[Implantation metastasis of colorectal adenocarcinoma in an anal fistula].

In our case report, we describe a rare form of metastatic colorectal carcinoma, in which tumor cells spread intraluminally and metastasis occurs with implantation mechanism far from the primary tumor. A 43-year-old male patient developed perianal abscess. After surgical intervention a fistula-in-ano appeared at the site of the abscess. Fistulotomy was performed in another hospital. A few months later, we admitted him to our department with an abnormal tissue proliferation appearing in the surgical area. Histology confirmed adenocarcinoma. Colonoscopy detected tissue proliferation in the sigmoid colon, causing a subtotal stenosis. Laparoscopic rectosigmoid resection and per anum tumor excision were performed. Detailed histological examination confirmed the same mucinous adenocarcinoma in the colon and the anorectal malformation. In this case, implantation mechanism is likely in the development of a synchronous tumor at the site of the fistula-in-ano. Implantation metastasis is considered rare, only a few cases have been reported in the international literature so far. We are not aware of any similar case reported from Hungary. Orv Hetil. 2023; 164(3): 110-113.