Result
Gross and microscopic examination of the radical parotidectomy specimen (S2) showed a 4 cm high grade carcinoma (AFIP grading for MEC) invading salivary gland, skeletal muscle and trapping hypertrophic nerves (Figure 1a&b). The tumour consisted of solid lobules, cords and trabeculae of moderate to highly pleomorphic carcinoma cells with focal keratinization (Figure 1c). Primitive glandular differentiation with foamy cells and abortive lumens staining positive for mucicarmine (Figure 1d) could be focally seen. Atypical mitoses (5/10 HPF) and necrosis were present. Immunohistochemical studies showed tumour cells positive for cytokeratin AE1/3, p63 and p40; negative for S100. Ki67 proliferative index varied from 15-30%. There was marked stromal sclerosis forming fibrous strands and bands. Heavy chronic inflammatory infiltrates including many Langerhans cells (CD1a positive, Figure 1d) and eosinophils were present in the background. The adjacent salivary gland showed non-sclerosing chronic sialadenitis with intralobular lymphocytic infiltrates and scanty lymphoepithelial lesions (Figure 1b). This high-grade carcinoma with mucoepidermoid differentiation and the accompanying features fit into the published criteria of SMECE. Ten regional lymph nodes dissected showed no malignancy.
Slide review of the initial excisional specimen (S1) was performed and showed a well-circumscribed low grade (AFIP grading) carcinoma hiding within a fibroinflammatory background (Figure 2a) and focally touching resection margin. Compared with S2, there was a similar SMECE picture but much less aggressive. It consisted of lobules and cords of squamoid cells with keratinization and minimal nuclear pleomorphism (Figure 2b). Cystic change and ductal differentiation with well-formed lumen positive for PAS (diastase resistant, Figure 2b&d) lined by foamy columnar mucinous cells were focally seen. Mitosis numbered about 1-2/10 HPF. There was no evidence of IgG4 associated disease. Immunohistochemistry profile (Figure 2d) was the same as that in S2 (Ki67 not available). There were distinctive sheets of Langerhans cells and eosinophilia forming eosinophilic abscesses mimicking Langerhans cell histiocytosis (Figure 2c). Definite tumour progression on recurrence from low (S1) to high grade carcinoma (S2) could be demonstrated with respect to cellular pleomorphism, necrosis, tumour differentiation and invasiveness. The AJCC pathological staging also progressed from pT2 to pT4a.
FISH using break-apart probe for CSF1 was performed to detect possible derangement of CSF1 gene with subsequent CSF1 over-expression. In most tumour cells there was 1 copy of the normal gene (fused green and orange signal giving orange-red color), and multiple green signals signifying duplicated CSF1 3’ end with deletion of the 5’ end in the other chromosome (Figure 3a). Illumina TruSight RNA pan-cancer assay detected no fusion transcripts in the specimen. On the other hand, expression of CSF1 gene in tumour cells could be demonstrated by RNA in-situ hybridization (RNA scope) as coarse and fine granules (Figure 3b).