INTRODUCTION

MASC is a low-grade malignant salivary gland cancer, first described by Alena Skálová in 2010¹, with various similarities to the secretory carcinoma of the breast including histology, immunohistochemistry and a genetic translocation².

Epidemiologically, it is usually encountered in adults with a mean age of 46 years-old³ and a slight male predominance^3,4. There have been 11 cases described in the paediatric population⁵.

There are three main differential diagnosis: AciCC which is positive for DOG1, does not have the ETV6-NTRK3 fusion gene and has basophilic granularity in the cytoplasm as an hallmark^2,3; mucoepidermoid carcinoma which is positive for p63 and is usually negative for S100 and mammaglobin in addition to having, in most cases, a t(11;19) translocation³ and, low-grade cribriform cystadenocarcinoma which has epithelial structures surrounded by an intact layer of p63 positive myoepithelial cells².

MASC is usually found in the parotid gland^3,4 followed by the oral cavity and submandibular gland³. The most common presentation is of a slowly growing painless nodule². In most cases it has good prognosis and an indolent course⁴. High grade transformation followed by an accelerated clinical course with poor prognosis has been described^3,4.

CLINICAL CASE

Our case is of a 28-year-old women with no relevant past medical history who presented in the ER with a week old, hard, 5x5mm, adherent and tender nodule in the left parotid region. There was no history of trauma or other symptoms.

Pre-operatory work-up included: blood analysis which showed no relevant alteration; ultrasound which showed a 8x5mm, lobulated, hypoechoic nodule with liquid/cystic content; cytology which described a low grade epithelial / myoepithelial tumour and an MRI describing a bilobulated, 8x5mm, nodule with well-defined borders and a thin wall in the left superficial parotid gland with anterior extension to the masseter muscle.

Thus, we performed a superficial left parotidectomy using a retro-tragal mini-lifting approach. There were no complications during surgery and the facial nerve was preserved.

Follow-up included a pathology exam in which the tumour was described as a brownish-yellow well delimitated nodule with a larger diameter of 0.4 cm. Histology (Figure 1) showed an encapsulated neoplasia (A) with a cribriform/microcystic pattern. Neoplastic cells had irregular nuclei, eosinophilic cytoplasm and secretion product in the gland-like spaces (B). Immunohistochemically (Figure 2) the neoplastic cells were positive for CK7 (C), mammaglobin (D), GATA-3 (E) and S100 protein (F) and negative for DOG-1 (G) and P63 (H).

Adjunctive radiotherapy with a total dose of 60Gy was also performed and, to date, the patient remains asymptomatic with no signs of recurrence.

Figure 1. Histology showed an encapsulated neoplasia (A) with a cribriform/microcystic pattern. Neoplastic cells had irregular nuclei, eosinophilic cytoplasm and secretion product in the gland-like spaces (B) 

Figure 2. Immunohistochemically the neoplastic cells were positive for CK7 (C), mammaglobin (D), GATA-3 (E) and S100 protein (F) and negative for DOG-1 (G) and P63 (H) 

DISCUSSION

MASC is, generally, a solitary², firm/rubbery mass with a white to grey cut surface³. Fluid-filled cystic spaces may be seen³. Perineural invasion and extension to extra-glandular tissues occurs often but lymphovascular invasion and necrosis are uncommon^2,3. Histologically it is a well-defined multilobulated tumor divided into segments by fibrous septa⁴. Neoplastic cells have uniform and round nuclei³ with centrally located small nucleoli and eosinophilic vacuolated cytoplasm, and can be arranged in microcystic, cribriform, tubular, papillary, follicular or solid nests pattern^3,4. Microscopic foci of invasion can be present although atypia is generally mild and mitotic activity is scarse^3,4.

Immunohistochemically, it is positive for a wide-spectrum of cytokeratins (AE1-AE3, CAM5.2, CK7, CK8, CK18, CK19), S100 protein, mammaglobin and, in most cases, GCDFP-15, SOX10, GATA-3 and vimentin^3,4. Basal cell / myoepithelial markers such as p63, calponin, CK14, SMA, and CK5/6 are virtually always negative. Most cases are negative for DOG-13

Genetically, a t(12;15)(p13;q25) translocation resulting in the ETV6-NTRK³ fusion gene is present⁴. ETV6 is a transcriptional regulator and NTRK³ is a membrane receptor kinase3. The fusion gene results in a ligand independent activation of the kinase thus promoting cell proliferation and survival^2,3. This fusion gene has not been described in other salivary gland tumours³ .

Imagiologic descriptions are lacking in the literature. On MRI the tumour shows hyperintensity relative to muscle in T1 and hypointensity relative to the parotid gland in T2².

The treatment of choice in MASC is complete surgical resection (standard care for low grade salivary carcinomas) ^2,3. Locoregional radiation is sometimes considered, mostly in cases with positive margins or perineural invasion³, although the impact on prognosis is not established². Systemic chemotherapy can be implemented if distant metastases are present³. Response to tyrosine kinase inhibitors has been described in other tumours with the ETV6 translocation but not in MASC³.