FINAL PATHOLOGIC DIAGNOSIS:

Kidney/retroperitoneum, mass, left, core biopsy: Renal carcinoid tumor (neuroendocrine tumor)

Primary renal carcinoid tumor is extremely rare, with less than 100 cases reported in literature. Neuroendocrine cells are not a normal part of the kidney. It is hypothesized that small rests of the cells may be acquired through metaplasia, a result of congenital abnormality, abnormal embryonal cellular migration, or represent metastasis of unknown primary. Most are found incidentally or a result of mass effect rather than carcinoid syndrome. Often, the tumors are hypodense and do not enhance.

In this case, there is equivocal but suspected enhancement on the CT, although given the size and appearance, a high suspicion for neoplasm is present. Rather than perform an expensive subtraction MRI, ultrasound was utilized to confirm the vascular nature of this neoplasm.

The ultrasound images are very helpful in demonstrating the important principal of careful Doppler interrogation, as the sonologist originally reported that no flow is present within the mass. However, after encouragement to perform a more careful evaluation, several areas of color Doppler and spectral arterial flow were identified in the solid portion of the mass, indicating that this is not a complex renal cyst compared by hemorrhage but rather a solid, vascular neoplasm. At that point, the patient could proceed directly to image guided biopsy for final pathologic diagnosis.