Nanostructure-initiator mass spectrometry (NIMS) FLT-MP/FLT ratiometric imaging of docetaxel- and vehicle-treated breast tumor mouse xenografts. Three-dimensional NIMS images of tumor sections from 3′-deoxy-3′-fluorothymidine (FLT)-treated mice shows significantly higher conversion of FLT to FLT-MP in vehicle-treated (left) versus docetaxel-treated mice (right). Tumor sections from one set of vehicle- and docetaxel-treated mice were placed in close proximity on a single NIMS chip and imaged using NIMS (Chip #1, top). As further validation, tumor sections from a second set of vehicle- and docetaxel-treated mice were placed in close proximity on a second NIMS chip and imaged using NIMS (Chip #2, bottom). A total of four independent tumors were imaged, two from docetaxel-treated and two from vehicle-treated mice. FLT-MP/FLT ratios were normalized to a maximum value of 1 for each NIMS chip.