The management of localized rectal cancer is done in two phases. The first phase, medical, combines a possible first chemotherapy, followed by a concomitant chemoradiotherapy. The second phase, 6 to 8 weeks later, consists of surgery of the rectum.~Magnetic resonance imaging (MRI) of the pelvis allows staging of the tumor and its possible lymph node extensions. MRI can also be used to assess tumor response after the first phase of treatment and before surgery. But MRI of the pelvis is partially imperfect.~PET-MRI is an examination that combines MRI with an injection of [18 F]fluorodeoxyglucose (18F-FDG). It is known to be more sensitive in assessing tumor stage and lymph node extensions compared to MRI and more sensitive in assessing tumor response before surgery.~The aim of this study is to determine predictive parameters for response to the first phase of treatment by accurately identifying the characteristics of tumors that have responded or not to treatment with chemotherapy and chemoradiotherapy.
With 50 patients included, this trial would be the largest pilot study evaluating the value of MRI PET in locally advanced rectal cancer. On the other hand, it would be the only pilot study performing several MRI PET during neoadjuvant treatment. Presumably, the response assessed at the 2nd MRI PET (before cycle 4 or induction chemotherapy) would be predictive of the overall response at the end of neoadjuvant treatment. Then, it would be possible to predict precociously the tumor response.