Background
Better understanding of the impact of metachronous metastases in locally advanced and recurrent rectal cancer may improve decision‐making. The aim of this study was to investigate factors influencing metachronous metastasis and its impact on survival in patients who have a beyond total mesorectal excision (bTME) operation.

Methods
This was a retrospective study of consecutive patients who had bTME surgery for locally advanced and recurrent rectal cancer at a tertiary referral centre between January 2006 and December 2016. The primary outcome was overall survival. Cox proportional hazards regression analyses were performed. The influence of metachronous metastases on survival was investigated.

Results
Of 220 included patients, 171 were treated for locally advanced primary tumours and 49 for recurrent rectal cancer. Some 90·0 per cent had a complete resection with negative margins. Median follow‐up was 26·0 (range 1·5–119·6) months. The 5‐year overall survival rate was 71·1 per cent. Local recurrence and metachronous metastasis rates were 11·8 and 22·2 per cent respectively. Patients with metachronous metastases had a worse overall survival than patients without metastases (median 52·9 months versus estimated mean 109·4 months respectively; hazard ratio (HR) 6·73, 95 per cent c.i. 3·23 to 14·00). Advancing pT category (HR 2·01, 1·35 to 2·98), pN category (HR 2·43, 1·65 to 3·59), vascular invasion (HR 2·20, 1·22 to 3·97) and increasing numbers of positive lymph nodes (HR 1·19, 1·07 to 1·16) increased the risk of metachronous metastasis. Nine of 17 patients (53 per cent) with curatively treated synchronous metastases at presentation developed metachronous metastases, compared with 40 of 203 (19·7 per cent) without synchronous metastases (P = 0·002). Corresponding median length of disease‐free survival was 17·5 versus 90·8 months (P < 0·001).

Conclusion
As metachronous metastases impact negatively on survival after bTME surgery, factors associated with metachronous metastases may serve as selection tools when determining suitability for treatment with curative intent.