Correlation of [18F]SiTATE-PET-tracer with somatostatin receptor expression in meningiomas: an exploratory ex vivo histological pilot study.

BACKGROUND: Radioactively labeled somatostatin analogs (SSAs) are used in PET imaging of meningiomas due to the high expression of somatostatin receptor (SSTR) subtype 2. 18F-labeled SSTR-tracers offer several economic and logistical advantages over 68Ga-labeled tracers, such as a cyclotron-based production, higher activities per synthesis, a lower positron energy resulting in improved spatial resolution, and a longer half-life. Owing to the advantages over the most commonly used 68Ga-labeled SSAs, [18F]SiTATE - although not yet approved by FDA or EMA - is currently in clinical use. The study compares [18F]SiTATE PET uptake with histological SSTR2-expression in meningiomas. RESULTS: A correlation of SUV values and immunohistochemical SSTR2-expression was performed in 12 patients (42% female, mean age 68.8 ± 7.2 years). This study demonstrates a strong and significant correlation between [18F]SiTATE uptake (SUVmean) and histological SSTR2-immune-reactivity-score (SSTR2-IRS) values (r = 0.80; p = 0.002). In addition, a significant difference in [18F]SiTATE uptake (SUVmean) was observed between low SSTR-expressing lesions (scores 0-1) and high SSTR-expressing lesions (scores 2-3; p = 0.006). Furthermore, it was noted that in four cases, the molecular tumor volume (MTV) was underestimated using a threshold value of SUVmin 4, presumably caused by a low SUVmax, while the application of a 50% isocontour threshold led to an underestimation of the MTV in two lesions that showed high SUVmax values. CONCLUSIONS: [18F]SiTATE PET shows a strong and significant correlation with SSTR2-expression in meningiomas. However, neither of the two threshold methods yielded MTVs comparable to MRI volumes in all cases. Further studies are necessary to identify the conditions under which each threshold value yields the best MTV.