Orbital Scintigraphy with the Somatostatin Receptor Tracer ^99mTc-P829 in Patients with Graves’ Disease

Abstract

Receptors for somatostatin (SST) (SSTR) are expressed on various tumor cells as well as on activated lymphocytes. Previous data have shown that ^99mTc-P829 binds with high affinity to many different types of tumor cells as well as to leukocytes via the human hSSTR2, hSSTR3, and hSSTR5 target receptors. Consequently, ^99mTc-P829 was successfully introduced as a peptide tracer for tumor imaging. In this study, we evaluated the orbital uptake of ^99mTc-P829 in patients with active and inactive thyroid-associated orbitopathy (TAO), accompanied by lymphocyte infiltration in the acute stage and by muscle fibrosis in the chronic stage of the disease. Methods: To evaluate its clinical usefulness in Graves’ disease, ^99mTc-P829 scintigraphy (≈740 MBq) was performed in 44 patients with TAO (median duration, 19 mo; range, 1–360 mo). The clinical activity of the orbital disease was graded by the NOSPECS (no signs or symptoms; only signs, no symptoms; signs only; proptosis; eye muscle involvement; corneal involvement; sight visual acuity reduction) classification of the American Thyroid Association, the clinical activity score (CAS), and the superonasal index (SNI). SPECT (360°) and planar studies were completed within 3 h after injection. Orbital (O) regions of interest (ROIs) were compared with temporoparietal and occipital (OCC) ROIs. Orbital uptake ratios in Graves’ disease were compared with data obtained from lung cancer patients with no eye disease (n = 22). Results: Overall, ^99mTc-P829 biokinetics were the same in Graves’ disease patients as in lung cancer patients, showing a rapid blood clearance and visualization of the facial bones within minutes of injection. In all control patients, the orbit appeared as a “cold area,” whereas visual orbital accumulation of ^99mTc-P829 was found in patients with active TAO (O/OCC ratios: 1.26 ± 0.04 vs. 1.69 ± 0.04; P < 0.01, respectively). Patients with active eye disease (n = 25) presented with an increased orbital uptake of ^99mTc-P829 compared with patients with inactive disease (n = 19; O/OCC ratio: 1.12 ± 0.05; P < 0.01). A statistically significant correlation was found between CAS and the orbital uptake (O/OCC ratio) values (r = 0.90), whereas no correlation could be documented regarding the NOSPECS classification as well as the SNI. Conclusion: In TAO, ^99mTc-P829 yields high orbital binding with good clinical correlation. The better image quality due to the high energy of technetium, the lower radiation dose for patients and personnel, and the short acquisition protocol favor SSTR scintigraphy with ^99mTc-P829 over ¹¹¹In-labeled compounds. The in-house availability of the radiotracer and cost-effectiveness are further advantages.