Salivary Gland Carcinoma Grading

About salivary gland carcinoma grading

Histologic grade correlates with stage, is an independent prognosticator, and helps optimize therapy — but a universal grading scheme is suboptimal given tumor-specific nuances. Most carcinoma types can instead be stratified into low aggression and high aggression categories, based on a combination of category, subtype, and category-specific grade. Head & neck neuroendocrine tumors use a separate, WHO 5th-edition mitotic/necrosis-based grade (Module B).

High-grade transformation (HGT), historically designated "dedifferentiation", is progression from a conventional, usually indolent phenotype to a pleomorphic, aggressive morphology. Wherever HGT is selected, the tumor is placed in the high-aggression category regardless of the underlying conventional subtype.

Table 1 — Risk stratification of salivary gland carcinomas

HGT = high-grade transformation. NOS = not otherwise specified. * Behavior varies with grading system or criteria. ** The cribriform subtype of polymorphous adenocarcinoma has a high propensity for regional recurrence. ^ Adenoid cystic carcinoma, though highly aggressive locally with capacity for distant spread, has somewhat lower risk for regional recurrence in the tubular/cribriform pattern. # Carcinoma ex pleomorphic adenoma behavior is determined by carcinoma type and extent. @ Salivary carcinoma, NOS behavior is determined by grade.

The source table was a two-column layout flattened to linear text; pairings for adenoid cystic carcinoma and for intraductal carcinoma apocrine type / salivary duct carcinoma were inferred, and single-category entries are treated as belonging only to the category shown. Verify against the source table image if it differs.

Mucoepidermoid carcinoma — AFIP grading scheme (Am J Surg Pathol 2019;43:885)

Grade: Low grade (0–4) · Intermediate grade (5–6) · High grade (7 or more).

Histologic grading of mucoepidermoid carcinoma using any single system appears inconsistent, with significant grading disparity reported between pathologists, so the system applied is reported alongside the grade. Low and intermediate grade map to low aggression (intermediate grade: behavior varies with grading system or criteria); high grade maps to high aggression.

Mucoepidermoid carcinoma — Brandwein et al. grading scheme (Am J Surg Pathol 2001;25:835)

Grade: Low grade (0) · Intermediate grade (2–3) · High grade (4 or more).

Mucoepidermoid carcinoma — Modified Healy grading system (qualitative)

Mucoepidermoid carcinoma — MSKCC grading system (qualitative)

Acinic cell carcinoma — proposed grading system (Am J Surg Pathol 2022;46:933)

Adenoid cystic carcinoma — solid component & high-grade transformation

Nodal metastasis is rare and 5-year survival favorable in the tubular/cribriform (low aggression) pattern, though all histologic patterns show high levels of local aggression and infiltration. The two systems are computed side by side and not reconciled; the tubular/cribriform vs. solid/HGT selection determines the Table 1 result.

High-grade transformation is defined as comedo-type tumor necrosis, frequent mitoses (often >10 per 10 HPF), and marked nuclear atypia, and is associated with high risk of lymph node metastasis, distant metastasis, and disease-related death (Am J Surg Pathol 2007;31:1683). It is treated as present only when all three features are checked.

Epithelial-myoepithelial carcinoma — high-grade transformation

Seen in approximately 20% of cases and infers a poorer prognosis (Head Neck Pathol 2013;7:S37). It usually involves the epithelial component: sheets and solid nests of markedly atypical cells with increased mitoses and necrosis. All three features must be checked for HGT to be flagged.

Polymorphous adenocarcinoma — optional quantitation

Papillary (>10%) and cribriform (>30%) components appear to be adverse prognosticators independent of subtype, and are optional to quantitate. The cribriform subtype is considered low aggression with favorable outcome but carries a high risk of regional (lymph node) spread.

Carcinoma ex pleomorphic adenoma

Subclassified by histologic type and/or grade and extent of invasion: minimally invasive, invasive, and intracapsular (non-invasive). An optimal cutoff to define minimal invasion is not yet defined, but 4–6 mm cutoffs are commonly used. The terms intracapsular and minimally invasive should only be applied to uninodular tumors (not carcinomas arising in multinodular recurrent pleomorphic adenomas) with a well-delineated interface, where the entire lesional border has been microscopically evaluated. Non-invasive and minimally invasive cancers appear to have a better prognosis than invasive cancers.

Carcinosarcoma appears to almost invariably arise in a precursor pleomorphic adenoma and should likely be regarded as a sarcomatoid-carcinoma subtype of carcinoma ex pleomorphic adenoma.

Table 2 — WHO classification of head & neck neuroendocrine tumors

Grade 1 NET corresponds to tumors previously termed carcinoid / well-differentiated neuroendocrine carcinoma; Grade 2 NET to atypical carcinoid / moderately-differentiated neuroendocrine carcinoma. Ki-67 is recommended but not required, and is not used for grading here.

Tumors exceeding the Grade 2 NET mitotic rate are usually more in keeping with neuroendocrine carcinoma (small cell or large cell type), which characteristically shows necrosis and mitotic counts exceeding 10 per 2 mm².

Merkel cell carcinoma is a cutaneous neuroendocrine carcinoma, often driven by Merkel cell polyomavirus. Primary parotid Merkel cell carcinoma is described, and further complicated by an earlier "Merkel cell" subtype of primary parotid small cell neuroendocrine carcinoma that, like Merkel cell carcinoma, expresses CK20 but is polyomavirus-negative and lacks a UV molecular signature. As with squamous cell carcinoma of parotid, the idea of a primary parotid Merkel cell carcinoma should be viewed with heavy skepticism — most are metastases from unknown primaries, often resolved by detailed clinical history and dermatologic examination.

Source

CAP Cancer Protocol, HN.Salivary.Res_1.0.0.0.REL_CAPCP.