I saw the sign: The black ring sign in carcinoma ex pleomorphic adenoma

Carcinoma ex pleomorphic adenoma (CA-ex-PA) is a rare salivary gland malignancy that may be difficult to diagnose preoperatively because of marked imaging heterogeneity. We present two cases of CA-ex-PA arising in the left submandibular and left parotid glands. On T2-weighted MRI, both tumors demonstrated the black ring sign, defined as a conspicuously thick hypointense rim surrounding an intratumoral component. The maximal rim thickness was 2.9 mm in Case 1 and 2.4 mm in Case 2. Mean apparent diffusion coefficient values were 1.33 × 10⁻³ mm²/s and 1.20 × 10⁻³ mm²/s, respectively. The T2-hypointense rim is thought to correspond histopathologically to fibrotic and hyalinized stromal changes associated with the preexisting pleomorphic adenoma component. The corona sign was absent in both cases despite pathologically confirmed invasive disease. These findings suggest that the black ring sign may support the preoperative diagnosis of CA-ex-PA when interpreted together with tumor morphology, diffusion characteristics, clinical findings, and radiologic-pathologic correlation. However, it should be regarded as a supportive rather than pathognomonic imaging feature.