ABSTRACT
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Imaging, using CT or MRI, has provided a major endpoint of response assessment for clinical trials
of conventional cytotoxic agents. Changes in bidirectional size (WHO criteria) or unidirectional
size (of the maximum dimension) RECIST are used as measures of tumor response. While widely
used, these measures are usually performed at the end of, typically, six courses of treatment, and do
not provide an early assessment of response. Size is usually considered to encompass the whole
observable volume, and does not provide an easy way of assessing the volume of viable tumor
remaining, where there may also be areas of necrosis or cyst. While MRI, based on the water
content of tissues, provides unrivaled morphological detail, it can also provide a number of
functional measures of tissues. Contrast agents can change the MR properties of water, providing a
means of probing the vascular function of tumors. While most contrast agents are passive, there is
considerable research effort directed at active agents that will report on their local environment or
cell surface receptors, or bioactive probes with signals that can be modulated by local biochemical
activity. Diffusion of water can be measured, probing the extracellular space of tumors, providing
information on cellularity and necrosis. These methods have been shown to inform on drug
delivery, and response.