Nilüfer Bıçakcı, Fatih Batı
Abstract Congenital in situ hepatic malrotation, often referred to as an “upside-down liver,” is a rare anatomical variant that can mislead Couinaud segment attribution and complicate longitudinal lesion mapping on 18F-FDG PET/CT. We report a 71-year-old man with biopsy-proven sigmoid colon adenocarcinoma who underwent serial 18F-FDG PET/CT examinations approximately every 6 months from August 2023 to December 2025 (approximately 28 months), during systemic therapy. PET/CT consistently demonstrated in situ hepatic malrotation with multifocal FDG-avid hepatic metastases. The dominant lesion measured 21 × 32 mm in the malrotated segment VI and showed interval progression in number, size, and maximum standardized uptake value (SUVmax). The most recent scan also revealed a new subcentimeter cavitary lung nodule in the superior segment of the right lower lobe with an SUVmax of 2.5, which was considered metastatic on imaging grounds in the clinical context, without histopathological confirmation. To prevent liver dome misregistration and mislocalization, interpretation relied on a fused multiplanar review, correlation with portal/hepatic venous landmarks, and verification on non–attenuation-corrected (NAC) images (with selective prone or respiratory-gated limited-bed acquisition when needed). Recognizing hepatic malrotation on PET/CT and documenting a vascular-landmark–based mapping and artifact-check workflow (multiplanar fusion + NAC review) are essential for accurate segment-based follow-up.