Bilateral atrophic kidney‐like tumors

To the Editor: Hes et al. recently reported three cases of renal cell tumor microscopically simulating atrophic kidney as a distinct entity. The tumors are characterized by grossly sponge-like appearance, microscopically the proliferation of microfollicles without atypia, two types of calcification, capsular formation containing smooth muscle, and prognostically benign in nature. We present here one young adult patient with tumors entirely identical to the previously reported cases, arising in bilateral kidneys without any family history. Bilateral renal tumors are found in 1 to 4% of reported sporadic renal tumor cases, and pathologists should bear in mind that this unique renal neoplasm may occur as bilateral tumors. Bilateral renal masses were incidentally discovered in a 19-year-old male during a medical workup for slight liver dysfunction. No physical or laboratory examination revealed any abnormal findings. No family history was identified. An abdominal computed tomography scan demonstrated homogeneous masses on both kidneys. Left partial nephrectomy and right tumor excision were performed. The patient has survived without recurrence or metastasis for 13 years since surgery. Grossly, the tumor of the left kidney measured 44 × 40 mm and that on the right 42 × 32 mm. Both tumors were well encapsulated, brown-colored, and had numerous minute cysts with sponge-like appearances (Fig. 1a). The surgical margins were negative in both tumors. Histologically, both tumors showed the very same appearances and were composed of small follicles variably dilated and forming microcystic structures (Fig. 1b). The follicles were filled with pale-staining eosinophilic secretion. The overall appearance at the low-power magnification showed remarkable similarity to atrophic kidney. The tiny follicles occasionally were packed together, giving an appearance of solid sheets under lowmagnification. The lining cells of the follicles had scant cytoplasm with darkly stained ovoid nuclei and indistinct nucleoli. Papillary infoldings within the follicles were occasionally observed, and the core of the papillae sometimes revealed calcifications resulting in psammoma bodies (Fig. 2a). The detached isolated tumor cells, floating in the colloid materials, resembled plasma cells (Fig. 2b). No mitoses and atypias were detected. Calcifications were often observed, and two types of calcification could be recognized (Fig. 2c). One type of calcification was that of the typical psammoma body mentioned above. The other one consisted of amorphous darkblue stained calcified deposits within the secreted materials. Muscular vessels were sparsely distributed in the tumor. Smooth muscle bundles are observed within the thick capsule, and spread into the stroma. No hemorrhage and necrosis were observed. Non-tumorous renal parenchyma in the left partial nephrectomy specimen showed no apparent abnormalities. Immunohistochemically, the tumor cells showed the labeling for vimentin, WT1, CD10, cytokeratin OSCAR (Fig. 2d), cytokeratin 7, PAX2, PAX8, and showed negativity for AMACR, CD57, Cathepsin-K, TTF-1, CA9 and RCC Ma. OSCAR corresponds to cytokeratins 7, 8, 18, 19 and its immunohistochemistry highlighted detached epithelial cells floating into follicular structures (Fig. 2d), while CK7 reactivity was restricted the lining cells of the small follicles. Desmin and alpha-smooth muscle actin were stained on the muscle bundles in the capsule, vessel wall, and sparsely in the stroma. One of the characteristic findings of this tumor is calcifications. Psammoma bodies are said to represent a process of dystrophic calcification. It is reported that it begins with thicka