Tumor Doubling Time Calculator

Volume doubling time is calculated from two serial imaging measurements of the same tumor, converting each longest-diameter reading into an estimated volume under a spherical-geometry assumption (4/3 x pi x radius cubed) and then finding how long it would take, at the observed exponential growth rate, for that volume to double. Because volume scales with the cube of diameter, even a modest-looking diameter increase between two scans can represent a much larger volume increase -- a diameter growing from 15 mm to 22 mm, for example, is roughly a 3.1-fold volume increase, not a 1.5-fold one, so a growth rate calculated from diameter alone systematically understates the underlying volume change. The number of days between the two scans also matters more to the doubling time estimate than either diameter reading alone, since the same measured volume ratio stretched over a longer interval implies a slower underlying growth rate, and stretched over a shorter interval implies a faster one -- a scanning interval recorded even a few days off can shift the calculated doubling time by a comparable margin to a real change in tumor size.

The specific growth rate (percent change per day, on the same exponential model) and the projected future diameter both follow directly from that same growth rate, extrapolated forward assuming growth stays exponential and unchanged from what the two scans captured -- an assumption that becomes progressively less reliable the further out the projection window and the target-diameter calculation are set, since neither of those two settings can influence the calculated doubling time itself; they only determine how far the same measured growth rate gets extrapolated forward. Real tumors frequently show S-shaped (Gompertzian) rather than pure exponential growth over long periods, treatment can change the growth rate entirely between scans, and imaging measurement variability of just a millimeter or two can swing the doubling time estimate substantially for slow-growing or minimally changed tumors. This tool computes growth kinetics from measurements you provide -- it does not estimate survival or prognosis, and a RECIST percent-diameter-change figure from two points is not equivalent to a formal RECIST response assessment across a full treatment timeline.