Discrepancy between measurements of tumors made independently by different physicians can be substantial. There are also differences between measurements of tumors made by the same physician on different occasions, but some physicians are significantly more consistent than others. The techniques by which measurements are made, namely, readings and tracings in the case of lesions shown on x-ray film, affect this relative consistency—some physicians may be relatively consistent by one technique but relatively inconsistent by another. Probability considerations concerning the wrong decisions that can be made regarding change of size indicate that descriptions of changes in small lesions are very unreliable.
Gurland J, Johnson RO. How Reliable Are Tumor Measurements?. JAMA. 1965;194(9):973-978. doi:10.1001/jama.1965.03090220029008