Role of Imaging in Response Assessment and Individualised Treatment for Sarcomas

The first systematic response evaluation criteria were established by WHO, based on the tumor size changes shortly after the computed tomography (CT) technique became available to the daily practice. RECIST, a simplified version of WHO criteria, and its newer version, RECIST1.1 are the currently available international response evaluation criteria in solid tumors and remains based on tumor size changes. While the introduction of molecularly targeted drugs has significantly improved the survival in patient with sarcomas, the evaluation of tumor response has become more complicated. Increasing number of studies have reported the lack of shrinkage in responding tumors and raised concerns of significant underestimation of responses using RECIST. The first such observation was made on gastrointestinal stromal tumor (GIST) treated with imatinib. In GISTs responding to imatinib, the degree of contrast enhancement on CT typically decreases significantly compared with the baseline, and, regardless of whether tumors shrink, heterogeneous hyperattenuating tumors become homogeneous hypoattenuating tumors with a smaller enhancing solid component. In current oncology practice, CT is a widely accepted method of evaluating tumor response. CT images are relatively simple to acquire and can be reasonably reproduced with no significant technical obstacles. FDG-PET is highly sensitive and specific in identifying responding sarcomas. It has mostly been used as a problem solver and for those with marginally resectable GIST. More recently, the utility of whole body MRI is undergoing exploration. This article discusses the traditional size-based response evaluation criteria, and introduces new evidence based response evaluation based on changes in morphology in addition to changes in tumor size on CT images, and whole body imaging is introduced at the end.

Response assessment in gastrointestinal stromal tumor

Imatinib is standard first-line treatment for patients with advanced gastrointestinal stromal tumor (GIST). Initial responses are not always accompanied by reductions in tumor size; consequently, other parameters should be considered in response assessments. Conventional size-based criteria such as Response Evaluation Criteria in Solid Tumors (RECIST) may underestimate responses to imatinib and have poor predictive value for outcome. Imatinib-responding tumors demonstrate decreased metabolic activity on positron emission tomography within the first weeks of treatment, often showing reduced density and greater homogeneity on computed tomography (CT) scans regardless of initial changes in tumor size. New criteria, based on reductions in tumor size or in tumor density on CT, seem more sensitive and specific for detecting early responses to imatinib, and more predictive of time to tumor progression and disease-specific survival. Compared with conventional size-based criteria, new CT-based criteria may potentially offer improved response assessment and be predictive of outcome in GIST. However, such emerging criteria should be further explored and validated in large, multicenter trials with imatinib and other kinase inhibitors in GIST and in other solid tumors.