HGT-AdCC, a rare entity is considered to be a disease of the elderly with male preponderance (Male to female ratio of 1.75:1) [8]. The first case of HGT-AdCC was described by Malhotra et al. in 2009 in an elderly male [9]. Seethala et al. noted the mean age of presentation for HGT-AdCC to be 61 years [8]. HGT-AdCC is a highly aggressive salivary gland neoplasm with high recurrence and poor prognosis [9]. Our case report differs from the previous observation of HGT-AdCC as an entity confined primarily to elderly males and also notes that parotid gland can be a site for primary HGT-AdCC. The overall 5-year and 10-year survival rates for low-grade AdCC ranges from 35-75 % and approximately 20% respectively. The 10-year survival rate for HGT-AdCC is 0%. In contrast, the overall 5-year and 10-year survival rate for the most common carcinoma of parotid gland, i.e., low-grade mucoepidermoid carcinoma is 75–95% and 50% respectively [10, 11]. The factors which affect the prognosis of AdCC of the parotid gland are the presence of facial nerve palsy, perineural spread, temporal bone involvement, histological type, nodal metastasis, distant metastasis, and recurrence [12, 13]. Facial nerve palsy can be seen in up to 70% of patients [14]. It results in poor overall and disease-free survival with a 5-year survival rate of 40%. PNI even if asymptomatic was earlier thought to be a worse overall prognostic indicator, however recent studies indicate that it is a marker of aggressive parotid malignancy and may not necessarily mean poor overall survival [15].

No large sample studies have been done on the survival rates of AdCC of parotid gland with temporal bone involvement. The occult perineural spread if undiagnosed preoperatively can lead to inadequate disease clearance and local recurrence. In a retrospective study over 20 years the incidence of parotid cancer with temporal bone involvement was only 1.1% (30 patients) out of 2700 parotid surgeries. In that case series, the parotid cancers to have occult spread to temporal bone facial nerve in decreasing order were adenocarcinoma (27%), squamous cell carcinoma (23%), salivary ductal carcinoma (14%), and AdCC (13%) with AdCC being one among the least commo n[16]. The commonly described pathway is via stylomastoid foramen with the perineural spread ascending into the vertical segment of facial nerve. In such cases, the temporal bone will show destruction pattern with other extensive local infiltration. This can be demonstrated on CT scan with erosion or widening of the foramen and canal [17]. In our case, the temporal bone showed isolated widening of the facial canal without any gross bony destruction hence this isolated finding could have been easily overlooked as the involvement of facial nerve in intratemporal route by malignant tumors arising from the extratemporal site is rare.

In a previous study of 16 patients done by Leonetti et al. the 5-year and 10-year disease free survival rates of AdCC with temporal bone invasion were 75% and 50% respectively [5]. These relatively high survival rates in this retrospective case series may be attributed to the absence of regional or distant metastasis on presentation and probable low-grade tumor coupled with aggressive therapy. Our patient had HGT-AdCC with temporal bone involvement and regional metastasis.

It has been described that cervical lymph node metastasis is rarely described and that distant metastasis is seen in up to 40% of patients [5]. This was in contrast to our case.

A three tired histopathological grading system for AdCC developed by Perzin and Szanto based on the percentage (%) of solid component [18, 19]. Presence of solid component in an AdCC is a poor prognostic factor. HGT-AdCC or dedifferentiated AdCC is defined as a poorly differentiated adenocarcinoma or high-grade carcinoma that has transformed from a low-grade malignant neoplasm. Histopathological findings with nuclear atypia, increased proliferative index, presence of necrosis and loss of myoepithelial cell, increased Ki 67 index on IHC can help in differentiating it from conventional low grade AdCC [8].

Table 2 depicts the patient demographic features, treatment, follow-up, and outcome of few other studies related to HGT-AdCC of parotid gland.

Table 2 Features of patients of HGT-AdCC of parotid gland

The notorious perineural spread can be seen at either macroscopic or macroscopic level or bot h[20]. In our case, it was seen at both levels. Perineural invasion is a complex process and the complete mechanism is still unknown. At the molecular level, there is a complicated interaction between several chemokines, glial cell-line derived neurotropic factors, Nerve growth factors, matrix metalloproteinases, tight connections of perineural cells, Schwann cells which in the end results in perineural invasion of the tumor [21].

Despite the poorly understood mechanism, patients with perineural invasion by cancer should undergo extensive surgery. Our patient had perineural spread till the tympanic segment of facial nerve and she underwent extensive surgery including modified LTBR. Such extensive surgery is required to improve the long-term outcome [5, 16].

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