Natural History

Many pathologic entities, particularly malignant diseases. may be arbitrarily resolved into a number of phases based on the way they manifest themselves clinically. That is, the natural history of a given disease may be described or summarized by breaking down its entire course of existence into a sequence of a few simple phases based on the way we experience the disease clinically. Carcinoma of the lung is especially amenable to such an analysis.

Pre-Detectable

Carcinoma of the lung always passes through a pre-detectable phase, beginning with its biological onset (the development of the first frankly malignant cell) and beginning when the disease may first be shown to exist whether through sputum cytology or chest radiography. Obviously, it is impossible to know the exact duration of this phase. It has been claimed that by the time a tumor is 10 mm in diameter it has already doubled in size 30 times, contains at least one billion cells, and has completed three-fourths of its anticipated existence. While this statement is perhaps overly simplistic (not taking cell type into account, for instance), the point remains that it is likely that during the majority of a lung tumor's existence it will be undetectable by any currently available diagnostic technique.

Detectable-Asymptomatic

Most cases of lung carcinoma are felt to enter a phase in which presence of the disease is potentially demonstrable, yet continues to be without symptoms. The disease is detectable if:

• The tumor is radiographically evident (5-10 mm in diameter), or
• Sputum is positive for malignant cells.

The duration of this "presymptomstic-detectable" phase is heavily dependent on the cell type involved and on location of the primary tumor. Sputum cytology can be positive for several years before symptoms occur in a progress from undetectable to unresectable within a few short months. Unfortunately, only about 5% of lung cancer diagnoses are made in this phase. These findings are typically made through incidental X-Ray findings during workup of an unrelated condition of through sputum and X-Ray screening of high-risk patients.

Symptomatic Phase

About 95% of all lung cancer diagnoses are made during the phase when the disease has become symptomatic. Carcinoma discovered at this point in its natural history is almost always well advanced. With very few but significant exceptions, symptomatic lung cancer carries poor prognosis. This is because the vast majority of symptoms in this disease are caused by either locally unresectable or metastatic tumor.