Clear cell papillary renal cell carcinoma (CCPRCC) is a recently recognized subtype of renal cell carcinoma entity after 2004 World Health Organization classification of renal tumors. CCPRCC has unique histomorphological and immunohistochemical characteristics. The distinction of CCPRCC from renal cell carcinoma (RCC) with clear cell morphology is crucial because the former is considered to have a favorable clinical outcome. CCPRCC may be interpreted in the past as other renal cell carcinomas, particularly low-grade clear cell RCC. In this study, the frequency of CCPRCC in previously diagnosed low-grade RCC and its clinicopathologic features were examined. A total of 126 cases of stage T1a with low nuclear grade RCC were identified from 625 consecutive RCCs removed by radical/partial nephrectomy over 12-year period (2000-2011). Archival tissue sections were retrospectively reviewed along with patient medical charts. Eight cases (1.3% of all RCC, 6.3% of pT1a low grade RCC) with characteristic histologic features of CCPRCC were confirmed by immunohistochemical studies. Seven cases were previously diagnosed as clear cell RCC and one as multilocular cystic RCC. Radiographically, CCPRCC favored a mid-pole location in the kidneys. At a median follow-up period of 52 months (range 20-114.5 months), there were no cases of local or distant recurrence. In conclusion, CCPRCC is not uncommon among small low-grade RCC tumors. CCPRCC can be correctly recognized by its unique histomorphological features and confirmed by immunohistochemistry studies, which is important due to the excellent clinical outcome following resection.

Cerebrospinal fluid (CSF) cytology provides valuable diagnostic and prognostic information for diseases of the central nervous system (CNS) and remains the gold standard for the detection of neoplastic meningitis. Metastatic involvement of the CSF by non-CNS neoplasms far surpasses that of primary brain tumors, although conventional glioblastoma multiforme (GBM) can occasionally be identified in the CSF. GBM with epithelial differentiation is an uncommon variant that may contain features such as adenoid structures, signet ring cells, or squamous metaplasia. Herein, we present a case of GBM with epithelial differentiation to highlight a potential diagnostic pitfall in CSF cytology. A 55-year-old man presented with neurological symptoms and a 6.4 cm left temporal lobe cystic mass. Primary resection revealed GBM with focal epithelial differentiation confirmed by cytokeratin, epithelial membrane antigen, and glial fibrillary acidic protein immunohistochemical studies. Four months following primary resection, the patient developed severe headache for which a lumbar puncture with CSF cytologic evaluation was performed. The cytospin preparation showed numerous malignant epithelioid cells with high nuclear-cytoplasmic ratio and prominent cytoplasmic vacuoles resembling metastatic carcinoma. However, the lesional cells were cytomorphologically identical to the epithelial component present in the patient's recently diagnosed GBM. This case illustrates the potential for GBM with epithelial differentiation to closely mimic metastatic carcinoma from a non-CNS site in CSF cytology, which expands the differential diagnosis and emphasizes the necessity of clinical correlation.

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