Parotid gland keratocystoma: a rare diagnosis of a neck lump

Abstract

Keratocystoma of the parotid gland is a rare benign tumour, with only a handful of case reports in literature. We report a case of a man in his 70s who presented to the head and neck clinic with a 2-month history of a gradually increasing left neck swelling. Fine-needle aspiration showed contents of an epidermal cyst; however, an MRI showed a solid/cystic mass with a low T2 signal, suggesting increased risk of malignancy. The patient went on to have a left superficial parotidectomy and level IIa supraselective neck dissection. The lesion contained laminated keratin and was lined by squamous epithelium, with nodal extension. Features were those of a keratocystoma; metastatic cystic well-differentiated squamous cell carcinoma was not favoured. This is the first reported case of a parotid keratocystoma with nodal extension of the lesion. This case report demonstrates that surgical excision and clinical monitoring can be a safe management strategy for these benign tumours.

Background

Clinicians’ understanding of salivary gland tumours is mostly based on clinical case series rather than high level evidence.¹ Diverse range of clinical and histological behaviours, interspersed with rarity of malignant tumours means that there is a lack of strong evidence, which can be used to provide recommendations for each individual histological subtype.¹ Approximately 300 malignant salivary gland tumours are registered in England and Wales each year.² The highest incidence of all tumours occurs in the parotid gland; however, submandibular and minor salivary glands have a higher incidence of malignancy.³

Keratocystoma of the parotid gland is a rare benign tumour, with only a handful of case reports on this entity, with a subsequent lack of data on its management and ideal length of follow-up.

Here, we report a case of a man in his 70s diagnosed with a left parotid keratocystoma, successfully managed with superficial parotidectomy, without evidence of recurrence observed 15 months postoperatively.

Case presentation

A patient in his 70’s was referred to the head and neck clinic via the 2-week wait suspected head and neck cancer pathway with an 8 week history of a left sided neck lump. On review in head and neck clinic, a gradually increasing left neck swelling was reported, with no associated increase in size with eating and drinking. The swelling was painless and there was no dysphagia, odynophagia or referred otalgia. There was no history of night sweats and a 10kg history of intentional weight loss over the preceding months was reported. The patient reported that he never smoked, and drank alcohol within the recommended limits of less than 14 units per week. Past medical history included hypertension and atrial fibrillation, with verapamil and apixaban prescribed for the two conditions, respectively.

On examination, there was a 2 cm left-sided level II firm swelling behind the ramus of the mandible. The facial nerve was clinically intact. The rest of the neck examination was unremarkable with no palpable lymphadenopathy. Examination of the ears, skin and oral cavity was unremarkable, with examination of the oropharynx consistent with a previous tonsillectomy. Flexible nasal pharyngolaryngoscopy was performed and revealed the nasopharynx, oropharynx, hypopharynx and larynx to be unremarkable.

Following the above findings, impression was of a tail of parotid tumour or a metastatic deposit in a level II (upper jugular) lymph node, with a high suspicion of malignancy. Fine-needle aspiration cytology (FNAC) revealed mature squamous epithelial cells on a grubby, keratinous background, in keeping with contents of an epidermal cyst. No malignant cells were identified. On telephone follow-up, however, 2 weeks after the initial appointment, further increase in size of the swelling and pain was reported.

As the presence of new-onset pain in the absence of any signs or symptoms of infection raised the possibility of this being a malignant salivary gland tumour, an urgent ultrasound scan (USS) was organised. This showed a 2.3 cm heterogeneous mass inferior to the tail of the left parotid gland with an irregular posterior margin; no lymphadenopathy was visualised (figure 1). Following the notion of the suspicious feature of irregular posterior margin, the patient went on to have an MRI scan. This showed a 2.1 cm heterogeneously enhancing mixed solid/cystic mass in between the deep and superficial lobes of the left parotid (figures 2 and 3). Appearances were felt to be potentially in keeping with a Warthin’s tumour, however, the low T2 signal suggested increased risk of malignancy.

Figure 1.

Ultrasound scan of left parotid gland showing a 2.3 cm heterogeneous mass inferior to the tail of the gland (arrow).

