Recognition of a hereditary colorectal cancer (CRC) syndrome is crucial and Lynch Syndrome (LS) is the most frequent immunohistochemistry (IHC)—screening for mismatch repair proteins (MMR) deficiency in CRC is therefore advocated. An unicentric cohort study was conducted in a central Oncological Hospital to assess its results. All patients under 70 years-old admitted between July 2017–June 2019 and submitted to surgery for CRC were included. Of 275 patients, 56.0% were male, median age 61.0 (IQR:54.5–65.0), with synchronous tumors in six. Histology revealed high grade adenocarcinoma in 8.4%; mucinous and/or signet ring differentiation in 11.3%; and lymphocytic infiltration in 29.8%. Amsterdam (AC) and Bethesda (BC) Criteria were fulfilled in 11 and 74 patients, respectively. IHC revealed loss of expression of MMR proteins in 24 (8.7%), mostly MLH1 and PMS2 (n = 15) and PMS2 (n = 4). Among these, no patients fulfilled AC and 13 fulfilled BC. BRAF mutation or MLH1 promoter hypermethylation was found in four patients with MLH1 loss of expression. Genetic diagnosis was performed in 51 patients, 11 of them with altered IHC. LS was diagnosed in four, and BC was present in three. One patient would not have been diagnosed without routine IHC screening. These results strengthen the important role of IHC screening for MMR proteins loss of expression in CRC.

A 54-year-old female presented to the Emergency Department due to 1 month lower right quadrant abdominal pain, with fever in the last 2 days. Three years before, she had been diagnosed with Familial Adenomatous Polyposis and had undergone prophylactic total proctocolectomy with ileal pouch, with no perioperative complications. Blood analysis showed an elevated C-reactive protein (7.8mg/dL). The abdominopelvic CT showed an irregular pelvic collection with air bubbles, lined by a thickened enhancing wall, lymphadenopathies and mesenteric fat densification. Endoscopic evaluation showed bulging of the ileal pouch wall. These findings were suggestive of a pelvic abscess and the patient was empirically treated with piperacillin/tazobactam, with clinical improvement. Two months later, the symptoms recurred. CT scan was repeated, showing persistence of the previously described pelvic collection, in communication with the intestinal lumen, associated with a soft tissue infiltrative tumour in the mesentery and at the site of the previous ileostomy. Endoscopic evaluation showed, in the closed end of the pouch, two fistulous openings leading to an ulcerated and friable lesion. Ultrasound-guided biopsy revealed fascicles with elongated, spindled cells of uniform appearance and pale cytoplasm, set in a collagenous stroma, without signs of cytologic atypia. Immunohistochemistry showed nuclear beta-catenin staining. These findings were suggestive of a desmoid tumour. After a multidisciplinary meeting, a derivation ileostomy was placed and systemic treatment with tamoxifen was began. This case illustrates the importance of considering the patient as a whole, including knowledge of past medical and family history, that may alert to alternative diagnosis. This case illustrates the importance of considering the patient as a whole, including knowledge of past medical and family history, that may alert to alternative diagnosis.