Lung nodule volume doubling time calculator
Compare a pulmonary nodule across two CT studies. Enter the prior and current volume (or mean diameter) and the scan interval to get the volume change and the volume doubling time (VDT), read against the Australian NLCSP growth definitions and the BTS 400- and 600-day thresholds, with a report line you can paste into your CT report.
or enter the interval directly — scan dates stay in your browser
Use the same volumetry software and reconstruction on both studies, and compare with the earliest available CT so slow growth is not missed. For a part-solid nodule, track the solid component.
Report line
How volume doubling time works
Volume doubling time is the number of days a nodule would take to double in volume if it kept growing at the rate measured between two studies. It uses the modified Schwartz exponential growth model. A shorter VDT means faster growth.
From mean diameters: VDT = interval × ln 2 ÷ (3 × ln(current ÷ prior diameter))
VDT is only defined when the volume has increased. The diameter form treats the nodule as a sphere and cubes any measurement error, so a 1 mm change on a 6 mm nodule is a volume change of almost 60%. Use automated volumetry on both studies wherever possible.
How the two frameworks read the result
| Framework | Result | Reading |
|---|---|---|
| NLCSP, volumetry | Increase over 25%, VDT under 600 days | Growing |
| NLCSP, volumetry | Increase over 25%, VDT over 600 days | Slowly growing if seen across more than one interval; otherwise stable |
| NLCSP, volumetry | Change within ±25% | Stable |
| NLCSP, volumetry | Decrease of 25% or more | Decreased |
| NLCSP, mean diameter | Increase over 1.5 mm | Growing within 24 months; slowly growing over more than 24 months |
| NLCSP, mean diameter | Change within ±1.5 mm | Stable (a fall of 1.5 mm or more is decreased) |
| BTS 2015 | VDT under 400 days | Offer further investigation |
| BTS 2015 | VDT 400–600 days | Yearly surveillance or biopsy, by patient preference |
| BTS 2015 | VDT over 600 days | Consider discharge or ongoing CT surveillance |
The NLCSP definitions apply to low-dose CT in the Australian National Lung Cancer Screening Program, where they feed the category and interval set by the Nodule Management Protocol. This calculator does not assign an NLCSP category or a PanCan risk; use the official protocol and the risk calculator it links to for that. For an incidental nodule outside screening, the Fleischner 2017 calculator gives the follow-up interval.
Worked example
A solid nodule measures 120 mm³ on the prior CT and 180 mm³ on a CT 182 days later. The volume ratio is 1.5, so the volume change is +50% and the VDT is 182 × 0.693 ÷ 0.405 ≈ 311 days. That is growing under the NLCSP definition and under 400 days on the BTS thresholds. With mean diameters instead, a nodule that goes from 6.0 mm to 7.0 mm over a year gives a volume change of about +59% and a VDT of about 547 days, but the 1.0 mm diameter change is still stable under the NLCSP diameter rule. That gap is why volumetry is preferred.
Common questions
How is the volume doubling time of a lung nodule calculated?
Volume doubling time (VDT) uses the modified Schwartz formula: VDT = interval in days × ln 2 ÷
ln(current volume ÷ prior volume). For example, a nodule that grows from 120 mm³ to 180 mm³
over 182 days has a VDT of about 311 days. VDT is only defined when the volume has
increased.
What VDT counts as growth in the Australian lung cancer screening program?
In the NLCSP Nodule Management Protocol, a volume increase of more than 25% with a VDT under
600 days is growing. An increase of more than 25% with a VDT over 600 days is slowly growing
only when it holds across more than one scan interval; otherwise it counts as stable. A change
within ±25% is stable, and a fall of 25% or more is decreased. When volumetry is not possible,
the protocol uses a mean-diameter change of more than 1.5 mm.
What do the BTS 400-day and 600-day thresholds mean?
The British Thoracic Society 2015 guideline treats a volume increase of 25% or more as
significant. A VDT under 400 days supports further investigation, 400 to 600 days supports
yearly surveillance or biopsy by patient preference, and over 600 days supports discharge or
ongoing CT surveillance.
Can I calculate VDT from diameters instead of volumes?
Yes, the calculator treats the nodule as a sphere, but any diameter error is cubed. A 1 mm
change on a 6 mm nodule is a volume change of almost 60%. Use automated volumetry on both
studies wherever possible.
Sources: Australian Government Department of Health, Disability and Ageing. National Lung Cancer Screening Program Nodule Management Protocol (2025), explanatory note 6. Callister MEJ, Baldwin DR, Akram AR, et al. British Thoracic Society guidelines for the investigation and management of pulmonary nodules. Thorax 2015;70(Suppl 2):ii1–ii54. RadSpeed is not affiliated with the NLCSP or the BTS. Confirm against the current documents.