<p>In clinical practice, there is a heavy reliance on imaging to assess tumor growth before determining treatment response. However, in patients with gastrointestinal stromal tumors (GIST) who receive a TKI, decreased tumor attenuation can signal treatment response even in the absence of tumor shrinkage.</p>

In GIST, the standard imaging modality that is used to assess treatment response is typically a computed tomography (CT) or magnetic resonance imaging scan. Some studies have found that fluorodeoxyglucose positron emission tomography imaging can show a response within the first few weeks after initiating TKI therapy. The standard-bearer for measuring tumor response, RECIST 1.1, is based entirely on serial assessments of unidimensional tumor measurements. We have known for many years that this strategy has inherent limitations and may not be the best indicator of treatment response. This is particularly true with the use of TKI-based therapies in GIST, where changes in the imaging characteristics of the tumor may, in fact, reflect beneficial response, even in the setting of an increase in tumor size.

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Alternative criteria have been developed to assess treatment response and may result in an improved assessment of therapeutic efficacy. The Choi criteria, for example, use Hounsfield units to look at changes in tumor attenuation on CT imaging. In addition, vascular tumor burden assessment is an emerging biomarker whereby tumor vasculature can be assessed using quantitative imaging measures and, in some cases, can be predictive of response. These newer tools for assessing treatment response are now increasingly being incorporated into clinical trials as secondary end points.

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Emerging imaging technologies in GIST such as dual-energy CT and novel radiotracers may potentially provide us with a more objective assessment of tumor response. This not only may help facilitate the selection of a therapy for a patient but also may offer a better sense of when to consider a change in treatment.

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As imaging technologies improve, the hope is that it may be possible to better differentiate whether or not a patient is benefiting from a particular therapy. In the same way that we are improving systemic therapies based on an improved understanding of underlying tumor biology, we are also improving our understanding of imaging characteristics that could be reflective of therapeutic response independent of tumor shrinkage.