Background
Malignant peritoneal mesothelioma (MPM) is a rare and aggressive cancer originating from the peritoneal lining, accounting for less than 10% of all mesothelioma cases. It is strongly linked to prior asbestos exposure, typically presenting after a latency of several decades. The clinical presentation is often vague—abdominal pain, distension, or altered bowel habits—making early diagnosis challenging. Presentation as an acute abdomen is exceptionally rare and can lead to misdirection in initial management.

Case Presentation
We report the case of a 60-year-old man with remote occupational asbestos exposure who presented with acute abdominal pain and localized tenderness, initially raising concerns for a surgical emergency. He was referred to the surgical team with presumed peritonitis. Initial imaging revealed omental thickening, peritoneal enhancement, and a pelvic soft tissue lesion. Upper and lower GI endoscopy were inconclusive. The diagnostic process was prolonged due to the non-specific presentation, with malignancy not initially suspected. Subsequent biopsy confirmed epithelioid peritoneal mesothelioma.

He was reviewed by the specialist peritoneal mesothelioma multidisciplinary team and commenced on dual immunotherapy (ipilimumab and nivolumab). During treatment, he developed right-sided neck pain and was found to have an internal jugular vein thrombosis—a rare thrombotic complication in the context of malignancy. He was started on anticoagulation. The disease later progressed, with increasing ascites requiring repeated paracentesis. He is currently receiving palliative care.

Conclusion
This case highlights the diagnostic complexity of MPM when it presents atypically, as in this patient mimicking an acute surgical abdomen. Initial misdirection delayed diagnosis, underscoring the importance of considering malignancy in unexplained abdominal presentations, especially with relevant exposure history. The development of an unusual thrombotic event further reflects the systemic and unpredictable nature of mesothelioma. A high index of suspicion, early multidisciplinary involvement, and awareness of atypical presentations are key to timely diagnosis and improved patient outcomes.