How to measure a lung nodule in a CT Scan DICOM image?
Aside from the whole discussion of pure signal processing: 
What exactly do you define as "the nodule". This is usually a biological entity with difficult to define borders. The nature of the nodule is sometimes invasive growth and therefore even in histological sections poorly defined. The CT itself has therefore a threshold higher than histology and therefore the true border of a nodule cannot defined easily. On the other hand, healthy tissue around the nodule might be compressed and appear dense in CT. This might be depending on the respiration phase the scan was triggert on (best results in midinspiration, according to newer data). Or inflammation might obscure the true border of the nodule.

Another aspect is interpolation of the images. The technique is usually a spiral-CT, so you don´t miss any lesions. The sections are therefore calculated. This results less defined borders of objects. If the focus would be on border detection or near histology resolution, high-resolution scans are usually required. This is what you do for interstitial lung disease. 
Unfortunately, they slice through the lung with a pretty large distance in between. Radiation dose would be extremely high, if you try to get a "full volume scan". But in this type of scan, you have to be aware, that You´s miss lesions between slices.

To come back to the initial question: I think you will have to validate your technique - whatever it is - with the gold standard. Which is histology sections. (Unfortunately, the lung isn´t easily sectioned...). Another option is an additional technique: like PET-CT (combining positron emission tomography with computer tomography), but the alignment algorithms are sometimes tricky.