Not just a GIST: an unclassifiable neoplasm in the genomics era

Unclassifiable primary tumors despite adequate tissue for pathologic examination are quite rare. We present a case of a 72-year-old female who was found to have an abdominal mass after she reported to the emergency department with complaints of abdominal pain with spasms, bloating and nausea. Computed tomography scan demonstrated a 12.3 × 15.7 × 15.9 large multilobulated mass, abutting and compressing the stomach, compatible with neoplasm. She underwent esophagogastroduodenoscopy with findings concerning for gastrointestinal stromal tumor. The patient underwent en bloc resection of the mass. The neoplasm was unable to be classified on pathologic examination despite a comprehensive workup and multiple consultations with specialized pathologists from local institutions, as well as national specialists. Final pathology was unclassified malignant neoplasm displaying calretinin expression only. This presents a difficult clinical entity to treat. Even in the genomics era, there are tumors that cannot be even broadly classified on pathologic examination.