Purpose or Case Report: Accurate tumor measurement is essential in initial assessment of solid tumors. Furthermore, it is vital when evaluating treatment response. Change in tumor size determines whether a treatment course is effective, if treatment should be prolonged, or whether a more aggressive treatment or chemotherapy drug should be administered. Currently endorsed and widely used guidelines for tumor volume measurement include response evaluation criteria in solid tumors (RECIST), a one dimensional measure (cm) of target lesions which is not routinely the longest axis; World Health Organization (WHO), a 2 dimensional measure of the long and one short tumor axis (cm2) but is not a measure of volume; and Childrens Oncology Group (COG), a 3 dimensional ‘volume’ (cm3) measurement but does not account for shape of the tumor.

Pediatric oncology patients are almost exclusively cared for in major academic or community hospital settings where modern CT and MR scanners routinely produce direct or reconstructed multiplanar images. Therefore an evolution of tumor measurement, to determine tumor volume, must be forthcoming.
Methods & Materials: A simulation experiment was designed to evaluate differences in calculated tumor volume v. RECIST, WHO, and COG criteria using reference spheres and ellipses ranging from 1-25 cm. To test different ellipse sizes a third dimension of 6cm and 12 cm were arbitrarily chosen. Tumor volume(V) was calculated using the following equations: sphere V=4/3πr3, ellipse V=8πr2 or 16πr2; RECIST=x or y, WHO=r2, 6r, or 12r, and COG=r3, 6r2, or 12r2 and percent relative error calculated.
Results: Graphical and numeric analysis comparing calculated volume of a sphere or ellipse with COG measurements shows that COG over-estimates spherical volume by a constant relative error of 63%. Relative error using WHO method, range 67%-169%, depends upon choice of long v. short axis. Relative error using RECIST technique also depends upon choice of axis, range 40%-169%.
Conclusions:
Currently endorsed RECIST, WHO, and COG criteria do not provide accurate estimates of tumor volume but only a scalar measure. The greatest range of potential error may be seen with WHO and RECIST, depending on measured axis.

In this work, results suggest that while COG, WHO, and RECIST provide some form of a scaled measurement, direct 3D volumetric tumor measurement is likely to provide improved accuracy in measurement of tumor burden thereby providing a meaningful basis upon which informed, thoughtful treatment decisions may be made.