[Application of magnetic resonance in "non-operative treatment" strategy for rectal cancer].

The primary curative modality for localized rectal cancer is total mesorectal excision (TME). Local control rate of rectal cancer has been improved after neoadjuvant chemoradiotherapy and even pathological complete response (pCR) has been demonstrated in a significant minority. Patients who achieve pCR to neoadjuvant chemoradiotherapy have an excellent prognosis compared with those without pCR. If the patients with complete response to neoadjuvant chemoradiation can be demonstrated by clinical findings and medical imaging (cCR), a non-operative management (NOM) strategy may be pursued to preserve sphincter function and avoid complications induced by TME, which is a new tendency in the treatment of rectal cancer in recent years. Assisting diagnosis of cCR by iconography is the important element of NOM practice. Selected patients should be followed up with intensive surveillance. The curative strategy must be carried out once the recurrence is detected. Imaging modalities, including magnetic resonance imaging (MRI), diffusion-weighted MRI, or proton emission tomography (PET), are limited in their ability to distinguish patients who have achieved cCR. Up to now, MRI, DW-MRI and 18F-FDG PET/CT before neoadjuvant chemoradiotherapy are not accurate enough to predict cCR and safely select patients for organ-sparing strategies. However, depth of tumor infiltration, extramural vascular invasion, circumferential resection margin, and location of rectal cancer can be demonstrated by high resolution MRI as independent risk factors in prediction of long-term survival of patients, which is a necessary manner of stratification treatment for rectal cancer. Therefore, patients who are defined as early rectal cancer with low risk factors are selected as candidates for NOM in recent studies in order to pursue low rate of local recurrence and distant metastasis. High resolution MRI assessment of tumor regression grade (mrTRG) can be used to assess response of rectal cancer to neoadjuvant chemoradiotherapy, which is associated with tumor burden. mrTRG is an imaging marker that indicates the difference in survival between good and poor responders and provides an opportunity for the multidisciplinary team to offer additional treatment options before planning definitive surgery. Functional imaging and even molecular imaging are needed in the future to screen suitable rectal cancer patients who are easier to achieve cCR from neoadjuvant chemoradiotherapy and to evaluate the efficacy of neoadjuvant chemoradiotherapy.