Aim: Radiopeptide therapy with beta emitter labeled 177 Lu/ 90 Y-DOTA(0)-Phe(1)-Tyr(3)-octreotide (DOTATOC) and more recently also alpha emitting 213 Bi-DOTATOC are promising new treatments for neuroendocrine tumors. No early predictors for treatment response have been recognized and tumor-shrinkage after radiation therapy appears slowly. In some solid tumors a decline in tumor perfusion was found predictive of final treatment response but the gold standard multiphase computed tomography (CT)

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... s a high radiation burden. Therefore we evaluated the ability of contrast-enhanced ultrasound (CEUS) to evaluate tumor perfusion as a response criteria. Materials and Methods: 14 patients with hepatic neuroendocrine tumor (NET) metastases were enrolled in the retrospective study. Eleven patients were treated with beta-emitting 177 Lu/ 90 Y-DOTATOC, either intravenous (i.v.) (n = 5) or intra-arterial (i.a.) (n = 6) and three patients received alpha-emitting 213 Bi-DOTATOC (i.a.). CEUS and contrast-enhanced CT (CE-CT) were performed before and 3 months after treatment. Results: CE-CT and CEUS presented comparable results in the baseline study and in the assessment of perfusion changes due to the different treatment regimes. A therapy related decrease in tumor perfusion is an early predictor of longterm morphologic response. Conclusion: CEUS is available and radiation free technique which showed comparable results for perfusion and diameter of liver metastases compared to CE-CT. Intensity reduction in an arterial phase CEUS can be seen as a positive sign indicating long term tumor response to treatment. Therefore CEUS may be considered as an imaging modality for monitoring early treatment after focal alpha and beta targeted therapy. Key Words: contrast-enhanced ultrasound, radionuclide therapy, treatment response, DOTATOC PET/CT. Liver metastases are often the life-limiting factors for patients with gastroenteropancreatic neuroendocrine tumors (GEP-NET). Most GEP-NETs, as well as their liver metastases are highly vascularised with a dense intra-mural vascular network [1]. Hence they show the typical peripheral contrast enhancement in the arterial phase of contrast-enhanced computed tomography (CE-CT) and contrast-enhanced ultrasound (CEUS). Moreover, they demonstrate an increased somatostatin receptor expression both in primary and metastatic lesions. Concerning therapeutic and diagnostic options, the PET tracer DOTA(0)-Phe(1)-Tyr(3)-octreotide (DOTA-TOC), a 68 Ga-labelled somatostatin analog, has been shown to have high sensitivity and specificity for GEP-NET detection and staging [2]. Treatment with 90 Y-or 177 Lu-DOTATOC (beta emitters) is emerging as a potent therapy in patients with GEP-NETs. Systemic (i.v.) treatment can induce partial remission in 25-30% of the patients [3]. Tumor uptake of DOTATOC can be enhanced by loco-regional (i.a.) administration [4]; however, only patients with limited tumor extent are eligible for this approach. In comparison to beta emitters, alpha emitters have a higher linear energy transfer and potentially can induce tumor necrosis by structural damage of the targeted cell rather than radiation-induced apoptosis as occurred with beta irradiation. Monitoring structural and functional characterisation of a tumor during therapy is important to tailor individual