The hypothesis to be proven with this study is that the use of PET/MR on the initial staging of rectal cancers in patients with extramural vascular invasion detected by MR will detect more lesions than conventional work-up and will significantly impact on therapeutic decision, improving disease free and overall survival.

The accurate staging of rectal cancer is essential to define therapy and for prognosis assessment. Imaging modalities usually provide useful information for pre-operative planning of primary tumour resection and may indicate the need of neoadjuvant treatment. It is recommended the use of magnetic resonance imaging (MRI) for local staging and computed tomography (CT) of chest, abdomen and pelvis for detection of distant metastasis. Patients with rectal cancer and vascular invasion might benefit from an intensive pre-operative staging in order to early detect distant metastasis, favouring a better therapeutic planning. There is no consensus regarding the use of PET/MR for initial staging of patients with rectal cancer. It has been shown that although changing pattern's in patients' stage, the use of PET/MR for colorectal cancers did not impact disease management. New studies are required for identifying the subgroup of patients with changes in the pre-operative MR that might benefit from the use of PET/MR for initial staging of rectal cancers.

Patients with rectal cancer will undergo pelvic MR, whole-body CT and whole-body PET/MR. According to the tumour characteristics on MR, there will be defined two group of patients: with EMVI-RM (group A) and without EMVI-MR (group B). The whole-body CT and PET/MR will be evaluated for the detection of loco-regional lymph nodes disease and distant metastasis. The total number of lesions and their respective sites will be recorded and compared for each method. The PET/MR management impact will be determined from the medical record or by direct contact with the treating clinician. The impact of PET/MR on management will be defined as high (the treatment modality or intent was changed), medium (the treatment modality or intent remained unchanged, although the method of treatment delivery or planned diagnostic procedure was changed), low (PET/MR results were consistent with planned management, and treatment modality or intent was unchanged), or none (the management plan was not changed, despite being inconsistent with the PET/MR stage-that is, PET/MR results were ignored). Overall survival will be used to evaluate prognostic significance. Clinical follow-up will be performed 3 monthly for 2 years. Imaging and, eventually biopsy, will be performed to evaluate symptoms or signs suggestive of residual or recurrent disease.

Inclusion Criteria:

• Age > 18 years;
• No contraindication to MRI (eletromagnetic devices, claustrophobia);
• No contraindication to PET/MR (hyperglycemia, claustrophobia);
• Ability to understand and the willingness to sign a written informed consent document.

Exclusion Criteria:

• Local resection of rectal tumor;
• Non-colorectal synchronic lesion;
• Previous treatment (chemo or radiation therapy) for rectal cancer;
• Renal insufficiency;
• Pregnancy, lactation or inadequate contraception
• Known allergy to contrast media (CT, MR or PET);
• Blood glucose level higher than 150 mg/dl.

Patients with histologically proven rectal cancer.