(P 103) GIST PATIENT AND TUMOR CHARACTERISTICS IN REFERENCE CENTERS AND NON-REFERENCE CENTERS: DATA FROM A NATIONWIDE REGISTRY

Objective: Since 2014 care for gastrointestinal stromal tumor (GIST) patients in the Netherlands is centralized in five GIST reference centers. Treatment is based on the European Society of Medical Oncology (ESMO) guidelines(1). The primary objectives of this study are to describe patient and tumor characteristics of patients treated in GIST reference centers and non-reference centers and to describe the patterns of care of Dutch GIST patients. Secondary objectives are to evaluate the rate of positive resection margins and tumor rupture and to compare the overall survival (OS) between intermediate risk and high risk patients treated in GIST reference and non-reference centers. Methods: This is a retrospective cohort study in Dutch GIST patients diagnosed between 2016 and 2019. Clinical information was obtained from the Netherlands Cancer Registry (NCR)(2). Follow-up was carried out up to 6 months after diagnosis. Mutation status was derived from the database from the Dutch Nationwide Pathology Databank, PALGA, and matched with the data obtained from the NCR database(3). A descriptive analysis was performed for patient and tumor characteristics and overall survival (OS) was estimated using the Kaplan-Meier method. Risk classification according to the revised AFIP-Miettinen criteria was determined for patients undergoing primary surgery(4). Results: Eighty-seven percent of patients (1352/1550) was diagnosed in a non-reference center. Thirty-six percent (493/1352) of these patients was referred to a reference center (Fig. 1). Referral rates for high-risk GIST and metastatic GIST patients were higher (62.2% (74/119) and 67.2% (90/134), respectively) (Fig. 1-2). The mutation analysis rate was 62.4% for patients with a GIST > 2 cm. A higher rate was seen for high-risk and metastatic patients (reference centers: 96.9%; non-reference centers: 87.6%; P<0.001). The use of systemic treatment was significantly lower for patients without referral to a reference center (high-risk: 71.1% versus 84.6%; metastatic: 52.2% versus 69.6%). However, more than half of high-risk patients that did not receive systemic treatment (15/27), had a non-imatinib sensitive mutation. Metastatic patients in non-reference centers more often received no treatment (22.7% versus 1.0%, P<0.001). With a median follow-up time of 34.8 months for intermediate-risk and high-risk patients and 14.7 months for metastatic patients median OS was not reached. Conclusion: One third of Dutch GIST patients is not referred to a reference center upon diagnosis. In high-risk and metastatic patients a high mutation analysis rate was observed both in reference and in non-reference centers. The majority of high-risk patients, both in reference and in non-reference centers, received systemic treatment. A significant number of metastatic patients in non-reference centers received no treatment. This could be due to a difference in case mix (e.g. performance status) or an actual difference in knowledge or access to treatment. A longer follow-up is needed to demonstrate a difference in median OS. These results emphasize the need of an experienced multidisciplinary tumor board to ensure mutation analysis and guide systemic treatment.