磁共振成像在直肠癌“非手术治疗”中的决策意义

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

全直肠系膜切除术仍旧首选的直肠癌局部治疗方案。新辅助放化疗的实施显著地提高了直肠癌局部控制率，部分患者甚至可以达到病理完全缓解，其无病生存期及总生存期也因此显著获益。这部分直肠癌患者如果通过临床及影像学判断为临床完全缓解（cCR），将可能不再需要全直肠系膜切除术，而进行＂监测与随访＂ ，即＂非手术治疗＂ （NOM）。NOM对于保全因手术而丧失或受损的肛门括约肌功能和避免手术治疗并发症具有重要的意义，是近年来直肠癌治疗的新趋势。影像学协助诊断cCR患者是NOM实施的重要环节。影像学方法，包括磁共振影像（MRI），扩散加权成磁共振成像（DW-MRI）和正电子发射计算机断层扫描（PET-CT），对于cCR的判断都存在局限性。首先，基线影像学（治疗前）尚不具备预判新辅助放化疗效果或识别cCR患者的能力。然而，通过高分辨率MRI所判断的肿瘤浸润深度、壁外血管侵犯与否、环周切缘情况及肿瘤位置，进行临床危险度分层，是直肠癌分层治疗不可或缺的一部分。因此，应用基线影像学所判断的危险因素筛选出更容易获得cCR的患者则显得更加重要。此外，新辅助放化疗后治疗效果的影像判断指标应与患者远期生存密切相关，即与NOM实施的根本目标相结合。通过高分辨率MRI所评估的肿瘤退缩分级（mrTRG）可有效识别新辅助放化疗后肿瘤负荷改变，并与患者长期生存显著相关。未来需要功能影像学乃至分子影像学来帮助筛选新辅助治疗更易获得cCR的群体并评价新辅助治疗效果。

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.