arranged in solid, trabecular, tubular or membraneous patterns, though a mixture of at least 2 types is common. Solid BCA is formed by small cells organized compactly, with peripheral palisading. In tubular and trabecular subtypes, groups of cells exist in narrow bands and ductal structures or a combination of both. These trabeculae may get interconnected to form a reticular jigsaw pattern. The membranous subtype is constituted by external cells in a stockade pattern and by an intense hyalinized basal membrane [28, 29].

Surgical excision—superficial or total parotidectomy remains the mainstay of treatment, though some authors have described simple excision with a cuff of normal salivary tissue. Ideally, however, extracapsular excision should be done, taking care not to disrupt the capsule to minimize the risk of recurrence, though recurrence rate is low (2%) [30] except for membranous variant (25–37%) (likely due to multicentricity rather than true recurrence [31]). Four per cent of membranous BCA undergo malignant transformation [32], hence, routine long-term follow-up is indicated, despite an overall good prognosis.

Conclusion

Basal cell adenomas are rare clinical entities, and atypical presentations can confound the diagnosis. While benign, the risk of malignant transformation and rare multicentric occurrence indicates a need to keep this condition in mind in case of parotid swellings and their surgical management.

Author contributions

DD—Concept and design, data collection, writing original draft preparation, writing review & editing, supervision. MNK—Writing review and editing, supervision. NC—Methodology, data collection, writing original draft preparation, writing review and editing. SJ—Data collection, writing review and editing, supervision.

Funding

No funding was received.

Availability of supporting data

Data transparent.

Declarations

Ethics approval

This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by Institutional Ethics Committee, Mangaluru – IEC KMC MLR 11/2023/466.

Consent for publication

Written informed consent was obtained from the patient’s legal guardian for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-chief of this journal.

Informed consent

Consent to participate and publish was obtained from the patient.

Competing interests

The authors have no relevant financial or non-financial interests to disclose.