Purpose Improvements in magnetic resonance imaging (MRI), total mesorectal excision (TME) surgery, and the use of (chemo)radiotherapy ([C]RT) have improved local control of rectal cancer; however, we have been unable to eradicate local recurrence (LR). Even in the face of TME and negative resection margins (R0), a significant proportion of patients with enlarged lateral lymph nodes (LLNs) suffer from lateral LR (LLR). Japanese studies suggest that the addition of an LLN dissection (LLND) could reduce LLR. This multicenter pooled analysis aims to ascertain whether LLNs actually pose a problem and whether LLND results in fewer LLRs. Patients and Methods Data from 1,216 consecutive patients with cT3/T4 rectal cancers up to 8 cm from the anal verge who underwent surgery in a 5-year period were collected. LLND was performed in 142 patients (12%). MRIs were re-evaluated with a standardized protocol to assess LLN features. Results On pretreatment MRI, 703 patients (58%) had visible LLN, and 192 (16%) had a short axis of at least 7 mm. One hundred eight patients developed LR (5-year LR rate, 10.0%), of which 59 (54%) were LLRs (5-year LLR rate, 5.5%). After multivariable analyses, LLNs with a short axis of at least 7 mm resulted in a significantly higher risk of LLR (hazard ratio, 2.060; P = .045) compared with LLNs of less than 7 mm. In patients with LLNs at least 7 mm, (C)RT plus TME plus LLND resulted in a 5-year LLR of 5.7%, which was significantly lower than that in patients who underwent (C)RT plus TME (5-year LLR, 19.5%; P = .042). Conclusion LLR is still a significant problem after (C)RT plus TME in LLNs with a short axis at least 7 mm on pretreatment MRI. The addition of LLND results in a significantly lower LLR rate.

IMPORTANCE Previously, it was shown in patients with low rectal cancer that a short-axis (SA) lateral node size of 7 mm or greater on primary magnetic resonance imaging (MRI) resulted in a high lateral local recurrence (LLR) rate after chemoradiotherapy or radiotherapy ([C]RT) with total mesorectal excision (TME) and that this risk was lowered by a lateral lymph node dissection (LLND). The role of restaging MRI after (C)RT with regard to LLR risk and which specific patients might benefit from an LLND is not fully understood.OBJECTIVE To determine the factors on primary and restaging MRI that are associated with LLR in low rectal cancer after (C)RT and to formulate specific guidelines on which patients might benefit from an LLND. DESIGN, SETTING, AND PARTICIPANTSIn this retrospective, multicenter, pooled cohort study, patients who underwent surgery for cT3 or cT4 low rectal cancer with a curative intent from 12 centers in 7 countries from January 2009 to December 2013 were included. All patients' MRIs were rereviewed according to a standardized protocol, with specific attention to lateral nodal features. The original cohort included 1216 patients. For this study, patients who underwent (C)RT and had a restaging MRI were selected, leaving 741 for analyses across 10 institutions, including 651 who underwent (C)RT with TME and 90 who underwent (C)RT with TME and LLND. MAIN OUTCOMES AND MEASURESThe main purpose was to identify the factors on primary and restaging MRI associated with LLR after (C)RT with TME. Whether high-risk patients might benefit in terms of LLR reduction from an LLND was also studied. RESULTSOf the 741 included patients, 480 (64.8%) were male, and the mean (SD) age was 60.4 (12.0) years. An SA lateral node size of 7 mm or greater on primary MRI resulted in a 5-year LLR rate of 17.9% after (C)RT with TME. At 3 years, there were no LLRs in 28 patients (29.2%) with lateral nodes that were 4 mm or less on restaging MRI. Nodes that were 7 mm or greater on primary MRI and greater than 4 mm on restaging MRI in the internal iliac compartment resulted in a 5-year LLR rate of 52.3%, significantly higher compared with nodes in the obturator compartment of that size (9.5%; hazard ratio, 5.8; 95% CI, 1.6-21.3; P = .003). Compared with (C)RT with TME alone, treatment with (C)RT with TME and LLND in these unresponsive internal nodes resulted in a significantly lower LLR rate of 8.7% (hazard ratio, 6.2; 95% CI, 1.4-28.5; P = .007). CONCLUSIONS AND RELEVANCERestaging MRI is important in clinical decision making in lateral nodal disease. In patients with shrinkage of lateral nodes from an SA node size of 7 mm or greater on primary MRI to an SA node size of 4 mm or less on restaging MRI, which occurs in about 30% of cases, LLND can be avoided. However, persistently enlarged nodes in the internal iliac compartment indicate an extremely high risk of LLR, and an LLND lowered LLR in these cases.