Progressing Sclerosing Mesenteritis (Mesenteric Panniculitis) Mimics Progression of Malignancy After Neoadjuvant Chemotherapy for Gastric Adenocarcinoma on Serial 18F-FDG PET/CT.

A 62-year-old man was diagnosed with a moderately differentiated gastric adenocarcinoma in the proximal stomach. A staging 18F-FDG PET/CT showed an intensely FDG-avid gastric mass, as well as a mildly FDG-avid misty nodular mesentery. After 3 cycles of neoadjuvant chemotherapy, a follow-up PET/CT showed partial response of the gastric primary, with increase in the size of nodules in the mesentery and increased FDG uptake, raising concern of secondary malignancy. Biopsy of the mesentery revealed xanthogranulomatous inflammation, consistent with sclerosing mesenteritis.