Giant seminal vesicle cyst: an unusual site for a malignant extragastrointestinal stromal tumour.

Mesenchymal tumours with clinicopathological and molecular profiles similar to gastrointestinal stromal tumours (GISTs) are, on occasion, found in extragastrointestinal locations. Extra GIST (EGIST) is a singular occurrence in the genitourinary tract. A 30-year-old man, catheterised following urinary retention, was found to have a complex pelvic retrovesical cyst on imaging. At operation, origin from the right seminal vesicle was evident with histopathology confirming a GIST. The patient received adjuvant therapy with tyrosine kinase inhibitor and is currently disease free at 2 years. This is only the second report of an EGIST at this anatomic locale. The current literature presents significant uncertainty in defining the true origin of EGISTs, particularly those in the pelvis. We propose the designation origin indeterminate stromal tumour (OIST), to facilitate disambiguation and advance accurate profiling of EGIST; a subject in evolution.