A rare case report of adenoid cystic carcinoma of parotid gland with perivascular and perineural spread

Adenoid cystic carcinoma is a slow growing malignant neoplasm of the salivary glands, mainly occurring in minor salivary glands and relatively rare in parotid glands. It has got a remarkable capacity for extensive subclinical spread to the adjacent structures. Here, we present a case report of adenoid cystic carcinoma of parotid gland with perineural and perivascular spread. To our knowledge, this is the first reported case of adenoid cystic carcinoma of parotid gland with perivascular spread along a major artery (ECA) and perineural spread along two different nerves (mandibular nerve and facial nerve)


DESCRIPTION
A 56-year-old male patient, with history of lower motor neuron type of facial palsy and swelling over the right angle of mandible from 8 months now presented with chief complaints of pain and increase in the size of swelling since 2 months.
On examination, facial asymmetry and swelling over the right parotid region were present. There was no deviation of tongue. Vision was normal. The patient was referred for ultrasonography of the right parotid region which showed a well-defined irregular hypoechoic lesion in superficial and deep lobes of right parotid gland. Figure 1 The lesion extends along the external carotid artery forming a soft tissue cuff around it beyond the confines of the parotid gland. Figure 2 Excision biopsy of the intraparotid portion of the tumor was done and the specimen was sent for histopathological examination which showed features of adenoid cystic carcinoma with predominant solid pattern with focal cribriform pattern and perineural spread. Figure 3 Further imaging evaluation with MRI neck was done which showed a well-defined T1/T2 hypointense lesion involving superficial and deep lobes of right parotid gland. The lesion is completely encasing the intraparotid portion of the right external carotid artery and its terminal branches, i.e. maxillary artery and superficial temporal artery. A cuff of tumor is also extending along the right ECA beyond the confines of the parotid gland-suggestive of perivascular spread. Nodular extension of the lesion into the masticator space is noted, extending along the mandibular (V3) division of right trigeminal nerve through the grossly widened right foramen ovale with right temporal extraaxial component causing indentation on right medial temporal lobe. Posteriorly, the lesion is extending into the Meckel's cave and cavernous sinus-suggestive of perineural spread along mandibular (V3) division of trigeminal nerve. Fatty atrophy of right-sided muscles of mastication and muscles of floor of mouth. The lesion is also extending along parotid, mastoid, tympanic, canalicular segments of the right facial nerve and greater superficial petrosal nerve-suggestive of perineural spread along facial nerve. The lesion, its perivascular and perineural extensions show intense enhancement on postcontrast study. Intensely enhancing nodular lesion is noted in right carotid space discrete from the above-mentioned lesion splaying right ECA and ICA which is suggestive of a lymph nodal deposit. Figures 4-8 The patient underwent positron emission tomography CT which did not reveal any distant metastasis

ABSTRACT:
Adenoid cystic carcinoma is a slow growing malignant neoplasm of the salivary glands, mainly occurring in minor salivary glands and relatively rare in parotid glands. It has got a remarkable capacity for extensive subclinical spread to the adjacent structures. Here, we present a case report of adenoid cystic carcinoma of parotid gland with perineural and perivascular spread. To our knowledge, this is the first reported case of adenoid cystic carcinoma of parotid gland with perivascular spread along a major artery (ECA) and perineural spread along two different nerves (mandibular nerve and facial nerve) Due to the inoperable stage of the tumor, radiotherapy was considered.

DISCUSSION
Adenoid cystic carcinoma (ACC) is a salivary gland neoplasm with aggressive nature and slow growth. They constitute about 21.9 % of all salivary gland malignancies including major and minor salivary glands. It has got a remarkable capacity for recurrence. 1,2 It can arise in any salivary gland, but mainly develops within the minor salivary glands and submandibular gland. In the parotid gland, ACC is relatively rare, accounting to about 2-3% of all tumors 3 . In head and neck region, some of the sites of origin are the tongue, larynx, paranasal sinuses, nasopharynx, lacrimal glands and external auditory canal. They occur mainly among females, between fifth and sixth decades of life.
As compared to its aggressive nature, the clinical presentation is rather indolent which belies its extensive subclinical spread to the adjacent structures. It presents as a slow growing mass with pain, facial nerve paralysis when involving the parotid gland and local invasion. They are locally aggressive with perineural and perivascular spread with late distant metastases. 4 Perivascular invasion has been reported in some cases of ACC and was found to be associated with a higher rate of metastasis. To our knowledge, this is the first reported case adenoid cystic carcinoma of parotid gland with perivascular spread along a major artery (ECA) and perineural spread along two different nerves (mandibular nerve and facial nerve).
Adenoid cystic carcinoma is histologically characterized by three patterns-cribriform, tubular and solid. 5 Most of the tumors show a mix of these patterns. Mitotic figures are more commonly visualized in solid patterns which have the worst prognosis.
ACCs are categorised into three grades. There are no solid areas in Grade I which is composed of cribriform and tubular patterns. Solid areas are seen in Grade II (less than 30 ) and Grade III (more than 30 percent). Atypical cells and mitoses are seen in Grade III 6 . Our case showed a predominantly solid pattern (more than 30%), thereby fitting into Grade III.
Metastasis to lungs is most common followed by liver. Unlike most malignancies, lymph nodal metastasis is rare.  Vascular endothelial growth factor (VEGF) overexpression has been seen in some tumors showing perivascular spread. 8 One possible explanation is that the VEGF secreted by the tumor cells gets attached to VEGF receptors in the blood vessel wall leading to perivascular tumor growth.
The mainstay of treatment is surgical, while radiotherapy can be considered for advanced stages and as an adjuvant to total surgical resection. As lymph nodal metastasis to regional lymph nodes is uncommon, neck dissection is usually not indicated. 3