Peritumoral Cortex Low-Enhancement Sign on Corticomedullary Phase CT: A Distinctive Indicator of Small Renal Malignancies.

Objective: To retrospectively evaluate the diagnostic potential of the peritumoral cortex low-enhancement (PCLE) sign on corticomedullary phase (CMP) CT images for differentiating malignant from benign lesions and clear cell renal cell carcinoma (ccRCC) from non-ccRCC among small renal masses (SRMs, ≤4 cm).

Materials and methods: After excluding cases with incomplete/poor-quality CT images and SRMs exhibiting cystic changes, visible fat, or infiltrative/completely endophytic/exophytic growth patterns, 603 histopathologically confirmed SRMs (507 malignant and 96 benign; 409 ccRCC and 194 non-ccRCC) from 595 patients across three institutions were retrospectively analyzed using thin-slice CT images (≤1.5 mm). PCLE was defined as a focal low-enhancement region of the peritumoral cortex at the tumor-cortex interface on CMP CT images, similar to the early dark cortical band sign. Diagnostic performance of PCLE for malignancy and ccRCC was evaluated. A separate cohort of 109 SRMs (91 malignant and 18 benign; 77 ccRCCs and 32 non-ccRCCs) from 108 patients, imaged with thick-slice CT (3 mm), was additionally analyzed using otherwise the same methodology.

Results: PCLE was identified in 331 SRMs (54.9%, 331/603), including 326 malignant and 5 benign SRMs (all oncocytomas >3 cm), and in 307 ccRCC and 24 non-ccRCC SRMs. For diagnosing malignant SRMs, PCLE demonstrated a sensitivity of 64.3% (326/507) and specificity of 94.8% (91/96). For ccRCCs, these values were 75.1% (307/409) and 87.6% (170/194), respectively. In tumors ≤3 cm, specificity reached 100% (78/78) for malignancies and 93.5% (129/138) for ccRCCs. Inter-observer agreement for PCLE was substantial (Cohen's kappa, 0.783). Findings from the thick-slice cohort closely mirrored those in the primary cohort obtained using thin-slice CT images.

Conclusion: PCLE observed on CMP CT demonstrated exceptional specificity for malignant and ccRCC among SRMs, especially in SRMs ≤3 cm, though sensitivity was limited, and false positives occurred in oncocytomas.