18F-FDG PET/CT for Posttreatment Surveillance Imaging of Patients with Stage III Merkel Cell Carcinoma

Abstract

The purpose of this study was to investigate the diagnostic and prognostic value of ¹⁸F-FDG PET/CT for surveillance imaging in patients treated for stage III Merkel cell carcinoma (MCC). Methods: This retrospective study included 61 consecutive stage III MCC patients who were clinically asymptomatic and underwent surveillance ¹⁸F-FDG PET/CT. Findings were correlated with either pathology or clinical/imaging follow-up. The median follow-up period was 4.8 y. Statistical analyses were performed. Results:¹⁸F-FDG PET/CT detected unsuspected recurrences in 33% patients (20/61) with lesion-based sensitivity, specificity, and accuracy of 92%, 93%, and 93%, respectively. The mean ± SD SUV for malignant and benign lesions was 7.5 ± 3.9 and 3.8 ± 2.0, respectively. Unknown distant metastases, as first recurrence site, were noted in 12 of 61 patients. Those with positive disease on ¹⁸F-FDG PET/CT within 1 y of definitive treatment had relatively worse overall survival (P < 0.0001). After adjustment on stage, risk of death increased with a higher SUV_max (hazard ratio for 1 unit = 1.17; P = 0.006) and with a higher number of positive lesions on ¹⁸F-FDG PET/CT (hazard ratio for 1 additional lesion = 1.60; P < 0.001). Conclusion: Postdefinitive treatment surveillance ¹⁸F-FDG PET/CT scanning detects unsuspected recurrences and has prognostic value. Inclusion of ¹⁸F-FDG PET/CT within the first 6 mo after definitive treatment would be appropriate for surveillance and early detection of recurrence. Our data merit further studies to evaluate the prognostic implications.

Keywords: 18F-FDG PET/CT; Merkel cell carcinoma; PET; prognosis; recurrence; surveillance.

Graphical abstract

FIGURE 1.

Asymptomatic 70-y-old woman with left arm MCC (s/p excision, left axillary lymphadenectomy, radiation to axilla) underwent surveillance ¹⁸F-FDG PET/CT scan 3.3 mo after treatment. ¹⁸F-FDG PET/CT (A; maximum-intensity projection, arrow) scan revealed solitary focal ¹⁸F-FDG uptake in left pelvis (B; fused PET/CT, SUV 9, arrow) in a nodular soft-tissue-density lesion in left adnexa (C; axial CT, arrow). USG pelvis showed solid mass in left ovary measuring 1.8 x 1.4 x 1.6 cm, corresponding to site of abnormality on ¹⁸F-FDG PET/CT. Patient underwent salpingo-oophorectomy and pathology was positive for MCC. s/p = status post; USG = ultrasound.

FIGURE 2.

Receiver-operating-characteristic (ROC) curve for benign versus malignant lesion prediction based on SUV_max. FPR = false-positive rate; TPR = true-positive rate.

FIGURE 3.

Swimmer plot illustrating information about local and distant recurrences, confirmed on pathology, during follow-up on surveillance ¹⁸F-FDG PET/CT scans since end of primary treatment for all included stage IIIA known primary (KP), IIIA unknown primary (UP), and IIIB MCC patients (n = 61).

FIGURE 4.

OS of patients based on findings and timing of ¹⁸F-FDG PET/CT scan (A, at 3-mo; B, at 6 mo, and C, at 12 mo after definitive treatment, respectively).