18F-FDG PET response and clinical outcomes after stereotactic body radiation therapy for metastatic melanoma

BACKGROUND: Clinical data that support stereotactic body radiation therapy (SBRT) metastatic malignant melanoma (MM) are limited. Furthermore, functional imaging with 18F-fludeoxyglucose positron emission tomography (PET) may offer a more accurate post-SBRT assessment. Therefore, we assessed the clinical outcomes and metabolic response of metastatic MM after SBRT. METHODS AND MATERIALS: Patients with MM who were treated with SBRT and had pre- and post-PET scans (>1) were included in this study. A total of 390 pre- and post-SBRT PET/computed tomography (CT) scans for 80 metastases were analyzed. The PET metabolic response was evaluated per the PET Response Criteria in Solid Tumors (PERCIST), version 1.0, criteria. Single-fraction equivalent dose (SFED) was calculated as per the standard. The Kaplan-Meier method was used for estimates of overall survival (OS) and progression-free survival. The cumulative incidence method was used to estimate metastasis control (MC). A Wilcoxon test was used to compare survival estimates. The prognostic factors for MC and OS were assessed using the Cox proportional hazards model, and the Likelihood Ratio was also used for comparisons between groups. RESULTS: A median of 6 PET scans (range, 2-6 scans) was evaluated for each metastasis. The median SFED was 42.8 Gy (range, 18-56.4 Gy) and the median biologically effective dose was 254.4 Gy(2.5) (range, 100.8-540 Gy(2.5)). Twenty percent of patients received chemotherapy and 59% received immunotherapy: granulocyte-macrophage colony-stimulating factor (64%) and ipilimumab (34%). MC was 94% and 90% at 1 year and 3 years, respectively. The OS was 74% and 27% and 1 year and 3 years, respectively. Complete response was achieved in 90% at a median of 2.8 months (range, 0.4-25.2 months). SFED >24 Gy correlated with improved MC (93% vs 75%, P = .01). Acute and late grade 3+ toxicities were 4% and 11%, respectively, with no grade 5 toxicity. CONCLUSIONS: Post-SBRT PET/CT for extracranial metastatic MM resulted in high rates of complete response at a median of 2.8 months, and durable MC was achieved with SFED >24 Gy. SBRT, in addition to surgery and ablation, should be discussed with patients with MM, especially those with oligometastases.