The volume doubling time (VDT) is an important volumetric parameter primarily used in lung cancer screening and follow-up by chest CT. Volume doubling time is defined as the time required for a growing nodule to double its volume. A longer VDT suggests a more benign course, whilst a short VDT is indicative of a more aggressive lesion with higher histological grade.
According to most studies a volume doubling time below 400 days represents a high likelihood of malignancy, whereas a VDT above 500 days is overwhelmingly characteristic of a benign nodule 1,2. With the growing use of software-based volumetry there is recently an increased reliance on VDT as a parameter in CT-based lung cancer screening 3.
Volume doubling time can be calculated manually by using an equation based on the modified Schwartz formula:
VDT = [ln2 × ∆T] / [ln(X2/X1)]
where X2 and X1 are the final and initial nodule volumes, ∆T ("delta T") represents the time (in days) between the two scans, and ln is the natural logarithm 4. In most cases the volume doubling time is however measured by software (increasingly by automated volumetry) or calculators (many of which are available online).
Practical points
CT acquisition (contrast enhanced vs non-contrast) and reconstruction parameters (most importantly slice thickness and reconstruction kernel), as well as window level and width have a great influence on volume doubling time measurements, thus these have to be consistent during a follow-up scan for a reliable evaluation of interval growth 1,5.
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