Introduction. Less than 1% of all colorectal cancers (CRC) are primary colorectal signet-ring cell carcinomas (SRCC), which represent an uncommon and aggressive histological subtype. Synchronous colonic metastases at initial diagnosis are very rare.

Case report. We report the case of a 65-year-old male patient who underwent routine colonoscopy following a positive faecal immunochemical test. The patient had no remarkable medical history and was asymptomatic.

A 3-cm semi-pedunculated polyp (Paris Isp, JNET type 3) and several smaller depressed lesions (Paris 0-IIa+c and JNET type 3) were observed in the descending colon.

Multiple biopsies were obtained. Histopathological examination of the biopsy specimens highlighted the presence of diffusely infiltrating signet-ring cells embedded within a mucinous stroma. Tumour cells were poorly cohesive, with most exhibiting prominent intracellular mucin vacuoles displacing the nuclei peripherally. Immunohistochemical staining of the specimens showed positivity for CK20 and CDX2, supporting a colorectal origin, and negativity for CK7 and GCDFP-15. Additional immunostaining revealed a preserved expression of E-cadherin, though focal loss was observed in infiltrative areas, suggesting a more aggressive biological behaviour. The Ki-67 proliferation index was markedly abnormal at 75%, and p53 showed strong diffuse positivity. Testing for mismatch repair proteins demonstrated intact expression of MLH1, MSH2, MSH6, and PMS2, indicating microsatellite stability. Further molecular analysis revealed wild-type KRAS and BRAF genes and no evidence of NRAS mutations.

There was no sign of extra-colonic metastases on the computed tomography. The endoscopic features, alongside the histopathological assessment and immunochemistry suggest primary colorectal SRCC with synchronous colonic metastases at initial diagnosis. The patient was referred for extensive hemicolectomy, regional lymphadenectomy and adjuvant chemotherapy.

Conclusion. The biological behaviour of SRCC is aggressive and unpredictable. The synchronous occurrence of primary SRCC and metastatic lesions further complicates the clinical management.