23 August 2006 Case of the Week #57


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We thank Dr. Ankur Sangoi, Stanford University, Stanford, California (USA) for contributing this case. We invite you to contribute a Case of the Week by sending an email to NPernick@PathologyOutlines.com with microscopic images (any size, we will shrink if necessary) in JPG or GIF format, a short clinical history, your diagnosis and any other images (gross, immunostains, EM, etc.) that may be helpful or interesting. We will write the discussion (unless you want to), list you as the contributor, and send you a check for $35 (US) for your time after we send out the case. Please only send cases with a definitive diagnosis.


The next case (Case #58) will be sent in 2 weeks, on 6 September 2006.



Case of the Week #57


Clinical history


The patient is a 3 month old girl with a right kidney mass who underwent nephrectomy.


Gross images: image


Microscopic images: well circumscribed; medium power; high power; infiltrative margin; glomerular entrapment; angiomatoid area


What is your diagnosis?


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Congenital mesoblastic nephroma, cellular type




Congenital mesoblastic nephroma, first described in 1967 (Pediatrics 1967;40:272) is the most common kidney tumor of infancy.


There are three types of congenital mesoblastic nephroma, with slightly different clinical presentation. The classic type (24% of cases) presents at an average age of 2-3 weeks of life with a mean size of 100g (Hum Path 1989;20:682). The cellular type (66% of cases), presents at 5 months of life and may weigh up to 1 kg. The mixed type (10%) of cases, has intermediate features. As most tumors present in infants, this diagnosis should be questioned if applied to patients over 2 years old (Murphy: Tumors of the Kidney, Bladder and Related Urinary Structures (AFIP Atlas of Tumor Pathology, Series 4, Vol 1); 2004, page 57).


Grossly, most congenital mesoblastic nephromas involve the renal sinus, and it is important to report involvement of the medial resection margin by tumor. The classic type resembles leiomyoma with a whorled cut surface, but also tongues of infiltrating tumor. The cellular type often has large cystic areas, hemorrhage and necrosis.


Microscopically, the classic type resembles infantile fibromatosis, with intersecting fascicles of bland spindle cells resembling myofibroblasts and interspersed thin collagen fibers. The cellular type resembles infantile fibrosarcoma. It has densely packed, plump atypical spindle cells with abundant cytoplasm, vesicular nuclei and nucleoli, and often frequent mitotic figures.


Tumor cells in both types are immunoreactive for smooth muscle actin, desmin and vimentin, and negative for keratin, except for entrapped epithelium.


The cellular type is often associated with t(12;15)(p13;q25), causing a ETV6-NTRK3 fusion gene, also seen in infantile fibrosarcoma involving infant soft tissues. The classic type lacks this translocation (Histopathology 2006;48:748)


Treatment consists of nephrectomy with wide margins. Chemotherapy is given for incomplete resection or tumor rupture during surgery. The cellular tumors may recur or rarely metastasize.


The differential diagnosis includes other tumors of infancy. Wilms tumor is common, but typically involves older children. It has prominent blastema and often nephrogenic rests, although previous treated tumors may have a well differentiated spindle cell stroma. Clear cell sarcoma presents at a similar age, but has a prominent chicken-wire vasculature and clear cells. The tumor cells have fine nuclear chromatin and a low mitotic rate, compared to the coarse chromatin and higher mitotic rate of congenital mesoblastic nephroma. Clear cell sarcoma is negative for smooth muscle markers.



Nat Pernick, M.D.
PathologyOutlines.com, LLC
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