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Detection of growth
Malignant nodules usually grow at a constant exponential rate, which can be expressed as the tumor's doubling time (ie, the interval required for it to double in size). An increase of 28% in nodule diameter indicates doubling
. In malignant lesions, the doubling time is between 25 and 450 days in most cases, with a median of 120 days.
Benign lesions are usually stable (ie, show no growth). Sometimes they grow slowly (ie, doubling time exceeds 500 days), but occasionally their doubling time falls within the time frame typical of a malignant tumor
. A long-standing surgical dictum states that a solitary pulmonary nodule is almost certainly benign if it is stable over a 2-year period (which implies a doubling time exceeding 730 days)
. Stability is determined by retrospective analysis of one or more chest films.
A doubling time of less than 20 days usually signifies that the nodule represents a nonmalignant, acute inflammatory process.
http://www.postgradmed.com/issues/1997/03_97/lilling.htm