(P 118) SOMATOSTATIN RECEPTOR TYPE 2 EXPRESSION IN SARCOMA

Objective: [177Lu]Lu-DOTA-TATE is a radiolabeled somatostatin analogue peptide receptor radionuclide therapy that has shown substantial benefit in patients with neuroendocrine tumors (NET). Somatostatin receptors are not NET specific, and sarcomas expressing the molecular target may be candidate tumors for this therapy. The aim of this study was to assess somatostatin receptor type 2 (SSTR2) expression in sarcoma by immunohistochemistry (IHC) and/or by 68-Ga-DOTATATE PET imaging.

Methods: SSTR2 expression was assessed by IHC in 74 patients with Ewing sarcoma, rhabdomyosarcoma, or synovial sarcoma. Intensity was scored numerically (0 to 3), and expression localization was described (membranous or cytoplasmic). SSTR2 expression was assessed by 68-Ga-DOTATATE PET imaging in an additional 8 patients. Krenning intensity score was applied, and determined to be positive (> 3) or negative (0 to 2). Clinical parameters, including demographics, stage, and relation to chemotherapy, were correlated.

Results: Six of 37 Ewing sarcoma samples (16%), 7 of 31 rhabdomyosarcoma samples (22%), and 4 of 6 synovial sarcoma samples (67%) were considered positive by IHC (having membranous positivity). The concordance rates in SSTR2 expression between prechemotherapy primary tumor and metastases, prechemotherapy and post-chemotherapy, prechemotherapy and relapse samples are 71%, 57%, and 82%, respectively. Of 8 patients with Ewing sarcoma who underwent 68-Ga-DOTATATE PET imaging, 2 had a Krenning score of 3. Interestingly, positive Ga-DOTATATE PET imaging did not correlate with IHC expression in all patients. A widely metastatic, multiply relapsed Ewing sarcoma patient with a Krenning score of 3 underwent treatment with [177Lu]Lu-DOTA-TATE, which has led to disease control for nine months.

Conclusion: SSTR2 is variably expressed in sarcomas. [177Lu]Lu-DOTA-TATE may be a therapeutic strategy for such patients.