Death from cancer is usually the result of dissemination of cancer cells from a primary tumor to secondary vital organs, and the formation of metastases. This process involves a series of steps, each of which have become targets of anticancer therapies such as intravasation of cancer cells into the bloodstream or lymphatics, delivery to organs (e.g., liver, lung, bone, brain, and lymph nodes), extravasation of cells into the organ parenchyma, cell proliferation to form secondary tumors, and development of new blood vessels to sustain continued growth (1). Importantly, single metastatic cells (2,3) or prevascular micrometastases (4) may also remain dormant within an organ, persisting until conditions are suitable for proliferation. Therefore, while surgical treatment of the primary tumor may be successful, undetectable dormant single metastatic cells or prevascular micrometastases can remain clinically silent for long periods and may eventually result in tumor formation and patient relapse (3,4).Metastasis to the brain can occur with many tumor types, including breast cancer, lung cancer, and melanoma. For breast cancer patients, the prevalence of brain metastases was historically estimated at 10 -16% with a 1-year survival rate of 20% (5). More recent studies, however, have demonstrated the prevalence of brain metastases in breast cancer patients to be closer to 22-30% (6), suggesting that its incidence may be increasing as a sanctuary site as systemic control improves. Brain metastases are typically treated with stereotactic radiosurgery or surgery with whole-brain radiation, supplemented with corticosteroid therapy for symptomatic relief. Patchell et al. (7) reported that surgery and whole-brain radiation can cure up to 90% of solitary brain metastases, which suggests that undiagnosed micrometastases or dormant cells are responsible for treatment failure. Thus, identification of micrometastatic and dormant brain metastatic tumor cells may facilitate an understanding of their biology and development of therapeutic interventions.For brain metastases of breast cancer, only a handful of experimental model systems have been reported. Yoneda et al. (8) performed six rounds of selection of human MDA-MB-231 breast carcinoma cells for brain metastasis in mice, followed by excision of the lesion and establishment of a cell culture. The resulting MDA-MB-231BR "brain-seeking" clone metastasized to the brain following intracardiac injection in 100% of the mice. Metastasis was identified histologically, which provided only one time point per animal. Clearly, studies of the metastatic process would greatly benefit from techniques that could dynamically monitor metastases from their earliest stage to endstage growth throughout entire organs or animals. This

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Liver metastasis is a clinically significant contributor to the mortality associated with melanoma, colon, and breast cancer. Preclinical mouse models are essential to the study of liver metastasis, yet their utility has been limited by the inability to study this dynamic process in a noninvasive and longitudinal manner. This study shows that three-dimensional high-frequency ultrasound can be used to noninvasively track the growth of liver metastases and evaluate potential chemotherapeutics in experimental liver metastasis models. Liver metastases produced by mesenteric vein injection of B16F1 (murine melanoma), PAP2 (murine H-ras-transformed fibroblast), HT-29 (human colon carcinoma), and MDA-MB-435/HAL (human breast carcinoma) cells were identified and tracked longitudinally. Tumor size and location were verified by histologic evaluation. Tumor volumes were calculated from the three-dimensional volumetric data, with individual liver metastases showing exponential growth. The importance of volumetric imaging to reduce uncertainty in tumor volume measurement was shown by comparing threedimensional segmented volumes with volumes estimated from diameter measurements and the assumption of an ellipsoid shape. The utility of high-frequency ultrasound imaging in the evaluation of therapeutic interventions was established with a doxorubicin treatment trial. These results show that three-dimensional high-frequency ultrasound imaging may be particularly well suited for the quantitative assessment of metastatic progression and the evaluation of chemotherapeutics in preclinical liver metastasis models.

Myxoma virus (MV) is a rabbit-specific poxvirus, whose unexpected tropism to human cancer cells has led to studies exploring its potential use in oncolytic therapy. MV infects a wide range of human cancer cells in vitro, in a manner intricately linked to the cellular activation of Akt kinase. MV has also been successfully used for treating human glioma xenografts in immunodeficient mice. This study examines the effectiveness of MV in treating primary and metastatic mouse tumors in immunocompetent C57BL6 mice. We have found that several mouse tumor cell lines, including B16 melanomas, are permissive to MV infection. B16F10 cells were used for assessing MV replication and efficacy in syngeneic primary tumor and metastatic models in vivo. Multiple intratumoral injections of MV resulted in dramatic inhibition of tumor growth. Systemic administration of MV in a lung metastasis model with B16F10LacZ cells was dramatically effective in reducing lung tumor burden. Combination therapy of MV with rapamycin reduced both size and number of lung metastases, and also reduced the induced antiviral neutralizing antibody titres, but did not affect tumor tropism. These results show MV to be a promising virotherapeutic agent in immunocompetent animal tumor models, with good efficacy in combination with rapamycin.

Imaging at a time period of equal liver parenchyma and vascular enhancement after contrast injection allows for clear delineation of liver-tumor borders, thereby enabling quantitative tumor-volume monitoring.