Figure 2.

Coronal section of MRI neck (T2-weighted sequence) showing a 2.1 cm heterogeneously enhancing mixed solid/cystic mass in between the deep and superficial lobes of the left parotid (arrow).

Figure 3.

Axial section of MRI neck (T2-weighted sequence) showing a 2.1 cm heterogeneously enhancing mixed solid/cystic mass in between the deep and superficial lobes of the left parotid (arrow).

On review in clinic, 6 weeks after initial appointment, the swelling continued to grow in size; a further USS showed increase in size of the mass by 6 mm with repeat FNAC suggesting features of an epidermal inclusion cyst; however a well-differentiated squamous cell carcinoma (SCC) could not be excluded.

The patient was discussed in the head and neck multidisciplinary team (MDT) meeting, where a recommendation was made for superficial parotidectomy to rule out a well-differentiated SCC. Following discussion with the patient and his daughter in clinic, the patient agreed to go ahead with the procedure.

Treatment

Three months postinitial presentation to head and neck clinic, the patient underwent a left superficial parotidectomy and level IIa supraselective nodal neck dissection. Medtronic nerve monitor was used for the procedure to identify and preserve the facial nerve. A modified Blair incision was used for access; cervicofacial flaps were raised over the parotid gland. Sternocleidomastoid (SCM) muscle was identified and the great auricular nerve was followed onto the parotid lump, with the main branch sacrificed to gain access to the tumour. The tragal pointer, posterior belly of digastric muscle and tympanomastoid groove were then exposed as landmarks for identification of the main trunk of the facial nerve. The soft tissue between the tragal pointer and SCM was divided to improve access. The accessory nerve and subsequently main trunk of the facial nerve were both identified with the nerve monitor and preserved. Intraoperative findings were of two palpable lumps on the parotid, one filled with sebaceous material and a level IIa round, firm lymph node.

Level IIa lymph nodes and both parotid lesions were excised with a cuff of parotid tissue using bipolar forceps, scissors and LigaSure bipolar electrosurgical device. All specimens were sent to histopathology. All branches of the facial nerve were electrophysiologically intact at the end of the procedure. The stump of the main trunk of great auricular nerve was buried into SCM and a size 14 Fr drain was inserted. The incision was closed in layers with 3/0 Vicryl and metal clips (neck) and 5/0 Ethilon (face).

The patient returned to the ear, nose and throat ward routinely after the procedure with no immediate complications.

Outcome and follow-up

The patient’s postoperative course was uncomplicated and his drain was removed on day 3 postoperatively. A subtle weakness of the marginal mandibular branch of the facial nerve was noted on the ward secondary to neuropraxia. The patient was allowed to restart his Apixaban and go home on day 3, post drain removal. Sutures and clips were removed in the community 7 days after the operation.

On review in head and neck clinic 8 days postoperatively, the patient was doing well with exception of a small amount of saliva leaking from the parotidectomy wound below the ear lobe, expected numbness of the ear lobe (following sacrifice of the great auricular nerve) and mild marginal mandibular nerve weakness which was improving.

Two weeks after surgery, the case was discussed in the head and neck MDT meeting with the histology results.

Histology

The surgical specimen consisted of a portion of parotid tissue within which was a cystic lesion, 30 mm across. The cyst contained cream-coloured soft material within it; the lesion was sampled in its entirety. Histologically the cyst was filled with laminated keratin and lined by bland, mitotically inactive stratified squamous epithelium. No other epithelial elements and no necrosis were identified (figure 4). The edges of the cystic areas were typically very smooth, although in places the epithelial edges were slightly irregular but still formed within a lobular-like configuration. In other areas, the lining was replaced by a foreign body giant cell reaction to keratin. There was no evidence of significant atypia or increased mitotic activity, including atypical mitoses. Ki67, a proliferation index marker, was low level and localised to the basal layer of the lesional epithelium (figure 4). Similarly, p63 and p53 were not overexpressed and were localised to the basal layer. Within the parotid gland and within close proximity of the main lesion, there was lesional extension within a single intraparotid lymph node.

Figure 4.

