Is 18F-FDG PET/CT Necessary During Vismodegib Therapy for Basal Cell Carcinoma? Quantitative Changes and Incremental Value Beyond CT

Abstract The role of 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) in monitoring basal cell carcinoma (BCC) during vismodegib therapy remains poorly defined, particularly in patients with extensive, multifocal, or Gorlin syndrome–associated disease. This study aimed to evaluate metabolic and anatomic treatment-response findings and the incremental diagnostic value of 18F-FDG PET beyond CT in this setting. Patients with histopathologically confirmed BCC who underwent 18F-FDG PET/CT during vismodegib therapy between December 2015 and August 2026 were retrospectively reviewed. Metabolic response was classified using PERCIST (Positron Emission Tomography Response Criteria in Solid Tumors)-like criteria based on serial maximum standardized uptake value (SUVmax) measurements, and tumor-to-liver ratio (TLR) was additionally calculated as a background-normalized metabolic parameter. Patients without suitable paired imaging for metabolic response assessment because of absent follow-up PET/CT, lack of an FDG-avid target lesion, or intervening local treatment were considered not evaluable for categorical metabolic response. Baseline and follow-up SUVmax and TLR values were compared using the Wilcoxon signed-rank test. Continuous variables were summarized using descriptive statistics, and categorical variables were reported as frequencies and percentages. Sixteen patients were included, of whom 5 had Gorlin syndrome. Primary lesions were predominantly craniofacial, occurring in 15 of 16 patients (93.8%), and 11 patients had orbital involvement. Baseline FDG avidity was present in 10 patients, whereas 6 had low or absent uptake. Among 8 patients with paired quantitative PET/CT measurements, the median baseline and follow-up SUVmax values were 5.45 and 4.50, respectively, with no significant interval change (Wilcoxon signed-rank test, p = 1.000). Similarly, paired TLR values showed no significant interval change (p = 0.834). Categorical PET response assessment was feasible in seven patients. Others were not evaluable because of absent follow-up PET/CT, no FDG-avid target lesion, or intervening local treatment. A CT-measurable target lesion was present in nine patients (56.2%), while PET provided incremental information beyond the CT component in three patients (18.8%), primarily through the identification of additional metabolically active lesions or disease findings not apparent on CT. Although 18F-FDG PET/CT may provide additional metabolic information in selected patients with advanced, multifocal, or Gorlin syndrome–associated BCC, its incremental diagnostic contribution beyond CT during vismodegib therapy appears limited. Selective use may therefore be more appropriate than routine serial imaging.