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Figure 17.36 Squamous cell carcinoma of the lung. Islands of invading malignant squamous cells are seen. A few well-developed cell nests
with keratinisation are evident.

slow-growing. Recent estimates on adenocarcinoma place 4. Large cell carcinoma: These are undifferentiated
this as the most frequent histologic subtype of lung cancer. carcinomas which lack the specific features by which they
Adenocarcinoma is further subclassified into 4 types: could be assigned into squamous cell carcinoma or
i) Acinar adenocarcinoma which has predominance of adenocarcinoma. Large cell carcinomas are more common
glandular structure and often occurs in the larger bronchi. in men, have strong association with cigarette smoking
ii) Papillary adenocarcinoma which has a pronounced and are highly malignant tumours. The tumour cells have CHAPTER 17
papillary configuration and is frequently peripherally large nuclei, prominent nucleoli, abundant cytoplasm and
located in the lungs and is found in relation to pulmonary well-defined cell borders. Variants of large cell
scars (scar carcinoma). undifferentiated carcinomas include giant cell carcinoma
with prominence of highly pleomorphic multinucleate
iii) Bronchiolo-alveolar carcinoma is characterised by cells and clear cell carcinoma composed of cells with clear
cuboidal to tall columnar and mucus-secreting epithelial or foamy cytoplasm without mucin.
cells growing along the existing alveoli and forming 5. Adenosquamous carcinoma: These are a small
numerous papillary structures (Fig. 17.38). Ultrastruc- proportion of peripheral scar carcinomas having clear
turally, these tumour cells resemble Clara cells or less often evidence of both keratinisation and glandular
type II pneumocytes. differentiation.
iv) Solid carcinoma is a poorly-differentiated adenocarci- The Respiratory System
noma lacking acini, tubules or papillae but having mucus- SPREAD. Bronchogenic carcinoma can invade the adjoin-
containing vacuoles in many tumour cells. ing structures directly, or may spread by lymphatic and
haematogenous routes.

Figure 17.37 Oat cell carcinoma of the lung. The tumour cells are arranged in sheets, cords or aggregates and at places form pseudorosettes.
The individual tumour cells are small, uniform, lymphocyte-like with scanty cytoplasm.