Baseline and on Treatment Biodistribution Variability of 18 F-FLT Uptake in Patients With Advanced Melanoma: Brief Communication.

PURPOSE: This prospective study evaluates the biodistribution of 18 F-FLT PET in patients with advanced melanoma before and after treatment with BRAF/MEK inhibitors. PATIENTS AND METHODS: Eighteen BRAF-positive unresectable stage IIIc or IV melanoma patients referred for 18 F-FLT PET/CT before (BL) and during (D14) BRAF/MEK inhibition were included. 18 F-FLT accumulation in the liver, bone marrow, blood, and muscle was quantified. RESULTS: Baseline interpatient 18 F-FLT uptake had a coefficient-of-variation between 17.5% and 21.5%. During treatment, liver uptake increased (SUV meanBL = 4.86 ± 0.98, SUV meanD14 = 6.31 ± 1.36, P < 0.001) and bone marrow uptake decreased (SUV meanBL = 7.67 ± 1.65, SUV meanD14 = 6.78 ± 1.19, P < 0.025). Both changes were unrelated to baseline metabolic tumor volume or tumor response. CONCLUSIONS: To assess 18 F-FLT PET, both liver and bone marrow uptake may be used as normal tissue references at baseline, but 18 F-FLT biodistribution significantly changes in longitudinal response studies when treated with BRAF/MEK inhibitors.

The Predictive Value of FDG PET/CT for Determining Progression-Free Survival in Advanced Stage III-IV BRAF -Mutated Melanoma Patients Treated With Targeted Therapy-What Can Be Learned From Progression?

PURPOSE: The aims of this study were to investigate whether (early) PERCIST response monitoring with 18 F-FDG PET/CT is predictive for progression-free survival (PFS) in unresectable stage III or IV melanoma patients treated with BRAF/MEK inhibitor (MEKi) and to define dissemination patterns at progression with a lesion-based evaluation in direct comparison to baseline to improve our understanding of 18 F-FDG PET/CT during BRAF/MEKi. PATIENTS AND METHODS: This prospective multicenter single-arm study included 70 patients with unresectable stage III/IV BRAF -mutated melanoma who underwent contrast-enhanced CT and 18 F-FDG PET/CT at baseline and 2 and 7 weeks during treatment with vemurafenib plus cobimetinib and at progression if possible. Tumor response assessment was done with RECIST1.1 and PERCIST. Follow-up PET/CT scans were visually compared with baseline to assess dissemination patterns. RESULTS: Using RECIST1.1, PFS was not significantly different between the response groups ( P = 0.26). At 2 weeks, PERCIST median PFS was 15.7 months for patients with complete metabolic response (CMR) versus 8.3 months for non-CMR ( P = 0.035). The hazards ratio (HR) for progression/death in non-CMR versus CMR was 1.99 (95% confidence interval [CI], 1.03-3.84; P = 0.040) and 1.77 (95% CI, 0.91-3.43; P = 0.0935) when adjusting for lactate dehydrogenase (LDH). At 7 weeks, median PFS for PERCIST CMR was 16.7 months versus 8.5 months for non-CMR ( P = 0.0003). The HR for progression/death in the non-CMR group was significantly increased (HR, 2.94; 95% CI, 1.60-5.40; P = 0.0005), even when adjusting for LDH (HR, 2.65; 95% CI, 1.43-4.91; P = 0.0020). At week 7, 18 F-FDG PET/CT was false-positive in all 4 (6%) patients with new FDG-avid lesions but CMR of known metastases. When 18 F-FDG PET/CT was performed at progressive disease, 18/22 (82%) patients had progression of known metastases with or without new 18 F-FDG-avid lesions. CONCLUSIONS: This study shows that PERCIST response assessment at week 7 is predictive for PFS, regardless of LDH. At 2 weeks, patients with CMR have longer PFS than patients with non-CMR, but different PET parameters should be investigated to further evaluate the added value of early 18 F-FDG PET/CT. Disease progression on PET/CT is predominated by progression of known metastases, and new 18 F-FDG-avid lesions during BRAF/MEKi are not automatically a sign of recurrent disease.