There are five common types of adenocarcinoma (actually I would say four since "mixed" doesn't seem to me a specific 'type") recognized in the WHO classification of lung cancer. But this classification also includes five rare but distinctive variants: fetal adenocarcinoma, mucin-producing adencarcinomas (mucinous "colloid" adenocarcinoma, mucinous cystadenocarcinoma, and signet ring adenocarcinoma), and clear cell adenocarcinoma.
While rare to exceedingly rare as pure tumors, the variant patterns are important to be aware of because they occur much more commonly as a component of mixed type adenocarcinomas, are associated with particular clinicopathologic features, and often require the exclusion of a different primary site. Another challenging variant is the nonmucinous lung adenocarcinoma which resembles colorectal adenocarcinoma but I'll review this variant in another post. Hope you all find this of use.
- Unencapsulated but well-circumscribed composed of closely packed branching tubules with little intervening stroma resembling fetal lung seen between 10 and 16 weeks' development (pseudoglandular stage).
- Cribriform arrangements can be prominent and cells arranged in cords and ribbons with peripheral palisading also described.
- The tubules are lined by "endometrioid" nonciliated pseudostratified columnar cells with frequent subnuclear or supranuclear vacuoles.
- Structures resembling squamous morules as in endometrioid adenocarcinoma are often present and the nuclei in these foci often appear optically clear (nuclear pseudoinclusions) due to biotin accumulation.
- Frequent mitoses. Variable necrosis--focal to extensive.
- Low-grade and high-grade variants are recognized which differ significantly in clinical and histopathologic parameters:
- Low-grade: mean age 34 years, about equal M:F, usually present at early stage, associated with a good prognosis; orderly arrangement of glands, focal necrosis (if present at all), small fairly uniform nuclei with inconspicuous nucleoli, morules common; slight to moderate loose fibromyxoid stroma, neuroendocrine differentiation almost always found, occasional AFP-positive cells, p53 protein overexpression uncommon, nuclear and cytoplasmic β-catenin expression
- High-grade: mean age 64 years, male predominance, usually advanced stage at presentation, associated with poor prognosis (42% DOD@24 mos.); disorderly arrangement of glands, extensive areas of necrosis common, large pleomorphic nuclei with prominent nucleoli, morules absent, abundant desmoplastic stroma, always AFP-positive cells, p53 overexpression common, membranous β-catenin expression
Morphologically similar to non-pulmonary mucinous carcinomas from GI tract, pancreas, ovary, and breast.
Mucinous (colloid) adenocarcinoma
- Rare as a subtype but mixed tumors frequently show mucinous morphology
- Appears same as in other organs in which it is more frequently encountered: dissecting mucin pools containing islands and strips of mucinous epithelium with malignant cytologic features
- In the lung, mucinous columnar cells can focally line alveoli in a lepidic pattern.
- The neoplastic goblet cells express enteric markers (CDX2, MUC2 and CK20) with inconsistent, focal and weak CK7 and TTF-1 expression--thus, clinical correlation is needed to diagnose this as a pure lung primary.
- Primary lung mucinous BAC does not show mucin pools invading stroma, is primarily composed of neoplastic mucinous cells lining alveolar walls, and does not show expression of CDX2 or MUC2.
- This neoplasm should be considered a low-grade carcinoma with more favorable survival rates compared to mucinous BAC or conventional mixed adenocarcinoma.
- Like the ovarian counterparts, this groups of tumors shows a histologic spectrum encompassing benign (mucinous cystadenoma), borderline (mucinous cystic tumor of borderline malignancy) and malignant (mucinous cystadenocarcinoma).
- The cytological and architectural criteria for differentiating these tumors are also similar to those in the ovary, except that invasion of surrounding lung is diagnosed as a mixed subtype of adenocarcinoma (for reasons I'm not quite clear but there it is).
- The mucinous epithelium in all of these cystic lesions differ from the mucinous adenocarcinoma type in that these express CK7 but inconsistently express CK20 and TTF-1. These are quite rare and have been studied for MUC2 or CDX2.
- Rare as a pure subtype and uncommon (around 2%) as a component of mixed adenocarcinomas.
- Younger average age when predominant as a component compared to average age of lung cancer patients in general
- Unlike the other subtypes, this variant is defined by cytology--the classic signet-ring morphology.
- Intracytoplasmic material stains with PAS diastase, mucicarmine, and Alcian blue @ pH 2.7--indicating both neutral and acid mucins
- Over 95% of tumors express CK7 and over 80% express TTF-1 but are negative for CK20; also positive for CEA, MUC1 and MUC5AC--a staining profile that helps distinguish this as a primary lung tumor versus metastasis from breast, gastric and colon primaries.
- This is the most aggressive mucinous-type lung adenocarcinoma.
- Tumors with at least 50% signet-ring component have a poorer 5-year survival rate compared to the typical adenocarcinoma prognosis.
- Extremely rare as a pure subtype--but over a third of mixed adenocarcinomas show at least some focal clear cell change.
- Tumor cells form sheets with at least focal tubal formation (the latter required for diagnosis) and may form papillae.
- Intracytoplasmic material is PAS-positive, diastase-sensitive--indicative of glycogen.
- Pure tumors have not been well-studied by IHC--one reported tumor was CK7+/CK20-/TTF-1+.
- Distinguishing primary lung clear cell adenocarcinoma from a renal metastasis requires
- clinical and radiologic correlation
- identifying a mixed growth pattern and intracytoplasmic mucin
- finding IHC markers specific for RCC