Response Evaluation After Radiotherapy

Accurate assessment of the response to radiotherapy is essential to avoid missing the opportunity for early salvage treatment for residual tumors as well as overtreatment in complete responders. The evaluation of radiotherapy response in liver tumors mainly involves the interpretation of imaging studies. Dynamic contrast-enhanced computed tomography (CT) or magnetic resonance imaging (MRI) are the most widely used image evaluation tools. Enhancement-based imaging response evaluation criteria, such as the European Association for Study of the Liver (EASL) and the modified Response Evaluation Criteria in Solid Tumors (mRECIST), are more sensitive than the size change-based RECIST and World Health Organization (WHO) criteria. However, early assessment of the tumor response is challenging. Understanding the time frame of imaging changes of the tumor and surrounding irradiated liver parenchyma is essential for accurate assessment. The typical imaging changes of the tumor are gradual reduction in size of the enhancing part and increase of necrosis over time. The focal liver reaction of the irradiated peritumoral liver tissue also changes over time from hyperemia in acute phase to gradual returning to normal enhancement pattern in chronic phase. Thus, accurate response interpretation requires a series of follow-up contrast-enhanced images with careful assessment of consecutive changes. In this chapter, we describe the imaging assessment of radiotherapy response in liver tumors, focusing on hepatocellular carcinoma (HCC).