Neoadjuvant preoperative chemo-radiotherapy (CRT) and radical surgery (Total Mesorectal Excision) has become the standard treatment for locally advanced rectal cancer. However, this therapeutic approach needs robust staging and re-staging techniques in order to avoid over- or under-treatment of different sub-groups of Patients. Magnetic Resonance Imaging (MRI) is the gold standard for pre-operative staging of rectal cancer, but the reported evidence on the role of MRI in assessing response to therapy (re-staging) based only on morphological evaluation has been shown presenting several limitations. Hence the reason for investigating novel MR imaging quantitative biomarkers, able to establish more objectively the response to therapy and to identify non-responders immediately after the start of neo-adjuvant treatment.

Different MR imaging biomarkers have been identified in oncologic imaging: a) Texture analysis (TA) of morphological T2 weighted images; b) Diffusion-Weighted Imaging (DWI); c) Intravoxel incoherent motion (IVIM) and d) perfusion MRI (pMRI).

a) Texture analysis is non-invasive method of assessing the heterogeneity within a tumor. Tumors with high intratumoral heterogeneity have been shown to have poorer prognosis, which could be secondary to intrinsic aggressive biology or treatment resistance. Different methods exist, including statistical-, model-, and transform-based methods. To date, studies that have been performed have focused in several areas, where the addition of texture to current methods may improve the detection, diagnosis, characterization, and response assessment. By highlighting certain features within a lesion of interest, texture analysis has the ability to improve assessment beyond direct visual analysis by a radiologist

b) Diffusion weighted imaging (DWI) is a diagnostic technique based on the measurement of Brownian motion of water molecules in tissues, which is different according to tissue characteristics. The apparent diffusion coefficient (ADC), a quantitative parameter measured on DWI, has been suggested to provide useful information regarding tumor cellularity, tumor aggressiveness, subtype characterization, and cancer treatment response. To date, in rectal cancer assessment, there is no demonstrated benefit of the use of DWI at primary staging, but there is growing evidence that the use of DWI in association with morphological T2 weighted sequences improves the performance of MRI in the assessment of tumor response after CRT and might be helpful in predicting responders versus non-responders. However, no consensus exists on the additional value of DWI for the assessment of a complete tumor response after CRT. Preliminary experimental evidences suggest that pre-CRT ADC in responders is significantly lower than that in non-responders and moreover a significant increase of the mean ADC after neoadjuvant CRT occur in the responder group with locally advanced rectal cancer. Lower ADC values were associated with a more aggressive tumor profile and ADC has the potential to become an imaging biomarker of tumor aggressiveness profile.

c) Intravoxel incoherent motion (IVIM) is an advanced analysis of DWI based on the acquisition of multiple diffusion b values. It represents a novel imaging approach in abdominal tumors, never being tested up to date in rectal cancer. Advanced analysis permits to evaluate microscopic movements in biologic tissues, including diffusion of water molecules and microcirculation of blood (perfusion). IVIMs are quantified by means of the apparent diffusion coefficient (ADC). ADC incorporates the effects of both diffusion and perfusion and is equal to the true diffusion coefficient D when diffusion is the only type of present motion. Perfusion fraction is strictly related to tissue vascularization. The possibility to accurately calculate this parameter could enable a non invasive evaluation of tumor perfusion. Perfusion parameters derived from IVIM modeling have been proposed as potential biomarkers of tumor vascularization. Indeed, the exact nature of what is measured with IVIM MR imaging deserves further investigation.

d) Perfusion MRI (pMRI) offers the possibility to analyze real tumor perfusion after contrast medium (Gadolinium chelate) administration. Perfusion MRI is an imaging modality that relies on the dynamic assessment of tracer uptake kinetics, subsequently quantified by means of pharmacokinetic models. These models describe, in terms of pharmacokinetic parameters, the wash-in and wash-out of contrast agent from the microvasculature into the surrounding tissues. Using a two-compartmental model Ktrans (wash-in rate), Kep (wash-out rate), Ve (extravascular extracellular space volume) and IAUGC90 (initial area under the gadolinium contrast agent concentration time curve at 90s) can be calculated. A preliminary study on a small cohort of patients has recently demonstrated a correlation between tumor response and Ktrans in tumor treated with antiangiogenetic drugs (Anti-VEGF, vascular endothelial growth factor). However, this data are limited to patients treated with experimental drugs and no data are currently available for tumors treated with classical neo-adjuvant CHT protocol.