Multimodal evaluation of xenograft tumors in mice with an in-vivo stereo imaging system and small-animal PET/CT.

In small-animal studies, calipers are the standard method used for measurement of external tumor size. However, as tumors are not usually prolate spheroids, this may lead to inaccuracies in the data. Also, correlations vary according to the kind and size of tumors. Tumors were generated by transplanting B16 mouse melanoma cells into the back of Balb/c nude mice. True volumes were measured by calipers, an in-vivo stereo imaging system, and as a standard uptake value (SUV) by (18)F-fluorodeoxyglucose ([(18)F]FDG)-PET. Correlations between measurements were analyzed. Correlation with the true volume was higher for measurements using the in-vivo stereo imaging system (r = 0.876) than with calipers (r = 0.744). Measurement of melanoma volume has a larger measuring error when performed using a caliper compared with measurements performed by stereo imaging when the volume of the melanoma is small. Correlation of the volume and PET-SUV by a caliper is low as the size of the melanoma increases. This same relationship exists with the comparison of stereo imaging and PET-SUV. The correlation between the SUV of [(18)F]FDG-PET and tumor volume with the melanoma is expected to be important in related future studies.