Inflammatory Mimicry: Epididymo-orchitis Masquerading as Malignancy on [18F]-FDG PET/CT Imaging

A 52-year-old male, a known case of adenocarcinoma of the stomach, underwent total gastrectomy and chemotherapy and was referred for ¹⁸F-fluorodeoxyglucose positron emission tomography–computed tomography (¹⁸F-FDG PET/CT) to rule out recurrence. ¹⁸F-FDG PET/CT showed a large FDG-avid lesion involving the entire right testicle with SUV_max of 15.1 along with the right hydrocele [Figure 1]. The findings were due to testicular cancer, lymphoma, and infection; therefore, further evaluation was advised. Ultrasonography showed a bully right testicle with multiple ill-defined hypoechoic regions. The epididymis was enlarged. There was a significant hydrocele with multiple septations and low-level internal echoes. The findings were suggestive of epididymo-orchitis. Subsequently, after antibiotic treatment and follow-up, ¹⁸F-FDG PET/CT demonstrated metabolic resolution of prior FDG-avid lesion in the right testis, confirming that it was a false positive result due to inflammation rather than cancer [Figure 2].

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Figure 1 A 52-year-old male, a known case of adenocarcinoma of the stomach, underwent total gastrectomy and chemotherapy, referred for ¹⁸F-fluorodeoxyglucose positron emission tomography–computed tomography (¹⁸F-FDG PET/CT) to rule out recurrence. (a) ¹⁸F-FDG maximum intensity projection and (b and c) transaxial and coronal PET/CT images show a large FDG-avid lesion involving the entire right testicle with SUV_max of 15.1 (arrow) along with right hydrocele. Findings can be due to testicular cancer, lymphoma, and infection; further evaluation was advised

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Figure 2 (a) Ultrasonography shows bully right testicle measures 3.7 cm × 3 cm × 2.9 cm. Multiple ill-defined hypoechoic regions are seen within the testicular parenchyma. The epididymis is enlarged. There is a significant hydrocele with multiple septations and low-level internal echoes. The left testis and epididymis are unremarkable. Findings are suggestive of epididymo-orchitis, possibly granulomatous etiology. (b) Follow-up ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography–computed tomography demonstrates metabolic resolution of prior FDG-avid lesion in the right testis

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¹⁸F-FDG PET is widely used in cancer for staging, restaging, assessing disease extent, monitoring treatment response, and prognostication.[1] While highly sensitive for detecting malignancies, its specificity can be affected by benign inflammatory conditions like orchitis, which can also show increased radiotracer uptake.[2] ¹⁸F-FDG uptakes in infective and inflammatory cells are related to increased expression of glucose transporters as well as various cytokines and growth factors. In addition, inflammatory cells like neutrophils and activated macrophages also demonstrate increased FDG accumulation, similar to cancer cells.[34] The pattern of ¹⁸F-FDG uptake in orchitis can vary, ranging from focal to diffuse involvement of one or both testicles. In some cases, the uptake may be asymmetric, with one testicle showing more intense uptake than the other.[56] Differentiating orchitis from testicular malignancy can be challenging based on ¹⁸F-FDG PET/CT findings alone. Both conditions can present with increased ¹⁸F-FDG uptake. However, certain imaging features can provide clues to the correct diagnosis. In cases of orchitis, the ¹⁸F-FDG uptake is often more diffuse, uniform, and symmetric, involving a significant portion of the entire testicular parenchyma. In contrast testicular cancer, the pattern of ¹⁸F-FDG uptake is often heterogeneous tends to present as focal, asymmetric lesions and generally show more ¹⁸F-FDG avidity in seminomatous tumors compared to nonseminomas and might be associated with lymph node involvement.[7] In addition, orchitis is often accompanied by scrotal edema, thickening of the tunica vaginalis, and reactive involvement of the epididymis, which can be detected on the CT component of the study. Clinical correlation and additional imaging modalities, such as ultrasound or magnetic resonance imaging (MRI), can help differentiate orchitis from testicular malignancy. Ultrasound can assess the testicular parenchyma and identify the presence of inflammatory changes, such as increased vascularity or heterogeneous echogenicity, which are more suggestive of orchitis.[8] MRI can provide excellent soft tissue contrast and may reveal characteristic features of orchitis, such as diffuse enlargement and heterogeneous signal intensity of the affected testicle.[9] Our case delves into the intricacies of false positive ¹⁸F-FDG PET/CT results due to orchitis. Careful evaluation of the imaging findings, clinical correlation, and the judicious use of additional imaging modalities for lesion morphology and distribution are helpful for accurate diagnosis and appropriate patient management.