Types of Carcinoma

"Malignant neoplasms of epithelial cell origin are called carcinomas... In squamous cell carcinoma the tumor cells resemble stratified squamous epithelium, whereas in adenocarcinoma the neoplastic epithelial cells grow in a glandular pattern."

• Robbins, Cotran & Kumar Pathologic Basis of Disease

Classification by Cell of Origin / Histologic Pattern

1. Squamous Cell Carcinoma (SCC)

• Arises from stratified squamous epithelium or from epithelium undergoing squamous metaplasia
• Hallmark features: keratinization (keratin pearls), intercellular bridges, individual cell keratinization
• Features are prominent in well-differentiated tumors, subtle in poorly differentiated ones
• Common sites: skin, oral cavity, esophagus, cervix, lung, larynx, anus
• Strongly associated with smoking (lung SCC) and HPV (cervical, oropharyngeal)

2. Adenocarcinoma

• Arises from glandular epithelium or forms glandular structures
• Tumor cells form tubules, acini, papillae, or secrete mucin
• Most common carcinoma type overall - especially in lung (now surpassing SCC), colon, breast, prostate, stomach, pancreas, endometrium
• In the lung, it is the most common type in women, never-smokers, and patients under 45 years of age

3. Basal Cell Carcinoma (BCC)

• Arises from basal cells of the skin (or adnexal structures)
• Most common cancer in humans overall
• Locally invasive but rarely metastasizes
• Common in sun-exposed skin of the face

4. Transitional Cell Carcinoma (Urothelial Carcinoma)

• Arises from transitional epithelium (urothelium) lining the urinary tract
• Common in bladder, renal pelvis, ureter
• Associated with smoking and certain occupational chemical exposures

5. Undifferentiated (Anaplastic) Carcinoma

• Cells show no recognizable differentiation pattern
• Designated undifferentiated malignant neoplasm when the cell of origin cannot be identified (approximately 2% of cases)
• Highly aggressive behavior

6. Small Cell Carcinoma

• A subtype of neuroendocrine carcinoma
• Predominantly in the lung; strongly linked to heavy smoking
• Oat-shaped cells with scant cytoplasm, high mitotic rate, early widespread metastasis
• Grouped as Small Cell Lung Cancer (SCLC) - treated differently from all other types

7. Large Cell Carcinoma

• A diagnosis of exclusion - undifferentiated malignant tumor without squamous, glandular, or neuroendocrine features
• Typically lung; poor prognosis

8. Neuroendocrine Carcinoma (NEC) / Carcinoid Tumors

• Arise from diffuse neuroendocrine cells
• Spectrum: well-differentiated (typical carcinoid) --> poorly differentiated (large cell NEC / small cell carcinoma)
• Can secrete hormones causing paraneoplastic syndromes

9. Adenosquamous Carcinoma

• Mixed type with both glandular (adenocarcinoma) and squamous (SCC) components in the same tumor
• Seen in lung, cervix, endometrium, pancreas

10. Sarcomatoid Carcinoma

• Carcinoma cells that acquire spindle-cell (sarcoma-like) morphology
• Includes spindle cell carcinoma and giant cell carcinoma (lung)

Summary Table

Broad Clinical Division (Lung as Paradigm)

• SCLC (Small Cell Lung Cancer) - neuroendocrine, responds to chemo but usually incurable
• NSCLC (Non-Small Cell Lung Cancer) - includes adenocarcinoma, SCC, large cell carcinoma; more likely to be resectable; targetable mutations (EGFR, ALK, ROS1, KRAS) guide therapy

"The four major histologic types of lung carcinoma are adenocarcinoma, squamous cell carcinoma, small cell carcinoma, and large cell carcinoma."

• Robbins & Kumar Basic Pathology (2021 WHO Classification)