Editors: Francesca Khani ( and Mahmut Akgul (

2022-06/February 7
Contributors: Leandro LL Freitas, Ana YK Grizotto, Athanase Billis

A 76-year-old man with an incidentally renal tumorin the lower pole of the rightside of a horseshoe kidney was submitted to a partial nephrectomy.


1. What is the correct diagnosis?

a) Small Cell Carcinoma

b) Primitive Neuroecrodermal Tumor (PNET)

c) Well differentiated neuroendocrine tumor (Renal Carcinoid)

d) Paraganglioma

e) Nephroblastoma

Well differentiated neuroendorine tumor (renal carcinoid)

Well differentiated neuroendocrine tumor (renal carcinoid) is a very rare tumor. Our case had the classic appearance of carcinoid tumors including areas with a morphologic pattern of anastomosing trabeculae and cords of cells composed of monotonous cells with “salt-and-pepper” chromatin with minimal stroma (Fig. 1).In some areas the stroma was prominent(Fig. 2). Focally, areas of solid growth with nest formation were present (Fig.3).In some areas the tumor formed glandlike lumina (Fig. 4). Synaptophysin was diffusely positive (Fig. 5), chromogranin focally positive, Cam5.2 positive, vimentin positive, CK7 negative, CK20 negative, TTF-1 negative, WT1 negative, racemase negative, and PAX8 negative.

In a study of 21 patients the age ranged from 27 to 78 years, with an average age of 52 years with no gender predominance. In this series, signs and symptoms associated with the tumor included back or flank pain, hematuria, enlarging abdominal mass, anemia and fever.But most of the patients were asymptomatic and tumors were discovered incidentally on workup for other conditions.None of the patients had clinical signs or symptoms of carcinoid syndrome. Most of the rumors were unifocal and only one was multifocal. Interestingly, our patient had horseshoe kidney smilar to the 4 of the 21 patients in the study.

Well differentiated neuroendocrine tumors may metastasize but the prevalence of metastatic spread is not known.The prognosis and the potential for metastases may be related to the mitotic activity and cytologic atypia. In the study of 21 patients, most of the cases demonstrated similar mitotic rates, ranging from 0-3/10 high-power field, the one case that demonstrate the highest mitotic rate (4 mitoses/10hpf) belonged to the patient who died of disease at 8 months. In other organs, neuroendocrine tumors are classified into three grades based on number of 2 mitoses/10 hpf or percentage of the Ki-67 index that provides prognostic information. Ki-67 index lower than 3% is considered low grade, between 3 and 20% intermediate,and greater than 20% high grade.In our case the Ki-67 index was 2.4%. Our patient had no metastases at the time of diagnosis and the follow-up since surgery is very short for any prognostic analysis.

The main differential diagnosis includes small cell carcinoma, primitiveneuroectodermal tumor (PNET), paraganglioma, and intrarenal neuroblastoma and nephroblastoma (Wilms tumor). Small cell carcinoma has a brisk mitotic rate, apoptotic activity, cellular molding and lack the typical organized architecture of the carcinoid tumor.PNET shows a monotonous cellular population containing bland, rounded nuclei, variable mitotic activity and is strongly positive for CD99. Paraganglioma commonly contain a nested pattern of neuroendocrine cells with granular cytoplasm surrounded by S100 positive sustentacular cells. Neuroblastoma contains a background of neurofibrillary stroma and demonstrates Homer-Wright rosettes. TTF-1 and WT1 were both negative in our case and excludes lung metastasis or adult nephroblastoma (Wilms tumor).

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4.Kim B, Kim H-S, Moon KC. Primary renal well-differentiated neuroendocrine tumors: report of six cases with an emphasis on the Ki-67 index and mitosis. Diagn Pathol 2019;14:12-7.

Leandro LL Freitas, Ana YK Grizotto, Athanase Billis
School of Medical Sciences State University of Campinas (Unicamp)


Kidney; well-differentiated; carcinoma