Diagnosis of ectopic tissue versus contamination by genetic fingerprinting in a routine surgical pathology specimen

A 62-year-old woman presented with recurring right upper quadrant pain and underwent a routine laparoscopic cholecystectomy. A large gallstone was found impacted in the fundus of the gall bladder. Interestingly, aside from noting mild inflammation of the gall bladder wall, microscopic examination of the specimen identified 2 fragments of benign thyroid tissue. Given the routine nature of the surgical procedure and lack of abnormality detected during the operation, the attending pathologist suspected extraneous tissue contamination ("floater") of the pathology specimen and submitted the block and slides to Molecular Pathology. The thyroid tissue-containing fragments and gallbladder wall were independently isolated and subjected to genetic fingerprinting using a standard forensic DNA identification panel. All fragments showed the identical fingerprint, strongly suggesting that they belonged to the same patient. The results indicated that the thyroid tissue was from an ectopic rest adjacent to the gall bladder, which has been reported only very rarely in the previous literature and illustrates the unusual use of molecular genetic testing to confirm the presence of ectopic tissue versus contamination.