低位直肠癌与局部复发性疾病风险增加和预后较差相关，具有独特的解剖学考虑因素以及不适用于中位和高位直肠癌的分期和治疗问题。尽管肿瘤组织学有助于推动所有直肠癌的分期和治疗，但对于可能累及肛管的低位直肠癌尤其重要，有助于确定低位直肠肿块是否应作为直肠癌进行分期和治疗（即、腺癌）或肛门癌（即鳞状细胞癌）。直肠癌的最佳分期和治疗取决于肿瘤位置和局部范围，这有助于指导管理决策，包括新辅助治疗和根治性手术治疗策略。肿瘤位于低位直肠以及局部累及肛管、括约肌和盆底有助于确定患者是否可以接受保留括约肌的切除术，例如低位前切除术与腹会阴切除术，以获得阴性手术切缘。存在与低位直肠特有的解剖结构和疾病传播模式相关的问题，包括如何确定和分期肛门括约肌受累、盆底直肠系膜筋膜状态以及直肠系膜外节点（如髂外和腹股沟）的节点状态淋巴结。由于这些原因，解释直肠癌分期 MRI 检查的放射科医生必须对低位直肠和肛管的独特解剖结构、低位直肠癌局部疾病扩散的细微差别以及低位直肠癌的治疗范例感到满意。 ©RSNA，2023 本文提供在线补充材料。本文的测验问题可通过在线学习中心获得。请参阅本期 Gollub 特邀评论。

Low rectal cancers, which are associated with increased risk of local recurrent disease and poorer prognosis, have unique anatomic considerations and issues for staging and treatment that do not apply to mid and high rectal cancers. Although tumor histology help drive the staging and treatment of all rectal cancers, it is of particular importance in low rectal cancers, which may involve the anal canal, to help establish whether a low rectal mass should be staged and treated as a rectal cancer (ie, adenocarcinoma) or an anal cancer (ie, squamous cell carcinoma). Optimal staging and treatment of rectal cancer are contingent on tumor location and local extent, which help guide management decisions including neoadjuvant therapy and curative surgical treatment strategies. Tumor location in the low rectum and local involvement of the anal canal, sphincter, and pelvic floor help determine whether a patient can undergo sphincter-preserving resection such as a low anterior resection versus abdominoperineal resection to achieve negative surgical margins. Issues exist related to the anatomy and patterns of disease spread that are unique to the low rectum and include how to determine and stage anal sphincter involvement, mesorectal fascia status at the pelvic floor, and nodal status of extramesorectal nodes such as the external iliac and inguinal lymph nodes. For these reasons, it is imperative that radiologists who interpret rectal cancer staging MRI examinations feel comfortable with the unique anatomy of the low rectum and anal canal, nuances of low rectal cancer local disease spread, and treatment paradigms for low rectal cancer. ©RSNA, 2023 Online supplemental material is available for this article. Quiz questions for this article are available through the Online Learning Center. See the invited commentary by Gollub in this issue.