99mTc-Tetrofosmin dual phase study for detection of parathyroid adenoma and comparison with 99mTcO4/99mTc-Tetrofosmin dual tracer technique

Objectives: Most studies done with the dual phase technique for the detection of parathyroid adenoma use 99mTc-Sestamibi as the radiopharmaceutical and employ the preferential tracer retention within the adenoma for diagnosis. Similar studies using 99mTc-Tetrofosmin are fewer and have revealed conflicting results in terms of Tetrofosmin revealing similar tracer retention within the parathyroid adenoma. The objective of this study was to determine whether 99mTc-Tetrofosmin reveals similar preferential tracer retention within our study group, as we use this radiopharmaceutical for myocardial perfusion scans and it is thus freely available to us. All patients also underwent a dual tracer technique for comparison using an additional image after 99mTcO4 administration.

Methods: Parathyroid scans done over a ten year period between 2009 and 2018 were reviewed and all patients with diagnosis of parathyroid adenoma were identified. Our protocol for all patients included the 99mTc-Tetrofosmin dual phase scan which involved injection of around 700 MBq of 99mTc- Tetrofosmin with images acquired at 5 minutes and two hours later. An additional image was acquired after administration of 180 MBq of 99mTcO4 at a separate siting to perform the dual tracer technique comparison. Few of these patients also underwent surgery for parathyroid adenoma using the gamma probe for the adenoma localization after 99mTc-Tetrofosmin injection prior to the surgery.

Results: A total of 45 patients were identified with a diagnosis of parathyroid adenoma. One of these patients was diagnosed to have three adenomas. Most of these adenomas (41 of 47, 87%) were diagnosed as inferior parathyroid adenomas based on their location inferior to or overlapping the inferior pole of the thyroid lobe. Thirty seven of the adenomas (82%) revealed preferential retention on the Tetrofosmin dual phase scan. All of these adenomas were identified using the dual tracer technique, where they were noted as areas of tracer uptake on the Tetrofosmin scan which was not demonstrated on the 99mTcO4 scan based on the subtraction technique. There was no discrepancy in diagnosis between the two techniques with both revealing a similar number of adenomas. Parathyroid surgery using the gamma probe was performed on 11 (24%) of these patients with successful localization of the adenomas. All of these turned out to be parathyroid adenomas on histology.

Conclusions: This study demonstrates the utility of 99mTc-Tetrofosmin dual phase technique for identification of parathyroid adenoma, as preferential retention of tracer was noted in 82% of the adenomas, which is comparable with that noted in studies done with 99mTc-Sestamibi. The study also demonstrated the utility of 99mTc-Tetrofosmin in intra-op localization of the adenomas using the gamma probe.