Concomitant thyroiditis and orchitis induced by immune checkpoint inhibitors detected on [¹⁸F]FDG PET/CT

Background The clinical management of malignant melanoma (MM) has undergone a significant revolution with the implementation of immune checkpoint inhibitors (ICIs). While these therapeutic agents stimulate the host immune system against cancer, they may also lead to immune-related adverse events (IrAEs). Positron emission computed tomography (PET/CT) with 2-deoxy-2-[1(8)F]fluoro-D-glucose ([F-18]FDG) has proven successful in detecting IrAEs in cancer patients undergoing ICI. In our case, we report a rare occurrence of ICIs-induced concomitant thyroiditis and orchitis detected on [F-18]FDG PET/CT. Case presentation We present a case involving a 61-year-old man referred to our hospital due to MM. Following surgical excision and sentinel lymph node mapping, he underwent an initial [F-18]FDG PET/CT, which yielded negative results. However, a follow-up PET/CT after 9 months revealed metastases in the lungs and lymph nodes. Subsequently, he initiated an ICI-based therapeutic regimen. After 3 months, he reported progressively worsening fatigue and the onset of testicular pain. A testicular ultrasound showed heterogeneous echotexture in both testicles with mildly increased vascularity. A subsequent PET/CT demonstrated complete regression of previously described pathological lesions in the lungs and metastatic lymph nodes. However, diffusely increased tracer uptake was observed in both the thyroid gland and testicles, findings absent in the pre-ICI examination. These were interpreted as IrAEs and promptly treated with corticosteroids, resulting in complete resolution of symptoms. Conclusions [F-18]FDG PET/CT plays a crucial role in staging and monitoring treatment response in cancer patients. When assessing subjects undergoing ICI-based therapies, particular emphasis should be given to detecting unusual IrAEs, as exemplified in our case.