3061 - Ablative MRI-Guided Radiation for GI Neuroendocrine Tumors: Assessment of Local Control and Toxicity

J. Peterson¹, J. M. Bryant¹, J. M. Frakes¹, M. Haider², J. Strosberg², S. Hoffe¹, and R. F. Palm¹; ¹H. Lee Moffitt Cancer Center and Research Institute, Department of Radiation Oncology, Tampa, FL, ²H. Lee Moffitt Cancer Center and Research Institute, Tampa, FL

Purpose/Objective(s): There are limited data regarding the efficacy of radiotherapy for treatment of gastroenteropancreatic neuroendocrine tumors (GEP-NETs). Radiotherapy can be used for treatment of unresectable primary tumors or for targeting solitary metastatic sites. The efficacy and toxicity in the use of high biologic effective dose (BED₁₀) MRI-guided radiotherapy (MRgRT) for GEP-NETs is unknown. Materials/

Methods: An IRB approved, single-institution retrospective study was performed of patients with diagnosis of GEP-NET treated with MRgRT from 2019 - 2023 to any site of disease. Clinical outcomes of interest included overall survival (OS) from date of diagnosis and progression free survival (PFS) and local control (LC) calculated from date of MRgRT. Acute and late radiation toxicity (CTCAE version 5.0) was characterized as less than or greater than 90 days from MRgRT, respectively.

Results: A total of 21 patients were identified with a median follow up from MRgRT of 17 months (IQR [12 – 33]). The median age was 55 years with most primary tumors located in the pancreas (71.4%), 5 (23.8%) in the small bowel, 1 (4.8%) in the ampulla, and 1 unknown. 19 (90.5%) patients were metastatic at the time of MRgRT. The MRgRT target was an abdominal lymph node in 11 (52.4%), pancreas in 5 (23.8%), liver in 4 (19.0%), and synchronous treatment of the pancreas and a lymph node in 1. The median biologically equivalent dose (BED₁₀) delivered during MRgRT was 100 Gy (IQR [76.7, 132]). 4 patients (19.0%) reported acute or late MRgRT toxicity; 2 (9.5%) reported acute Grade 3 toxicity and 1 had late MRgRT toxicity in the form of Grade 2 leuko/lymphopenia. The median OS and LC were not reached; 1 patient had local failure at 16 months resulting in a crude local control rate of 95.2%. Median PFS was 25 months (95% CI [16, Not reached]). BED₁₀ >100 did not significantly correlate with Grade 3 toxicity (p = .07).

Conclusion: MRgRT was well tolerated in this cohort of GEP-NET patients and LC was excellent across all tumor classifications. Because of the relative indolent biology of GEP-NETs, mitigation of late toxicity may be best approached with MRgRT and these outcomes should be updated with additional follow up. MRgRT should be considered as a reasonable option for patients unsuitable for primary tumor resection or metastasectomy.

Abstract 3061 – Table 1

Patient Characteristics	No. (%) (n = 21)
Age at Dx (y), median [range]	55 [36—83]
Sex
Male	13 (61.9)
Female	8 (38.1)
Primary Site
Pancreas	15 (71.4)
Small Bowel	5 (23.8)
Unknown	1 (4.8)
Primary Tumor Size (cm), median [IQR]	3.8 [2.8—5.0]
Tumor Grade
1	6 (28.6)
2	8 (38.1)
3	4 (19.0)
Unspecified	3 (14.3)
MRgRT Target
Lymph Node	11 (52.4)
Pancreas	5 (23.8)
Liver	4 (19.0)
Pancreas and Lymph Node	1 (4.8)
Fractionation Schedules
BED10 (Gy), median [IQR]	100 [76.7—132]
All RT Toxicity	4 (19.0)
Acute Toxicity	4 (19.0)
Abdominal pain
Grade 2	1 (4.8)
Grade 3	3 (14.3)
Nausea
Grade 1	1 (4.8)
Grade 2	2 (9.5)
Grade 3	1 (4.8)
Vomiting
Grade 1	1 (4.8)
Grade 2	2 (9.5)
Chronic Toxicity
Leuko/Lymphopenia
Grade 2	1 (4.8)