Objective To evaluate the diagnostic value of ¹⁸F-fluorodeoxyglucose (FDG) PET/CT imaging in patients with secondary renal lymphoma (SRL) and its differential analysis with renal immune disease (RID).

Methods The ¹⁸F-FDG PET/CT images and clinical characteristics of 12 patients with SRL confirmed by histopathological examination or clinical comprehensive diagnosis in Tianjin First Central Hospital from December 2017 to December 2020 were analyzed retrospectively. The patients in the SRL group comprised 7 males and 5 females, aged 50±15 years. At the same time, 10 patients with positive ¹⁸F-FDG PET/CT imaging and final diagnosis of RID were selected, including 2 cases of IgG4-related kidney disease and 8 cases of antineutrophil cytoplasmic antibody-associated vasculitis. These 5 males and 5 females, aged 60±10 years, comprised the RID group. Ten healthy subjects with normal kidneys by ¹⁸F-FDG PET/CT imaging were selected as the healthy control group (5 males and 5 females, aged 55±10 years). The ¹⁸F-FDG PET/CT features of renal and extrarenal invasion were observed and recorded. The maximal standardized uptake value (SUV_max) of renal lesion or renal cortex, the ratio of SUV_max in renal lesion or renal cortex to mean standardized uptake value (SUV_mean) of liver, and SUV_max of extrarenal invasions were measured and calculated. One-way ANOVA was used to compare SUV_max and ratio of standardized uptake value (SUV_ratio) among the three groups, and independent sample t test was used to compare SUV_max of extrarenal ¹⁸F-FDG uptake in the SRL group and RID group.

Results The 12 patients with SRL exhibited multifocal masses (n=6) and diffuse nephromegaly (n=6) in bilater kidneys with abnormally increased ¹⁸F-FDG uptake. In 10 patients with RID, the ¹⁸F-FDG PET/CT imaging of the affected kidneys showed bilateral diffuse nephromegaly with increased ¹⁸F-FDG uptake of renal parenchyma, which was similar to diffuse nephromegaly in SRL. However, in terms of extrarenal involvement, the patients in the RID group had common sites of involvement, and the patients in the SRL group were mostly associated with the involvement of different tissues and organs of the whole body, with irregular distribution. Statistical analysis showed that SUV_max and SUV_ratio of renal lesions in the SRL group were significantly higher than those in the RID group (21.88±12.04 vs. 9.09±3.51 and 11.38±6.52 vs. 3.67±1.12, respectively), and SUV_max and SUV_ratio of renal lesions in the two groups were significantly higher than those in the healthy control group (SUV_max=3.23±0.39, SUV_ratio=1.47±0.25; F=17.189, 18.361; both P<0.001). ¹⁸F-FDG SUV_max of extrarenal lesions in the SRL group was significantly higher than that in the RID group (27.67±15.09 vs. 7.55±3.70, t=−3.889, P=0.001).

Conclusion ¹⁸F-FDG PET/CT has a good clinical value in the diagnosis of SRL and the evaluation of systemic involvement, and it is helpful in distinguishing SRL from RID.