Diverse spectrum of uncommon tissue involvement in IgG4-related diseases on ¹⁸F-FDG PET/CT

Objectives: IgG4 related disease (IgG4-RD) is a rare, multi-organ involving fibro-inflammatory disease that is categorized as an orphan disease. It is referred to as the great mimicker because of the diverse range of tissue involvement and widely varying clinical presentation, often overlapping with those of common malignancies. Accurate identification of IgG4-RD is vital as it has a very favorable response to medical treatment with steroids. ¹⁸F-FDG PET/CT is a valuable modality in the assessment of these patients, as it can identify multi-organ involvement as well as provide the initial diagnostic clue in otherwise confounding cases. The aim of this exhibit is to familiarize the readers with the diverse patterns of IgG4-RD on ¹⁸F-FDG PET/CT whole-body imaging, which can improve diagnostic accuracy.

Methods: Medical and imaging records of patients with histopathologically confirmed IgG4-RD who underwent ¹⁸F-FDG PET/CT either before or after establishing the diagnosis were retrospectively reviewed. Cases with challenging imaging findings, typically involving uncommon sites, were short-listed for the exhibit along with a brief review of the clinical findings. The clinical information is provided to help the reader understand and appreciate the degrees of overlap with common malignancies. Uncommon presentations and patterns of multi-organ involvement reinforce the diverse nature of the disease and require prior acquaintance for pattern recognition later.

Results: System/ Organ wise involvement of IgG4-RD on ¹⁸F-FDG PET/CT is presented, including IgG4-RD of the lymph nodes, pancreas, gall-bladder, bile-ducts, esophagus, salivary glands etc. Each case is discussed with the clinical scenario and special emphasis is given to patients who were referred for PET/CT based on a different clinical suspicion and the possibility of IgG4-RD was raised on the ¹⁸F-FDG PET/CT imaging findings.

Conclusions: ¹⁸F-FDG PET/CT has a role in the initial diagnosis, biopsy site planning, assessment of disease severity and extent and treatment response evaluation in patients with IgG4-RD. The whole-body evaluation can help in providing a diagnostic clue to plan further investigations as well as for tissue sampling. Since several patients with histopathologically confirmed IgG4-RD may have normal serum levels of IgG4, identifying the disease sites for accurate sampling becomes even more important. This exhibit illustrates the utility of ¹⁸F-FDG PET/CT, highlighting the range and patterns of tissue involvement in IgG4-RD.