(A) H&E section of an area of the lesion, lined by keratinising stratified squamous epithelium and filled with laminated keratin. In addition, there was very little associated inflammation in this area. In (B–D) are Ki67, p53 and p63 immunohistochemical stains respectively. The Ki67 index is low and located at the basal layer of the cyst lining. Both p53 and p63 are not overexpressed and have a similar distribution to Ki67.

Following MDT discussion, the cystic lesion was deemed to represent a rare parotid entity called keratocystoma with a single pseudopod extension into an immediately adjacent intraparotid lymph node. Ancillary investigations aided reassurance of the lesion’s benign nature and also ensured that a metastatic SCC was excluded.

The keratocystoma was narrowly excised with a 0.5 mm margin clearance. Clinical follow-up was recommended. The patient was informed of the results via a telephone appointment.

Follow-up

The patient was followed up in clinic again 5 months postoperatively where his only concern was numbness of the left ear lobe, which was an expected finding. The parotidectomy wound healed with no evidence of recurrence of the lesion.

On further follow-up, 15 months after the operation the patient had full facial nerve function with no evidence of locoregional recurrence of the parotid lesion. The patient was reassured and discharged from head and neck clinic due to the benign nature of the pathology.

Discussion

Keratocystoma is a rare benign salivary gland tumour with the parotid gland the most affected site.⁴ There are a very limited number of these cases reported worldwide. This entity will be enlisted in the WHO Classification of Head and Neck tumours, fifth edition.⁴ Its aetiology remains unclear; however, it is speculated that this tumour may arise from salivary ducts which undergo squamous metaplasia. Complete surgical excision remains the most accepted treatment.^{5 6}

Despite its benign nature, it is essential to histologically distinguish keratocystoma from other lesions comprising squamous cells, especially malignant tumours. The paramount tumour to consider in the differential diagnosis is a well-differentiated SCC, which in most cases would be metastatic from a cutaneous primary in the head and neck. This may be challenging to distinguish on a histological basis; however, SCC usually has an infiltrative architecture whereas a keratocystoma’s architecture is more circumscribed. This was an important differential to consider in our patient, given that his age is well outside the published range of previous reported cases of keratocystoma, which tends to present in younger cohort.^{6 7} Another malignant tumour in the differential is a mucoepidermoid carcinoma. Along with squamous cells, this tumour also comprises intermediate cells and mucocytes (including goblet cells). The mucocytes especially can be highlighted by special stains such as an alcian blue and periodic acid-schiff.^{5 6}

Benign lesions also enter the realm of differentials, one of which is a Warthin’s tumour. This is usually prevalent in male smokers in their sixth decade and is characterised by two components; oncocytic epithelial cell lining and lymphocytic stroma. It typically looks different from keratocystoma but can undergo infarction and subsequent squamous metaplasia. This diagnosis appears unlikely in our case due to the lack of necrosis or a pre-existing lesion. Another plausible benign differential is sialometaplasia which in contrast has a lobular architecture and usually lacks keratinisation.^{5 6}

Keratocystoma is typically not multifocal. Our case shows extensions of lesional tissue in pseudopods including adjacent lymph node extensions. This may reflect multifocal disease. Lymph node extensions of a keratocystoma have not yet been recorded in the literature and this is the first reported case of this phenomenon. In this regard, immunohistochemistry helps to substantiate the lesion’s benign nature. This was demonstrated by the localisation of Ki67 staining to the basal layer as well as no overexpression of p53 and p63.⁶ However, despite the reassuring morphology and immunohistochemical profile, correlation with the clinical and radiological findings with long-term clinical follow-up was recommended.

Learning points.

• Keratocystoma of the parotid gland is a rare benign tumour, which needs to be histologically differentiated from metastatic cystic well-differentiated squamous cell carcinoma (SCC).

• Keratocystoma can locally infiltrate into adjacent lymph nodes, with this being the first reported case of nodal extension.

• Surgical excision and clinical monitoring is a safe management strategy for these benign tumours.

• Differential diagnosis of these tumours includes metastatic SCC, Warthin’s tumour, epidermal cyst, mucoepidermoid carcinoma and sialometaplasia, with special histological stains used to aid diagnosis.

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