About the Lung Nodule Growth Rate Calculator
When a pulmonary nodule shows up on two CT scans, radiologists compare the diameters to judge whether it is growing, shrinking, or unchanged. Because a small nodule is close to spherical, its volume scales with the cube of its diameter - so a modest increase in diameter can represent a much larger increase in volume. This calculator applies the standard Schwartz volume doubling time (VDT) formula to two diameter measurements and the interval between scans to estimate how quickly the nodule's volume is changing.
The formula
Assuming a spherical nodule, volume V is proportional to diameter d cubed. If V1 and V2 are the estimated volumes at the first and second scan and t is the number of days between them, the volume doubling time is:
VDT = t x ln(2) / ln(V2/V1) = t x ln(2) / (3 x ln(d2/d1))
A shorter VDT means the nodule's volume is doubling faster - it is growing quickly. A longer VDT means slower growth. If the two diameters are equal, no growth is detectable over that interval; if the follow-up diameter is smaller, the calculator reports apparent regression rather than a doubling time.
Interpreting the doubling time
Teaching thresholds commonly cited in radiology: a VDT under about 400 days is considered concerning and more suggestive of a malignant process; 400 to 720 days is indeterminate and usually prompts continued short-interval imaging; longer than roughly 720 days (about two years) is traditionally associated with a benign, slow-growing process. These are population-derived bands, not a diagnosis for any one nodule - subsolid (ground-glass) nodules in particular can grow very slowly yet still be malignant, so the classic two-year-stability rule is applied more cautiously to them.
Getting accurate inputs
- Use the same measurement plane and technique on both scans - ideally the same greatest-diameter value recorded by the radiologist, on comparable slice thickness.
- Enter the true interval between the two scans rather than a rounded estimate; a few weeks of error can noticeably shift the doubling time for closely spaced scans.
- Small nodules are harder to measure precisely - a 1 mm difference on a 5 mm nodule is a much larger relative change than the same difference on a 20 mm nodule.
When to consult a professional
This calculator performs the standard doubling-time arithmetic; it does not diagnose, stage, or recommend a management pathway. Nodule follow-up intervals and biopsy decisions should follow your radiologist's or pulmonologist's assessment, which weighs nodule density, shape, location, and your personal risk factors alongside the growth rate.