186 - Characterizing Radiation Target Volumes and Patterns of Failure in Pediatric Neuroendocrine Tumors Using <sup>68</sup>Ga DOTA-Octreotate PET/CT
Presenter(s)
E. L. McKone1, R. O. Kowalchuk1, M. Dorr2, J. D. Cameron3, M. A. Connors2, W. Allen-Rhoades4, P. J. Schoettler4, J. Rawwas5, D. R. Johnson6, G. B. Johnson6, A. Mahajan1, S. K. Ahmed7, and N. N. Laack II1; 1Department of Radiation Oncology, Mayo Clinic, Rochester, MN, 2Mayo Clinic, Rochester, MN, 3Department of Quantitative Health Sciences, Mayo Clinic, Rochester, MN, 4Department of Pediatric Hematology/Oncology, Mayo Clinic, Rochester, MN, 5Children's Minnesota Hematology Oncology, Minneapolis, MN, 6Mayo Clinic, Division of Nuclear Medicine, Rochester, MN, 7Department of Radiation Oncology, Mayo Clinic AZ, Phoenix, AZ
Purpose/Objective(s): Metaiodobenzylguanidine (MIBG) scintigraphy is standard for detecting and planning radiotherapy (RT) for pediatric neuroendocrine neoplasms, although low sensitivity and poor spatial resolution is problematic. We initiated a non-randomized prospective single institution trial of patients =30 years old with high-risk neuroendocrine neoplasms to assess the utility of FDA-approved 68Ga-DOTATATE PET/CT in defining RT gross and clinical target volumes (GTV, CTV).
Materials/Methods: Patients were enrolled at initial diagnosis or recurrence. Standard multimodality therapy preceded standard-of-care consolidative RT with treated volumes defined by post-induction MIBG. PET scans were obtained during RT planning. Two double board-certified Nuclear Medicine Radiologists performed clinically blinded review of PET and MIBG scans. PET-defined GTV and CTV were contoured. Differences between PET and MIBG-defined metastatic site (MS) target volumes informed the primary endpoint. Patterns of failure (POF) for MS were assessed from the end of RT during ongoing 2 year follow up and reported as patient level progression free survival (PFS) and MS level local control (LC).
Results: Ten patients (5 male) with neuroblastoma of median age (range) 3.5 years (1-12) enrolled. A median (range) of 21.6 Gy (21.6-45) in 12 fractions (12-25) was delivered. With median follow up of 17.3 months, 1-year PFS for patients enrolled during initial treatment (n=8) was 59.2% (95% CI: 32.3-100).
Six patients with 17 PET-avid MS were assessed for the primary endpoint. There was a median (IQR) of 3 (1-4) MS per patient. The median (IQR) absolute volume difference between PET-defined and treated GTVs was -0.37 cc (-2.00, -0.06), with a relative difference of -24.03% (-51.28, -2.66). The median (IQR) absolute volume difference between PET-defined and treated CTVs was -1.03 cc (-20.8, 1.00), with a relative difference of -12.44% (-55.45, 14.64). POF were analyzed for all patients (n=10) with a median (IQR) of 13 (10-15) MS per patient and 118 MS in total. With a 17.3 month median follow-up, 1 and 2-year LC rates were 85.5% (95% CI: 78.9-92.6) and 56.1% (42.5-74.1), respectively. For irradiated MS (n=35/118, 29.7%), 1 and 2-year LC were 70.9% (95% CI: 57.3-87.9) and 60.8% (95% CI: 42.0-88.1), respectively. For MS not irradiated due to induction response as per MIBG (n=73/118, 61.9%), 1 and 2-year LC rates were 93.2% (95% CI: 87.0-99.9) and 54.8% (95% CI: 36.6-82.1), respectively. There were 55 PET avid metastatic lesions identified, 7 (12.7%) of which were not apparent on MIBG at any timepoint. Of these, 20/55 (36.4%) relapsed, including 1 not seen on MIBG.Conclusion: DOTATATE PET may improve MS RT target delineation with better localization and reduced target volumes. POF demonstrate only 60.8% LC after RT with relapse in 54.8% of non-irradiated MS at 2-years. PET may identify MS requiring treatment intensification post induction. Further efforts are needed to define clinically meaningful PET findings at the pre-RT timepoint.