Fluorescencelabeled human tumor cells can be easily observed noninvasively in superficial organs. However, if tumor growth occurs in internal organs, such observations are difficult and in most cases impossible to detect.The tissue depth limitation of this technique becomes even more evident during visualization of Denatonium benzoate microscopic brain tumors.In the brain, excitationemission of fluorescently labeled tumor cells is not only limited by tissue depth, but also by light penetration through the skull.Other tumor cell visualization techniques can be employed to circumvent this limitation of fluorescence labeling.This imaging modality can be used for noninvasive and sequential detection of human microscopic tumors in mice.This technique has been used for the detection of dormant human liposarcoma tumors in the renal fat pad of mice and for monitoring the growth of human glioblastoma tumors stereotactically inoculated in the brain of mice. The luciferase reporter allows for the reliable detection of a signal from tumors that are less than mm in diameter, as verified by histology.The sensitivity of this method makes it useful for the detection of microscopic tumors in organs that are otherwise challenging for Trilostane realtime imaging, such as the brain.Small animal MRI imaging provides clear definition of the geometry of a microscopic tumor, and can be used in combination with luciferase imaging. The enzymatic activity of luciferase is rapid and transient, and can be detected only following intravenous injection of the substrate. Only viable and metabolically active tumor cells can be detected.Therefore, this method allows for realtime monitoring of tumor cell presence and viability during the dormancy period as well as throughout the angiogenic switch.The presence of a luciferase signal during the dormancy period of microscopic human tumors confirms the previous conclusion that dormancy does not result from tumor cell cycle arrest or eradication.In all cases, an increase in luminescence intensity was followed by the growth of tumors.Therefore, this imaging modality is a sensitive method for the detection of microscopic human tumors during the dormancy phase and throughout the switch to the angiogenic phenotype.Although this imaging modality has great potential for aiding our understanding of the biology of the angiogenic switch, it cannot be employed clinically for the detection of microscopic tumors.Other imaging methods have been recently reported for noninvasive longitudinal detection of small micrometastases and single cancer cells in a mouse brain.Individual cancer cells trapped within the brain microcirculation were detected using MRI and validated using highresolution confocal microscopy.At least two different approaches can be utilized to separate these two tumor cell populations in vitro or in vivo.Previous studies used established human tumor cell lines which were described by the literature as nontumorigenic or as having a no take phenotype.These studies demonstrated that mice inoculated with human liposarcoma cells developed three kinds of tumors: nonangiogenic tumors.Because nonangiogenic tumors were neither grossly visible nor palpable, they have previously been called no take or nontumorigenic.Moreover, the spontaneous switch of dormant tumors to the angiogenic phenotype would have been missed if the length of the study was less than a year.Therefore, some of the existing human tumor cells lines have been wrongfully termed nontumorigenic when indeed they can form tumors, but only after existing for months as microscopic tumors.