Longitudinal follow-up and outcomes of pediatric and adult patients with SDH-deficient GIST

11542 Background: Gastrointestinal stromal tumor (GIST) are gastrointestinal non-epithelial neoplasms most commonly driven by somatic mutations in KIT or PDGFRA. Approximately ten percent of GIST are due to germline mutations in SDHx or epigenetic loss of expression of SDHC. While KIT and PDGFRa driven GIST respond to treatment with tyrosine kinase inhibitors, effective systemic therapies for SDH-deficient GIST have not yet been identified. SDH-deficient GIST are indolent tumors that typically progress slowly over time. A better understanding of the natural history of patients with these neoplasms is critical to identify effective treatments and improve patient care. A cohort of patients with SDH-deficient GIST is followed at the NIH Clinical Center and through the NIH Pediatric and Wild-type GIST Clinic. The aim of this study is to characterize the long-term outcome of this cohort of patients. Methods: Data from patients with SDH-deficient GIST enrolled in a study Natural History and Biospecimen Acquisition for Children and Adults with Rare Solid Tumors (NCT03739827) from January 2019 through January 2023 at the NIH were evaluated. In addition to review of medical records and imaging, when available, tumors were characterized by sequencing of SDH genes. Germline analysis of SDH genes was offered to consenting patients and families. Results: Clinical information and specimens were collected from 77 GIST patients (median age at diagnosis 21.5, [range 7-57] years; 72.7% (56) female, 21.3% (21) male) were classified by molecular subtypes: 33.8% SDHC epimutation, 28.6% SDHA, 22.1% SDHB, 14.3% SDHC and 1.3% SDHD. Median age at presentation of SDHC epimutation was younger (14.5 years, range: 8-56) when compared to SDHA (27.5 years, range: 7-55), SDHB (22 years, range: 8-54), SDHC (18 years, range: 10-57) and SDHD (39 years, N = 1). Most commonly, patients had at least one surgery (55.8%) while a lower proportion had four or more (7.8%). Primary tumors occurred in the stomach with 26% having metastases at presentation the most common location being the liver (10.4%). Overall, 64.9% of patients were treated with imatinib, 39% were treated with sunitinib and 15.6% received other additional systemic therapies. Within the cohort there were 13% of patients with paragangliomas and 6.5% with chondromas while only 9.1% had a family history of GIST. Conclusions: Longitudinal follow-up of an SDH-deficient GIST patient cohort will allow for better understanding of the natural history including treatment history and survival of patients with this rare disease and will help to optimize strategies for treatment and follow-up of these patients.