Typically, PRCC tends to appear homogeneous while clear cell RCC is likely to be in a heterogeneous form when the tumor is less than 3 cm in diameter. Comparatively, in cases of tumors larger than 3 cm in diameter, PRCC is generally heterogeneous with areas of necrosis and hemorrhage compared to chromophobe RCC. Solid, small PRCC tumors (<3 cm in diameter) are more easily viewed on nephrographic, excretory phase images rather than on unenhanced, corticomedullary phase images. Magnetic resonance imaging (MRI)
Magnetic resonance imaging (MRI) is recommended instead of CT for patients with an allergy to iodinated contrast materials. As some renal tumors do not enhance significantly on CT, MRI examination is required to be performed with more sensitive contrast enhancement. On MRI, the distinct features of PRCC are fibrous capsules and homogeneously low single intensity on both T1- and T2-weighted images. Specifically, PRCC exhibits hypointensity due to its dense collagenous matrix or deposition of calcium and hemosiderin within the tumor. Such visual features help PRCC to be differentiated from clear cell RCC, which has heterogeneously higher single intensity shown on T2-weighted images. PRCC displays the smallest tumor-to-cortex enhancement at corticomedullary and nephrographic phases when juxtaposed with clear cell and chromophobe RCCs. Grading System
The WHO/ISUP system is histological tumor grading system for renal cell carcinoma, suggested by the International Society of Urologic Pathologists (ISUP) in 2012 to diagnose tumor grades based on nucleolar prominence. Currently recommended by the WHO, this four-tiered WHO/ISUP grading system has also been validated for PRCC.
Pathogenesis
Different molecular mechanisms are involved in PRCC development, which further result in distinct histologic features and clinical outcomes.Type 1 PRCC is caused by a genetic mutation or a gain in chromosome 7 where the MET gene is positioned, resulting in the promotion of oncogenic pathways in renal epithelial cells. Typically, the MET gene is upregulated for renal tissue repair and regeneration by encoding the receptor tyrosine kinase c-MET of hepatocyte growth factor. However, activation of the oncogenic pathway in the MET gene will manifest invasion, anti-apoptosis, angiogenesis, and metastasis.Type 2 PRCC is associated with irregularity of several signaling pathways, which includes CDKN2A silencing, mutation in chromatin-modifying genes, and a GpG island methylator phenotype (CIMP). CDKN2A is a tumor suppressor gene, while loss of its expression results in enhanced tumorigenesis and metastasis. Moreover, mutation of gene involved in chromatin remodeling (SETD2, BAP1, or PBRM) may lead to higher rate of TFE3/TFEB fusion. Additionally, CIMP papillary renal cell carcinoma tumors exhibited somatic FH gene mutation, which is closely associated with HLRCC syndrome. Diagnosis
Currently, cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging (MRI) is known as the best option for diagnosing papillary renal tumors. Computed tomography (CT)
Contrast-enhanced computed tomography (CT) is most commonly used to identify the subtypes of RCC. PRCC can be differentiated from other types of RCC due to its distinguishing features, displaying a small hypovascular renal tumor on T2 weighted images.