Even today most lung nodules detected on CT are managed and followed up based on diameters measured manually in the standard imaging planes. It has been long known however that this approach has many shortcomings and pitfalls. Volumetry and comparisons based on nodule growth are less prone to be influenced by nodule geometry or inaccuriacies of caliper measurements, and also illustrate that due to the relative sphericity of most lesions small changes in diameter can result in large difference in volume.
Volume doubling time (VDT) is a parameter first proposed decades ago that demonstrates the kinetics of nodule volume changes, and thus lends itself to aid decision making and management. I have therefore made two calculators available today to quantify this parameter together with percent volume growth.
The volume-based calculator needs nodule volume as an input parameter (this is ideally obtained via semi-automated volumetry software). The reality is however that a large proportion of imaging departments worldwide do not have such tools available. Therefore I have provided another, diameter-based formula, which can calculate VDT from nodule diameters measured in three planes (the formula is the same as the one used in the NELSON clinical trial). This can also be useful if for some reason automated volumetry is not possible or incorrect (e.g. some subpleural and subsolid nodules). As a further alternative there is a simple volume calculator, which from calculates volume from the largest diameter assuming sphericity. The obtained volume can be then used as an input parameter for the volume-based VDT calculator. Note however that this approach has a considerably larger margin of error than true volumetry.
In any case, it has to be stressed that VDT is just one of the parameters and signs one has to be on the lookout for while assessing nodule behavior. Nodule morphology, percent volume growth, absolute volume, and individual factors have to be also considered while making decisions on follow-up and further diagnostic studies.