Figure 1. Fig. 1

Oligometastasis is a relatively recent concept in the field of esophageal cancer. Its definition remains somewhat unclear, especially for esophageal squamous cell carcinoma (ESCC). With regard to adenocarcinoma of the esophagogastric junction, oligometastasis was recently defined as recurrence limited to 1 organ with ≤3 metastases or 1 extra- regional lymph node station based on the Delphi consensus of Western experts [1]. Regarding non-small cell lung cancer, 2 randomized phase II studies have been performed, and they showed that local treatment had some benefit compared to systemic therapy alone [2,3].

Kang et al. [4] examined the details of 321 patients who had recurrent disease after R0 or R1 esophagectomy for ESCC. Their multivariate analysis showed that the body mass index, minimally invasive esophagectomy (MIE), pN status, residual tumor, post-recurrence treatment, and number of recurrences were independent post-recurrence prognostic factors. Furthermore, they demonstrated that the post-recurrence survival curves were clearly divided among patients with 1, 2–3, or ≥4 recurrences. Based on their data, they proposed that oligometastasis in ESCC should be defined based on 1, 2, or 3 recurrences. Although post-recurrence treatment strategies were not adequately analyzed in this setting, I believe that this new definition of oligometastasis in ESCC will facilitate further studies in the future.

R1 resection and pN status are well-known prognostic factors in ESCC. In this study, the authors showed that these 2 factors were significant prognostic factors for post- recurrence survival. This finding may be intriguing to readers. I speculate that it is because R1 or pN2/3 status indicates the ontologically poor behavior of a tumor even after recurrence. In contrast, Kang et al. [4] also found MIE to be a prognostic factor. To date, a few randomized controlled trials have shown that MIE had non-inferior or identical outcomes to open esophagectomy in terms of overall survival in patients with esophageal cancer [5,6]. Could MIE be a prognostic factor only during the post-recurrence period? Future research should investigate this possibility.

I think that the main limitation of this study is that only a quarter of the patients received neoadjuvant treatment. These days, patients with locally advanced ESCC are already supposed to receive neoadjuvant chemotherapy or chemoradiotherapy. In the near future, neoadjuvant treatment with immune checkpoint inhibitors may become the standard treatment for esophageal cancer. These changes would influence the post-recurrence treatment strategy. In addition, because immune checkpoint inhibitors are available as a first-line treatment for esophageal cancer, post-recurrence treatment strategies have already changed, and some of these patients could survive longer than what was possible before. Therefore, we should improve and update the treatment strategy for recurrence after esophagectomy while using the new definition of oligometastasis for ESCC proposed by Kang et al. [4].

Author contributions

All the work was done by Shinji Mine.

Conflict of interest

No potential conflict of interest relevant to this article was reported.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.