Congenital Mesoblastic Nephroma: A Rare Pediatric Neoplasm / Kongenital Mezoblastik Nefrom: Nadir Bir Pediatrik Tümör

Abstract Congenital mesoblastic nephroma is a stromal neoplasm of infancy. It has been referred to as mesenchymal, cystic or leiomyomatous hamartoma. Th ese tumors are centered around the hilus of the kidney. Mesoblastic nephromas need to be distinguished from other pediatric renal neoplasms as these lesions are treated by complete surgical excision without chemotherapy unless gross residual tumor remains. Here, we describe the gross and microscopic features of mesoblastic nephroma in a twenty-day old infant. ÖZ Konjenital mezoblastik nefrom, bebeklik döneminin stromal neoplazisidir. Mezenkimal, kistik veya leyomyomatöz hamartom olarak kabul edilmektedir. Bu tümörler böbrekte hilus civarında yerleşim gösterir. Mezoblastik nefromlar makroskopik rezidü olmadığı takdirde kemoterapiye gerek kalmadan cerrahi eksizyon ile tedavi edildikleri için diğer pediatrik renal tümörlerden ayırt edilmelidir. Burada, 20 günlük bir bebekte saptanan mezoblastik nefromun makroskopik ve mikroskopik özellikleri tanımlanmaktadır.


INTRODUCTION
Stromal tumors account for 15% of all pediatric renal tumors. Congenital mesoblastic nephroma (CMN) is a stromal neoplasm of infancy and has characteristic features that help to diff erentiate it from metanephric stromal tumor (MST) (1). CMN is centered around the hilus of the kidney and nearly all involve the renal sinus. Histopathologically, CMNs are predominantly monomorphic neoplasms composed of spindled mesenchymal cells of fi broblastic or myofi broblastic lineage. Th ese are divided into two major types: classic and cellular (2). Almost all CMNs are predominantly localized to the kidney and perinephric or hilar soft tissue at the time of detection. Recurrences and metastasis occur in approximately 5 to 10% of all tumors. It is a very rare tumor and very few cases have been reported in the literature. Here we present a case in a 20-day-old infant.

CASE REPORT
A twenty-day-old female child presented with abdominal swelling. Ultrasonography revealed a solid mass in the right kidney. A provisional diagnosis of Wilms tumor was made and right nephrectomy was done. Histopathological sections from the nephrectomy specimen showed features of mesoblastic nephroma with immunohistochemistry confi rming the diagnosis.
Th e kidney measured 9x7x5 cm. On cut section, the cortex and medulla could not be seen, and the kidney was replaced by a grey-white homogenous lesion having a whorled myomatous appearance. A thin segment of compressed normal kidney was seen. A small cyst measuring 1 cm in diameter was also noted ( Figure 1).
Microscopy revealed a monomorphous lesion composed of interlacing fascicles of spindled cells. Th ese cells were plump with moderate amount of cytoplasm and elongated vesicular nuclei. Dilated thin walled vascular spaces were also noted. Th e tumor margins were irregular, with radiating bands extending into the normal renal parenchyma ( Figure 2). Entrapped renal glomeruli and tubules were also seen in the tumor ( CMN is diagnosed in the fi rst few months of life. It may even present in utero, causing non-immune hydrops and polyhydramnios (2).
Mesoblastic nephromas grossly appear as solitary, unilateral masses with fi rm, bulging cut surfaces oft en indistinguishable from nephroblastomas that have been treated with chemotherapy. Cysts, haemorrhage and necrosis are common. However, some lesions are quite fi rm. Most CMNs are centered around the hilus of the kidney and mostly involve the renal sinus, and careful surgical and pathological examination of the medial aspect of the nephrectomy and its medial margin is therefore of paramount importance.
Histologically, they are composed of spindled mesenchymal cells of fi broblastic or myofi broblastic lineage and are thought to arise from secondary mesenchyme. Two histological subtypes have been described: classic and cellular type (1). Th e classic type resembles infantile fi bromatosis with interlacing fascicles of spindle cells, resembling fi broblasts, myofi broblasts, interspersed with scant collagen fi bres. Tumor margins are highly irregular with radiating tongues of cells extending into the parenchyma and perirenal soft tissues (2). Abnormal metaplastic changes in the tubules or glomeruli adjacent to or entrapped by the lesion are present. Only 24% of CMNs are of the classic type. Th e cellular type, which is more common, is characterized by increased cellular density and high proliferative rate, imparting a sarcomatous appearance to the tumor. Mitotic activity is more conspicuous than the classic type (1). Cellular and classic patterns coexist and they are described as mixed mesoblastic nephroma. Extramedullary hematopoiesis can be observed. Skeletal muscle diff erentiation is not a feature of CMN. Our case showed the classic pattern. CMNs are positive for vimentin, actin and desmin. Th e onion skin rings of muscle (collarettes) around tubules and blood vessels, juxtaglomerular hyperplasia, angiodysplasia and desmin negativity, which are characteristic features of MST, were absent in our case distinguishing it from MST (3). Th e characteristic branching vasculature and diversity of patterns which are specifi c to CSSK help to distinguish it from CMN. Our case could be distinguished from Wilms tumor that had been treated with chemotherapy by the   absence of blastematous foci on histology. Wilms tumor is also seen aft er six months of age.
All CMNs behave clinically in a benign fashion and it is therefore important to diff erentiate them from aggressive mimics as nephrectomy is curative (4). Th ere is a case reported to undergo spontaneous regression (5).
Th is communication highlights a case of CMN, an underreported pediatric renal neoplasm with a good prognosis aft er total excision.