Case 7 -

Tubulocystic Carcinoma of Kidney Mathilde Sibony MD Hôpital Tenon Paris, France

Clinical History: In a 56-year-old man explored for prostate hypertrophy, a mass in the inferior pole of the left kidney was detected by ultrasound. On TDM and MRI, the tumor was 3cm in diameter and partially microcystic. A needle biopsy (slide 1) and tumorectomy (slide 2) were performed. Case 7 - Slide 1 Click to view with ImageScope Click to view with a Web-Based Viewer Case 7 - Slide 2 Click to view with ImageScope Click to view with a Web-Based Viewer Pathological Findings: On the biopsy, small or medium sized tubulo-cystic spaces were distributed in a fibrotic stroma. The tubules were lined by flattened or cuboidal eosinophilic cells, with occasional hobnail features and separated by delicate septa. Nuclei were regular, round or oval with some prominent nucleoli. The diagnosis of tubulocystic carcinoma was proposed and a partial nephrectomy of the left kidney was performed. Macroscopically the tumor appeared not to be encapsulated but was well demarcated from the normal parenchyma with a spongy appearance. Microscopically the histology of the resected tumor was similar to that seen in the biopsy. Diagnosis: Tubulocystic carcinoma of the kidney Discussion: Tubulocystic carcinoma (TCCa) of the kidney is a recently described entity [1], and as such was not included in the latest WHO 2004 classification [2]. It is a rare tumor which has been described under different names during recent decades. Pierre Masson illustrated it as : "epithelioma bellinien" in 1956 [3]. In the 3rd AFIP series published in 1994, renal cell carcinomas collecting duct types were illustrated either by high grade solid tumors or by multicystic tumors. In the latter, dilated tubules were lined by regular cuboidal or hobnail cells [4], as in our case. This tumor has since been considered to be a low grade collecting duct carcinoma by McLennan et al. [5], Amin et al. [6] and Srigley et al. [7]. These authors proposed an "evolving concept" of collecting duct carcinomas, with low-grade cystic carcinoma possibly of collecting duct origin at the low end of the spectrum. Farah et al. recently added a well documented case of such a tumor [8]. At the 93th USCAP meeting (Vancouver, 2004), Amin et al reported a series of 29 similar tumors and renamed them "tubulocystic carcinoma of the kidney " [9]. In their series, the tumor had a "bubble-wrap" appearance on the cut surface and a remarkable uniformity on macroscopic and microscopic aspects, characterized by a complex and multicystic pattern. All cases were unencapsulated and composed of tubules of variable size and of micro/macro cysts lined by cuboidal to flattened eosinophilic epithelial cells. Some areas contained cells with hobnail features. Nuclei often showed small visible nucleoli. Immunohistochemistry showed positivity for CK8, CK18, CK19, CD10, P504S ; some tumors were positive for CK7 and a few were positive for CK34 bE12. These tumors occured predominantly in men (6:1) and demonstrated a very low rate of recurrence or metastases (2/29). Of interest, the two metastatic tumors displayed areas of clear cells. Four cases were added to the original series of 29 and a number of these were analyzed at the ultrastructural and immunohistochemical methods [10]. The cells displayed characteristics of proximal convoluted tubule cells (brush border, expression of CD10 and P504S) and of intercalated cells of the collecting duct (short microvilli, cytoplasmic interdigitation, expression of CK19 and parvalbumin), thereby demonstrating the absence of an exclusive relationship with collecting duct or Bellini duct tumors. At the end of 2005, McLennan and Bostwick, re-evaluated their first series of 13 cases published in 1997 and wrote an extensive review on "recently described rare renal tumors" [1]. They divided these into two separate entities : 5 cases of mucinous tubular and spindle cell carcinoma composed of anastomosing tubular structures with mucin and 8 cases of tubulocystic carcinoma composed of cystic tubules, separated by a fibrotic stroma. Two main differential diagnoses for cystic renal neoplasms must be considered [11]. Cystic nephroma (CN) is a benign lesion occurring after the age of 30, mainly in females (8:1). Macroscopically the tumor is unique, encapsulated entirely composed of multiple cysts. Cysts are usually large and lined by a single layered cuboidal or flat epithelium of eosinophilic cells with or without "hobnail" morphology. Neither tubules, nor epithelial cells are seen in the septae. The stromal part is variable : usually fibrotic, it can be focally more cellular and often has an ovarian-like appearance [12]. This latter fact is classically considered to be a useful feature for the differential diagnosis. Some authors suggest that mixed epithelial stromal tumor may correspond to a solid type cystic nephroma and [13]. TCCa and CN have overlapping morphologic features. Both tumors are composed of cysts lined by a single cell layer and separated by a fibrotic stroma, although in TCCa the cysts are smaller than in cystic nephroma and the septae are fibrotic and delicate without ovarian-type stroma. These findings raise the possibility that these tumors belong to the same spectrum of low grade cystic renal tumors, though it is noteworthy that tubulocystic carcinoma occur more often in men and cystic nephroma in female. Multilocular cystic renal cell carcinoma (RCC) is surrounded by a fibrous pseudocapsule, made of non communicating cysts separated by thick fibrous and hyalinized septae [14]. Nests of clear cells are found in septae. The cysts are usually lined by one or more layers of clear cells. It is a subtype of clear cell RCC which is now recognized as a new low grade entity with very good prognosis in the last WHO classification [2]. In conclusion tubulocystic carcinoma of the kidney is a distinctive, newly recognized subtype of renal cell carcinoma which appears to be have a relatively good prognosis although metastatic behaviour may occasionally be seen. The morphological similarities between cystic nephroma and tubulocystic carcinoma allow us to speculate that they may belong to the same spectrum of cystic renal neoplams. References: MacLennan, G.T. and D.G. Bostwick, Tubulocystic carcinoma, mucinous tubular and spindle cell carcinoma, and other recently described rare renal tumors. 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