Positron Emission Tomography (PET) Response to Stereotactic Body Radiation Therapy (SBRT) for Metastatic Malignant Melanoma

Abstract

After treatment of metastatic lesions with SBRT, scarring and fibrosis may lead to misclassification of treatment response when using non-metabolic imaging. Thus, the metabolic information provided by PET may offer a more accurate assessment of treatment efficacy. However, early radiation changes on PET may falsely lead to concern for progressive disease. Here, we report our SBRT experience with long-term PET follow-up in patients with metastatic melanoma. Thirty-two metastatic melanoma patients treated with SBRT who had evaluable baseline and post-SBRT PETs were identified in our prospectively maintained database from 2008 to 2011. PET metabolic response was evaluated per PERCIST 1.0 criteria: Complete response (CR) was a decrease in the maximum standard uptake value corrected for lean body mass (SUL) to 1.5 times the liver mean + 2 standard deviations, partial response (PR) was a 30% decrease in SUL, progressive disease (PD) was> 30% increase in SUL and stable disease (SD) was any lesion not fitting these criteria. Local control (LC) included CR, PR, and SD. Fifty-six SBRT-treated lesions and 201 pre- and post-SBRT PET scans were analyzed. Median follow-up (f/u) was 1.9 years. Sites of treatment included: 15 musculoskeletal, 14 liver, 14 lung, 11 abdominal, 2 extra abdominal lymph nodes. Median single-fraction equivalent dose (SFED) was 43 Gy (range = 18-56 Gy). A median of 5 PET scans (range = 2-6) were evaluated for each lesion. LC was 89% and 75% and overall survival was 54% and 23% at 1.5 and 3 years, respectively. Median time to CR was 2.8 months (0.7-15 months). CR was achieved in 49 lesions (86%), and 43 lesions maintained CR at last f/u. Median f/u for lesions in continuous CR was 22 months. Overall, rates of PR, SD, and PD were 7%, 5%, and 13%, respectively. SFED > 24 Gy correlated with LC (RR = 1.4, P = 0.006). At initial f/u (median 2 months), the rate of CR was 60%, and 9 lesions (16%) had increased SUL. These 9 lesions resolved to 5 CR, 2 SD, and 2 PD with subsequent f/u. CNS metastases developed after SBRT in 35% (median 6 months). Three patients were alive, two with no evidence of disease, at last follow-up. SBRT produces a complete metabolic response and prolonged local control in the majority of treated patients. Even after an initial rise in SUL, long-term local control is high. These data support the use of SBRT to overcome the inherent radioresistant nature of metastatic melanoma.