Given the poor prognosis for peritoneal metastasis relative to metastatic disease in visceral organs, its tumor-node-metastasis (TNM) classification (8th ed. published in 2017) was revised to M1c (metastasis to the peritoneum with or without organ involvement) and separated from M1a (metastasis to one organ) and M1b (metastasis to more than one organ). Moreover, the stage classification of peritoneal metastases was stage 4C