FDG Uptake in WT1 Peptide Vaccination Adjuvant-induced Granuloma

We report a woman in her 20s with alveolar rhabdomyosarcoma treated with Wilms tumor 1 (WT1) peptide vaccination administered as an investigational immunotherapy. Serial PET/CT demonstrated intense FDG uptake at the injection site with progressive skin thickening and calcification. Remarkably, FDG uptake continued to increase after vaccination cessation, peaking at 49 months before gradually declining. Unlike transient inflammation seen with conventional vaccines, WT1 vaccination induces prolonged FDG uptake due to the oil-based adjuvant Montanide, causing chronic inflammation and granuloma formation. Recognition of this characteristic course is essential to avoid misdiagnosis as cutaneous metastasis.