Immune Checkpoint Inhibitor-Induced Myelitis Presenting with "Hot Cord" Sign on ¹⁸F-FDG PET/CT: A Case Report

Immune Checkpoint Inhibitor (ICI)-induced myelitis is a rare but potentially severe neurological immune-related adverse event. Its diagnosis can be challenging because conventional Magnetic Resonance Imaging (MRI) findings may be subtle or limited. A distinctive case was reported of ICI-induced myelitis characterized by diffuse spinal cord hypermetabolism on 18F-FDG PET/CT, demonstrating a "hot cord" sign and highlighting the potential value of metabolic imaging in neurological immune-related adverse events.

A 55-year-old man with clear cell renal cell carcinoma developed progressive neurological symptoms after two cycles of toripalimab combined with axitinib. Contrast-enhanced spinal MRI demonstrated patchy T2 hyperintensity and enhancement at the T11-T12 levels and within the cauda equina. 18F-FDG PET/CT subsequently revealed diffuse increased tracer uptake throughout the spinal cord, with a maximum standardized uptake value (SUVmax) of 6.06, producing a characteristic "hot cord" appearance. Cerebrospinal fluid analysis demonstrated inflammatory changes, including elevated cell count and protein levels. Based on the clinical history, neurological manifestations, imaging findings, and laboratory results, ICI-induced myelitis was considered the most likely diagnosis. The patient received high-dose glucocorticoid therapy, after which his neurological symptoms substantially improved.

This case demonstrates a characteristic diffuse "hot cord" pattern of ICI-induced myelitis on 18F-FDG PET/CT. PET/CT may provide complementary metabolic information to MRI and could facilitate the recognition of ICI-associated inflammatory spinal cord injury, particularly when MRI abnormalities are limited or nonspecific.