Intracellular lipid in papillary renal cell carcinoma (pRCC): T2 weighted (T2W) MRI and pathologic correlation.

OBJECTIVE: To evaluate if pRCCs demonstrate intracellular lipid (i-lipid) at chemical-shift (CS) MRI, and assess T2W-MRI and pathologic characteristics.
METHODOLOGY: Sixty-two patients with a pRCC diagnosis underwent MRI over 11 years (IRB-approved). Two radiologists independently assessed for presence of i-lipid on CS-MRI and homogeneity on T2W-MRI. Inter-observer agreement was assessed via an intraclass correlation and results were compared using the Chi-square test. Discordant cases were reviewed to establish consensus. T2W SI-ratios (SI.tumor/SI.kidney) and CS-SI index were compared using independent t-tests and Spearman correlation. Two pathologists re-evaluated the histopathology.
RESULTS: Nine of the 62 pRCCs (14.5%) demonstrated i-lipid; agreement was moderate (ICC = 0.63). Pathology review depicted clear cells in four tumours and foamy histiocytes in five tumours. 25.8-35.4% (ICC = 0.65) of tumours were homogeneous on T2W-MRI. No pRCC with i-lipid was considered homogeneous (p = 0.01-0.04). Overall, T2W SI-ratio and CS-SI index were 0.89 (±0.29) and -3.63 % (-7.27 to 11.42). pRCC with i-lipid had significantly higher T2W SI-ratio (p = 0.003). There was a correlation between the CS-SI index and T2W SI-ratio, (r = 0.44, p < 0.001).
CONCLUSIONS: Intracellular lipid is uncommonly detected in pRCCs due to clear cell changes and foamy histiocytes. These tumours are associated with heterogeneously-increased SI in T2W-MRI. KEY POINTS: • A minority of pRCCs demonstrate intracellular lipid in CS-MRI. • Quantitatively, intracellular lipid in pRCCs is minimal (<25%). • Intracellular lipid in pRCCs are from clear cell heterogeneity or foamy histiocytes. • pRCCs with intracellular lipid are heterogeneously hyperintense at T2W-MRI. • pRCCs that are homogeneously hypointense at T2W-MRI do not contain intracellular lipid.