Nodule Volume Doubling Time (VDT)

Assuming exponential growth, the volume doubling time is VDT = ln(2)·Δt / ln(V2/V1); for diameter pairs the volume ratio is inferred as (d2/d1)³. The British Thoracic Society 2015 guideline and the NELSON screening trial use a ≥25% volume increase as significant growth (the approximate 95% reproducibility limit of CT volumetry) and stratify VDT: below 400 days (or clear growth) triggers further diagnostic workup, 400–600 days supports ongoing yearly surveillance or biopsy by patient preference, and above 600 days supports discharge or ongoing surveillance. Very short doubling times (<25 days) are biologically implausible for malignancy and usually reflect inflammation or measurement error. For two-dimensional measurements, a change of ≥2 mm exceeds the roughly ±1.3 mm reproducibility of manual callipers and is the guideline enlargement criterion for ground-glass nodules. A negative VDT indicates regression, which is generally benign behaviour.