PATHOLOGY



Benign tumors, composing 5 per cent of the total, are usually diagnosed on routine chest x-rays, and symptoms, if present, are usually related to bronchial obstruction. The commonest central tumor is the bronchial adenoma, which usually appears benign but is potentially malignant and rarely produces features of the carcinoid syn­drome. The most common peripheral tumor is the pulmonary hamartoma, which has a characteristic “popcorn” pattern of calcification.

Primary malignant neoplasms of the lung can be classified on the basis of their cell type, as sum­marized in Table 24-1. The relative incidence of each cell type varies from study to study. Cur­rently, squamous cell carcinoma and adenocar­cinoma each represent one third of the cases, with the remainder made up of large cell and small cell tumors. Bronchoalveolar carcinoma is considered a variant of adenocarcinoma. Small cell undiffer­entiated carcinoma or oat cell carcinoma has the greatest propensity to metastasize early in its course, leading most clinicians to assume dissem­inated disease at diagnosis even without objective evidence.
Starting as a single malignant cell (10 p size), 30 volume doublings are required to produce a tumor of 1 cm diameter, the smallest size detect­able on a chest x-ray . Ten further doublings produce a tumor 10 cm in diameter, but most patients die before the tumor reaches this size. Small cell carcinoma has the fastest doubling time and the worst prognosis. While adenocar­cinoma has the longest doubling time, it has a worse prognosis than squamous cancers because of early extrathoracic spread.

Metastatic spread of neoplasms to the lung is common, involving the parenchvma, endobron­chial mucosa, chest wall, pleural space, or me­diastinum. Direct extension is the least common mode of spread, occurring with breast, liver, and pancreatic tumors. Hematogenous spread is com­mon with renal, thyroid, and testicular tumors and bone sarcomas and presents with asympto­matic discrete nodules on chest x-ray. Lymphan-gitic spread presents as an infiltrate or diffuse re-ticulonodular pattern on chest x-ray and causes severe dyspnea, usually out of proportion to the x-ray findings. This pattern is typical of spread from adenocarcinoma of the breast, stomach, pan­creas, ovary, prostate, and lung.