Lung cancers are divided into two readily identified sub groups, small cell and non small cell lung cancer, upon which treatment strategies are based, though further subgroups are also identified (WHO classification). NSCLC is further subdivided into squamous and adenocarcinoma, each of which has different clinical and morphological appearances. Their behaviour is generally similar, though a primary squamous cell carcinoma frequently grows to quite a size before metastasising, whereas adenocarcinoma may metastasise while the primary tumour is still relatively small.

An interesting sub group of adenocarcinoma spreads along alveolar walls in a “broncho-alveolar” pattern. When small these have a particularly good prognosis. However, if they have consolidated a complete lobe resembling a lobar pneumonia, they tend to continue to spread like a pneumonic process exfoliating cells to other lobes. At this stage it is rarely curable. The broncho-alveolar spread may also resemble interstitial lung fibrosis leading to a delay in diagnosis, which is often eventually made on thoracoscopic or open lung biopsy.

Small cell lung cancer is believed to be the malignant end of a spectrum of tumours showing neuro-endocrine differentiation. These tumours consist of highly anaplastic small cells with a high nuclear/cytoplasmic ratio. They are highly malignant and virtually all have metastasised by the time of diagnosis. The intermediate cell variant behaves similarly to the more typical small cell carcinoma. Small cell tumours make up approximately 25% of lung cancers. Many tumours have mixed components resembling both small cell and non small cell components. These in general behave as small cell carcinoma. Frequently after chemotherapy small cell components have responded well leaving residual non small cell tumour. For this reason consolidation radiotherapy should be considered for residual tumour after standard small cell chemotherapy.