Lung Nodule Growth Rate Calculator — Volume Doubling Time
Enter two measurements from CT follow-up and get the volume doubling time (VDT), the daily growth rate, and an honest rule-of-thumb interpretation of the growth speed. Works from volumes (mm³), diameters (mm), or mass measurements. Educational only — this is not a diagnosis.
Volume doubling time from two volumes
Formula (Schwartz, Cancer 1961): VDT = t × ln(2) / ln(V2/V1).
Diameter → volume → doubling time
CT reports often give diameter only. Each diameter is converted to a sphere volume (V = π/6 × d³) before VDT is computed. Real nodules are rarely perfect spheres — radiology volumetry is more accurate.
Mass doubling time
For solid masses tracked by mass (e.g. quantitative CT): MDT = t × ln(2) / ln(M2/M1) — the same exponential formula.
How to interpret the result
| VDT | Rule-of-thumb interpretation |
|---|---|
| < 400 days | Fast growth — higher concern. Under 400 days is the most frequently used threshold that triggers further workup for indeterminate nodules. |
| 400 – 600 days | Intermediate — indeterminate. A gray zone; interpret only with a radiologist. |
| > 600 days | Slow growth — benign-leaning. Most studies find VDTs above 500–600 days overwhelmingly characteristic of benign nodules. |
Honest note: sources disagree on the benign boundary (500 vs 600 days) — we use the conservative 600 days and show this table as a rule of thumb, never a diagnostic cutoff. Benign nodules can grow fast; cancers can grow slowly or stay stable.
Frequently asked questions
What is volume doubling time (VDT)?
Volume doubling time is the number of days a lung nodule takes to double its volume under an exponential growth model. It is a standard follow-up metric in lung-cancer screening and pulmonary nodule surveillance, computed from two volume measurements and the days between the two CT scans. Longer VDT suggests slower, more benign-leaning growth; shorter VDT suggests faster, more concerning growth.
What is the volume doubling time formula?
The standard formula (Schwartz, Cancer 1961) is VDT = t × ln(2) / ln(V2/V1), where t is the interval in days between the two CT scans, V1 is the first volume and V2 the second volume in mm³. The calculator also reports the equivalent daily growth rate, (V2/V1)^(1/t) − 1, as percent per day.
What does a worked example look like?
A nodule measured 100 mm³, then 200 mm³ after 150 days. VDT = 150 × ln2 / ln(2) = 150.0 days, with a daily growth rate of 0.4632% per day. Because 150 days is under the 400-day threshold, the tool labels this fast growth — a rule-of-thumb flag to discuss with a radiologist, not a diagnosis.
What VDT is concerning for a lung nodule?
A widely used rule of thumb: VDT under 400 days = fast growth with higher concern (the most frequently used threshold that triggers further workup for indeterminate nodules); 400–600 days = intermediate/indeterminate; over 600 days = slow growth, benign-leaning. Radiopaedia notes VDT above 500 days is overwhelmingly characteristic of benign nodules, so 500–600 is a gray zone where sources differ. These are rules of thumb, not diagnostic cutoffs — benign nodules can grow fast and cancers can grow slowly.
My CT report only gives diameter, not volume. Can I still compute VDT?
Yes — use the Diameter tab. It converts each diameter to a sphere volume with V = π/6 × d³, then applies the standard VDT formula. Example: 6 mm → 7 mm over 180 days gives volumes 113.10 mm³ and 179.59 mm³ and a VDT of 269.79 days. Note this assumes a spherical nodule; real nodules are often non-spherical, so radiology volumetry software is more accurate.
What does a negative VDT mean?
A negative VDT means the nodule got smaller between scans — its absolute value is the halving time in days. Example: 200 mm³ → 100 mm³ over 120 days gives a VDT of −120.0 days, i.e. the nodule halved its volume every 120 days. Shrinkage is usually reassuring, but interpretation still belongs to a clinician.
What is mass doubling time (MDT)?
Mass doubling time is the same exponential-growth calculation applied to solid mass measurements instead of volume: MDT = t × ln(2) / ln(M2/M1). It is used when mass (e.g. from quantitative CT) is tracked rather than volume. Example: 50 mg → 100 mg over 200 days gives an MDT of 200.0 days.
Is this calculator a diagnosis?
No. Volume doubling time is one feature among many that clinicians use; nodule management follows the Fleischner Society guidelines, which combine size, morphology, patient risk factors, and growth. Always discuss CT findings with your doctor or radiologist — do not make medical decisions from this calculator alone.
Is my data sent to a server?
No. All calculations run locally in your browser with JavaScript. Nothing is uploaded, stored, or shared — the page works offline once loaded.
Related tools
- Adrenal Washout Calculator — adrenal nodule washout percentage on CT
- BMI Calculator — body mass index