Diffusion-weighted magnetic resonance imaging (DWI) is a microstructural imaging technique that characterizes tissue based on differences in the movement of water molecules. These differences can be quantified using the apparent diffusion coefficient (ADC). The ADC has been shown to differentiate post-treatment inflammation and necrosis from recurrent or persistent tumoural tissue in rectal cancer with high specificity [6]. Pre-treatment ADC assessment with DWI has a sensitivity of 100% and specificity of 86% for detection of pCR (p=0.003) whilst treatment induced changes in ADC (measured at week 2 chemoradiation) have a sensitivity and specificity of 100% for predicting pCR (p=0.0006) [5]. Prediction of response should be improved further with the use of PET imaging. A staging 18F-FDG (18F-2-fluoro-2-deoxy-D-glucose fluorodeoxyglucose ) PET/CT has been demonstrated to provide new findings compared to contrast-enhanced CT of the thorax/abdomen/pelvis resulting in a change in the stage of disease and alteration of treatment strategy in 14% of patients [7]. F-FDG PET/CT can be more predictive of histological response and outcome than anatomic imaging alone [8]. Whilst PET Response Criteria in Solid Tumors (PERCIST) has not been shown to have predictive power on the response to neoadjuvant therapy, PET Residual Disease in Solid Tumor (PREDIST) criteria has been shown to correlate to pCR (p = 0.004) [9].