A Post-surgical Collection Is Not Just a Collection: A Case Report of Colorectal Cancer Metastasis.

Subdiaphragmatic metastases in colorectal cancer are rare and can present as persistent intra-abdominal collections post-surgery. An elderly male in his mid-70s with colorectal adenocarcinoma (pT4a pN1c R0) developed a chronic subdiaphragmatic collection after undergoing an emergency right hemicolectomy for an obstructing tumor. Initially presumed to be a benign post-surgical complication, the collection persisted despite standard management. Subsequent investigation revealed metastatic adenocarcinoma, highlighting an uncommon metastatic pattern. This case highlights the need for a high index of suspicion in post-surgical patients when conventional explanations fail to explain clinical progression. Early consideration of metastatic disease and timely tissue sampling can significantly impact management and prognosis. This case reinforces the importance of clinical vigilance, advanced imaging, and multidisciplinary collaboration in recognizing rare metastatic patterns and optimizing treatment strategies in complex oncologic care.