Aims PD‐1 inhibitors facilitate immune response against certain tumour types, including melanoma. These drugs have led to prolonged survival but can also result in autoimmune‐type side effects, including gastrointestinal inflammation. The histopathological effects of this medication class have not been well studied. Methods and results We identified 37 gastrointestinal tract biopsies from 20 patients taking a PD‐1 or PD‐L1 inhibitor and evaluated clinicopathological findings. Diarrhoea was the most common symptom, and endoscopic findings ranged from mild erythema to erosion/ulceration. Common histological findings included lamina propria expansion, villous blunting (if applicable), intra‐epithelial neutrophils and increased crypt/gland apoptosis, although intra‐epithelial lymphocytes were rarely prominent. A few cases showed crypt rupture with resultant histiocytic/granulomatous response. Most patients responded to drug cessation and/or steroids, but follow‐up endoscopies were not performed. Conclusions PD‐1/PD‐L1 inhibitors can cause gastritis, enteritis and colitis, similar to other immunomodulatory antibodies (such as CTLA‐4 inhibitors and PI3Kδ inhibitors), but the histological findings vary somewhat among drug classes. Clinical history, lack of prominent intra‐epithelial lymphocytes and crypt rupture may help to distinguish PD‐1 inhibitor gastroenterocolitis from mimics, which include other medication effect, inflammatory bowel disease, graft‐versus‐host disease, cytomegalovirus infection and autoimmune enteropathy.

ARID1A plays a key role in pancreatic acinar homeostasis and response to injury. Furthermore, ARID1A restrains oncogenic KRAS-driven formation of premalignant proliferative IPMN. -deficient PDACs are poorly differentiated and have mesenchymal features conferring migratory/invasive and stem-like properties.

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Aims Appendiceal well‐differentiated neuroendocrine tumours (NETs) are usually incidental and clinically benign. Several studies have reported different risk factors for nodal metastasis. The aim of this study was to investigate our appendiceal NETs (App‐NETs) to determine the factors associated with malignant behaviour. Methods and results For 120 App‐NETs, we reviewed the clinical presentation and follow‐up, including serum chromogranin A (CgA) levels, and compiled several microscopic variables. Pathological factors were compared with nodal status and time to biochemical recurrence (elevated serum CgA level) by the use of Cox regression. We also reviewed similar App‐NET data in the Surveillance, Epidemiology, and End Results (SEER) Programme. Among our 120 cases, seven patients had positive lymph nodes, and nine developed subsequent elevation of CgA levels; none developed distant metastases or died of disease. Only three patients had grade 2 NETs; none had nodal disease, and one developed an elevated CgA level. Increasing tumour size was associated with an increased risk of nodal disease [odds ratio (OR) 4.99, P = 0.0055). All seven node‐positive cases were ≥13 mm. Factors associated with elevated CgA levels included age (OR 1.04, P = 0.041), pT4 disease (OR 10.22, P = 0.033), and nodal disease (OR 24.0, P = 0.012), but not size (OR 2.13, P = 0.072). Of the 1492 reported App‐NETs in the SEER database with data on tumour size, 137 (9%) were pN1; only five of these (4%) were coded as being <5 mm. Conclusions Small (<5 mm) App‐NETs that do not invade the serosa or mesoappendix appear to be overwhelmingly benign and low‐grade, requiring neither Ki67 staining nor synoptic reporting. Given their indolent behaviour, different nomenclature or staging may be more appropriate for these NETs.

Solitary fibrous tumor is a rare, benign spindle cell neoplasm that was first described in the thoracic pleura. This tumor is now known to occur at many extrapleural sites. There are established criteria for the diagnosis of malignant solitary fibrous tumor including ≥4 mitotic figures per 10 high-power fields, increased cellularity, cytologic atypia, infiltrative margins, and/or necrosis. Although all solitary fibrous tumors have the potential to recur or metastasize, those with malignant histologic features tend to behave more aggressively. We report a case of solitary fibrous tumor, with malignant histologic features, in a 21-year-old woman which arose from the serosal surface of the sigmoid colon.