The value of PET/CT for the diagnosis and evaluation of immunotherapy in occult primary renal cell carcinoma: a case report.

Skin and soft tissue diffusion metastasis (also known as occult cancer) is rare in renal cell carcinoma (RCC). Here, we report an extremely rare case of a 67-year-old male patient with occult primary RCC who developed metastases to the gums, skin, and diffuse soft tissue. The primary renal lesion was missed by computed tomography (CT), ultrasound, and 18F-fluorodeoxyglucose positron emission tomography (PET)/CT, and the diagnosis was confirmed by biopsy of gums and subcutaneous nodules. Subsequent enhanced CT revealed a lesion in the left kidney. The patient had progression-free survival of 16 months after treatment with axitinib and pembrolizumab. Pseudoprogression and tumor heterogeneity pose major challenges in the evaluation of immunotherapy. PET/CT is indispensable especially for cases with multiple metastases, widespread distribution of lesions, and major heterogeneity. In this case, the total lesion glycolysis was calculated by PET/CT and was used to evaluate systemic tumor load before and after immunotherapy, which was calculated as the product of the metabolic tumor volume and the mean standardized uptake value of the target lesion, which increased the accuracy of assessing diffuse lesions. Total lesion glycolysis can be used as a new method to quantitatively evaluate the efficacy of immunotherapy.