Current status on response to treatment in locally advanced rectal cancer: what the radiologist should know.

The assessment of tumor response, after neoadjuvant radiochemotherapy (nCRT), allows stratifying the patient in order to consider the proper therapeutical management. Histopathology analysis of the surgical specimen is considered the gold standard to assess tumour response and the definition of a complete cancer response is related to the clinical and endoscopic features, by direct evaluation of the rectal wall. However, imaging studies, especially Magnetic Resonance Imaging (MRI) have provided additional parameters, as the evaluation of nodal or mesorectal status. MRI provides a radiological tumour regression grade (mrTRG) that is correlated with the pathologic tumor regression grade (pTRG). Functional MRI parameters have additional impending in early prediction of the efficacy of therapy and can be valuable in drug development processes. Some of functional methodologies are already part of clinical practice: diffusion-weighted MRI (DW-MRI) and perfusion imaging (dynamic contrast enhanced MRI [DCE-MRI]). Other technologies, such as radiomics with MRI are still in the experimental phase. An adequate radiological report describing the restaging of rectal cancer after nCRT should be a "structured report" to improve communication in a multidisciplinary team.