In vivo detection of malignant thymic masses by indium-111-DTPA-D- Phe¹-octreotide scintigraphy

Many tumors with neuroendocrine characteristics express high amounts of somatostatin receptors that enable in vivo imaging with [¹¹¹In-DTPA-D- Phe¹]-octreotide. In this study, we have analyzed the feasibility in detecting and characterizing thymic masses by somatostatin receptor scintigraphy (SRS). Methods: Eighteen patients (13 women, 5 men, ages 18-78 yr; mean ± s.d. = 42.1 ± 17.6 yr) were enrolled in this study. Eleven patients were studied during diagnosis and seven during routine follow-up. In seven patients, myasthenia gravis was the presenting symptom. SRS was performed within 4 wk after CT and/or MRI. Planar and tomographic images were acquired within 24 hr after the injection of approximately 111 MBq of [¹¹¹In-DTPA-D-Phe¹]-octreotide. The scintigraphic results were categorized according to the histologic findings. Results: Histology diagnosed 10 mixed epithelial/lymphoid thymomas (8 with prevalent epithelial component), 2 thymic carcinomas, 1 thymic carcinoid, 1 lymphangioma and 4 thymic hyperplasias. Two thymoma were Stage I, 3 were Stage II, 2 were Stage III and 5 were Stage IV, as was the thymic carcinoid. Indium-111-DTPA-D- Phe¹-octreotide concentrated in primary and/or metastatic sites of thymic tumors, thereby enabling successful external gamma imaging of sites greater than 1.5 cm in size. Tumor-to-lung (T/L) ratios were as high as 7.6-fold (range 1.7-7.6). Untreated thymomas showed higher T/L (4.34 ± 1.57) than treated ones (2.68 ± 1.18). No uptake was detectable in the four patients with benign thymic hyperplasia and the patient with the lymphangioma. Conclusion: Indium-111-DTPA-D-Phe¹-octreotide is avidly concentrated within thymic tumors, but it is not concentrated by thymic hyperplasia, which allows differential diagnosis. Thus, in patients with myasthenia gravis, SRS may have a role in characterizing thymic masses, thereby overcoming the limits of cross-sectional imaging modalities.