FDG-PET Superior to CT for Predicting Melanoma Relapse After Immunotherapy

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The long-term outcome of patients with melanoma may be predicted by FDG-PET imaging and also influence treatment duration.

Most patients with a partial response (PR) on a computed tomography (CT) scan and a complete metabolic response (CMR) on a fluorodeoxyglucose (FDG)-positron emission tomography (PET) scan should be considered to have achieved complete response and at low risk for relapse of melanoma, according to study findings published in the Journal of the European Academy of Dermatology and Venerology.

In patients with metastatic melanoma, treatment with anti-PD1 immune checkpoint inhibitor has produced sustainable clinical activity. Nevertheless, strong predictive factors for long-term outcomes and relapse risk remain to be identified. The study was designed to compare FDG-PET imaging to CT scans for distinguishing absence of tumor vs residual tumors in patients with metastatic melanoma treated with anti‐PD1 immunotherapy between October 2014 and October 2017. All 26 patients (15 men and 11 women) were considered to be in complete remission; had CT scans and FDG-PET imaging performed within 2 months of immunotherapy treatment discontinuation; and had measurable and well-identified targets on imaging scans. Data on demographics, treatment, disease features, and outcomes were obtained from medical records.

The primary study outcome was relapse during follow-up. Response on CT imaging was evaluated using the response evaluation criteria in solid tumors (RECIST): progressive disease (PD), stable disease (SD), PR, and complete response (CR). FDG-PET imaging was classified as stable metabolic disease, progressive metabolic disease, residual FDG avidity (RFA), and CMR. For categorical variables, investigators used Kaplan-Meir estimates and a log-rank test, as well as Cox models and a likelihood ratio test for continuous variables.

According to CT-scans, 9 of the 26 patients in complete remission had a CR, with 2 relapsing after 3 and 9 months; 15 participants had a PR with 4 relapses after 7, 13 (2 patients), and 14 months. Of the 2 patients with an SD on CT-scans, none relapsed. No PDs were seen. According to FDG-PET evaluations, 20 of the 26 patientsin complete remission had a CMR and only 1 relapsed after 9 months. Six of the participants had an RFA and 5 relapsed after 3, 7, 13 (2 participants), and 14 months. None had an SMD or a PMD.

Discrepancies were noted between the CT scans and PET scans in 12 out of 26 patients. Ten patients showed a PR on their CT scan but a CMR on their FDG PET scan and none of these patients relapsed. Univariate analysis showed that RFAs on FDG-PET scans were significantly associated with occurrence of relapse (hazard ratio [HR], 31.26; 95% CI, 3.415-286.2; P =.00231), although CMRs appeared to indicate a protection from relapse (HR, 0.032; 95% CI, 5.9¹⁰⁻⁵-0.35; P =.00231). After adjustments were made for sex, age, and RECIST score, FDG-PET scan remained a good predictor of relapse risk (P =.015).

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Despite limitations such as a small sample size,  investigators concluded, “FDG-PET imaging could play a crucial role in predicting the long-term benefit of treatment and in making the decision of treatment discontinuation.”  They wrote that further prospective studies of FDG-PET imaging every 2 months after immunotherapy initiation “would be interesting to determine as early as possible the stage of the CR.”

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Mesnard C, Bodet-Milin C, Eugène T, Nguyen JM, Khammari A, Dréno B. Predictive value of FDG-PET imaging for relapse in metastatic melanoma patients treated with immunotherapy [published online March 27, 2020]. J Eur Acad Dermatol Venereol. doi: 10.1111/jdv.16358