Estimate volume doubling time from two CT measurements.
VDT = (Time × ln2) / ln(V2/V1). Volume is estimated assuming a spherical nodule: V = (4/3)π(d/2)³. Malignant nodules typically have a VDT of 20–400 days. VDT < 20 days suggests infection; VDT > 400 days suggests benign etiology. Note that a 26% increase in diameter represents a doubling of volume.