The incidence of PTLD in kidney transplant patients is approximately 1–10%, and it occurs most frequently during the first year after transplantation. Immunosuppression and EBV infection, are two major factors associated with the progression of PTLD  .The Gastrointestinal tract, allograft kidney and abdominal cavity are common sites of PTLD, but it is rare in the native kidney, which has been described in only three studies [10,11,12]. In previous reports, both US and CE-CT can clearly show the tumor and be used to characterize the tumor. The tumors showed persistent hypo-enhancement throughout the examination with CE-CT. The enhancement pattern of the tumor is the main difference from clear cell renal cell carcinoma, which present avid early hyper-enhancement and early wash-out throughout the examination [13, 14].
In our case, the tumor showed avid early hyper enhancement on CEUS; however, this was not visible on US and CE-CT. There are three possible explanations for this difference in our case: First, since the microvasculature of the atrophic renal parenchyma was reduced compared to a normal kidney, less iodine contrast agent entered the renal parenchyma . In addition, the tumor was small in size, and there was much less iodine contrast agent entering the tumor. As a result, the tumor presented no enhancement on CE-CT, which caused a failure to distinguish tumors from surrounding cysts. Second, because CEUS contrast material is purely intravascular, it better correlates with the microvessel density of a tumor. Therefore, CEUS is even more sensitive for detection of hypovascular lesions than contrast-enhanced CT . Although the native kidney perfusion reduced in our case, the microvascular density of the tumor is relatively more abundant than the peripheral renal parenchyma. As a result, the tumor appeared hyper enhanced compare to the surrounding renal parenchyma on CEUS. Third, the CE-CT scan often started at 35 s after the beginning of the injection, but the lesion showed peak enhancement at 33 s during CEUS examination. This may result in missing the peak enhancement phase of the tumor on CE-CT. In contrast, real-time CEUS can continually provide information about blood perfusion for the renal lesion after injection, which can offer more diagnostic clues to differential the tumor from adjacent cysts.
US-guided biopsy is a common clinical method to obtain tissue specimens for histopathological analysis . However, this technique may be unsuccessful when the tumor is poorly differentiated from adjacent structures. CEUS is well placed to address this problem because of its capacity to differentiate between the altered vascularization of a tumor and surrounding structures. CEUS could help further confirm the tumor border and guide the needle to the target area. CEUS also can be used to differentiate enhanced active area from non-enhanced necrotic area. By directing the biopsy needle toward enhanced areas of the lesion, the sample from necrotic parts of the lesion can be reduced . In a previous report, using the contrast agent, the lesion detection rate was increased from 77.3% with US to 92.0% with CEUS during the biopsy, with a 95.2% success rate for CEUS-guided biopsies of these lesions [18, 19]. According to these finding, we performed the biopsy in our patient under the guidance of CEUS. Finally, the pathology of the biopsy specimen confirmed the diagnosis of diffuse large B cell lymphoma.
In summary, CEUS can provide more useful information than CE-CT to detect and diagnose PTLDs derived from atrophic native kidneys; CEUS-guided biopsy can improve the diagnostic accuracy and success rate of percutaneous biopsy. We believe that CEUS and CEUS-guided biopsy may be an effective method for early screening and diagnosis of native kidney PTLD in kidney transplant patients.