FDG-PET as a predicitve marker for primary resistance to imatinib (IM) in patients with gastrointestinal stromal tumors (GIST).

Abstract

10051 Background: A small number of patients (pts) with advanced GIST show primary resistance to IM, defined as progressive disease (PD) at first CT evaluation. Early recognition of resistance could prohibit side effects and save costs of ineffective treatment. IM can induce a rapid decrease in (18)F-fluorodeoxyglucose (FDG) uptake in GIST. Aim of this study is to investigate if early change in tumor FDG uptake predicts primary IM resistance. METHODS: Consecutive pts with metastatic or locally advanced GIST had FDG-PET scans before and 1 week after start of IM. Maximum Standardized Uptake Value (SUVmax) was determined and mean SUVmax for a maximum of 5 lesions per pt was calculated. Relationship between PET response (according to EORTC guidelines) and CT response (according to RECIST and Choi) after 2 months of treatment, was investigated. RESULTS: 22 male and 14 female pts were included, mean age 62 years (range 23-81). 29 pts were treated in a palliative and 7 in a neo-adjuvant setting. 6 pts were not evaluable for PET response: 4 pts had no FDG-avid lesions, 2 pts had discordant PET scan protocols. 81 tumor lesions in 30 pts were quantified. Mean SUVmax on the first scan was 7.4 (SD 3.8, range 2.2-18.4) and on the second scan 3.0 (SD 2.1, range 0.1-11.8) (P<0.001). 26 pts had a metabolic response, 4 pts had metabolically stable disease. After 8 weeks of treatment 29 and 23 pts were evaluable according to RECIST and Choi criteria respectively (see table). PET response had a high positive predictive value (PPV) for clinical benefit (complete response (CR) or partial response (PR) or stable disease (SD)) according to RECIST: 92% (95% CI 74-99%) and Choi criteria: 95% (95% CI 76-100%). However, the false negative rate was 11% (95% CI 2-30%) resp. 9% (95% CI 1-30%) and pts with PD had a PET response. CONCLUSIONS: Although early FDG-PET response has a high PPV for clinical benefit from IM in pts with GIST, FDG-PET scans can not be used to predict primary resistance. [Table: see text].