Diffusion-weighted MR imaging of ameloblastomas and keratocystic odontogenic tumors: differentiation by apparent diffusion coefficients of cystic lesions

BACKGROUND AND PURPOSE: Ameloblastomas and keratocystic odontogenic tumors are major aggressive odontogenic tumors in the maxillomandibular regions, but the differentiation between these 2 tumors is frequently ineffective based on only conventional CT and MR imaging findings. Here, we evaluated diffusion-weighted MR imaging for the differentiation of these 2 odontogenic tumors.MATERIALS AND METHODS: We prospectively studied 9 patients with ameloblastoma and 7 patients with keratocystic odontogenic tumor using diffusion-weighted MR imaging. Apparent diffusion coefficients (ADCs) of the nonenhancing and solid lesions in these tumors were determined with use of 2 b factors (500 and 1000).RESULTS: Two types of nonenhancing lesions were identified; one with high signal intensity on fat-suppressed T2-weighted images (type A) and the other with low or intermediate intensity (type B). The type A nonenhancing lesions were observed in all the ameloblastomas, but they were evident in only 2 keratocystic odontogenic tumors. It is interesting to note that the ADCs of the nonenhancing lesions in the ameloblastomas were significantly higher than those of the nonenhancing lesions in the keratocystic odontogenic tumors (2.48 ± 0.20 × 10⁻³ mm²/s vs 1.13 ± 0.56 × 10⁻³ mm²/s; P < .001). The ADCs of the solid lesions in the ameloblastomas (1.39 ± 0.15 × 10⁻³ mm²/s) were significantly lower than those of the nonenhancing lesions in the ameloblastomas and were similar to those of the nonenhancing lesions in the keratocystic odontogenic tumors.CONCLUSION: ADC determination may be used as an adjunct tool for differentiation between ameloblastomas and keratocystic odontogenic tumors.

A meloblastomas and keratocystic odontogenic tumors are major aggressive odontogenic tumors in the maxillomandibular regions. Both tumors display common radiologic features such as unicystic or multicystic lesions with extensive thinning and expansion of the overlying cortex. They are locally invasive and prone to recur after inappropriate surgical excision.¹ Furthermore, the effective preoperative differential diagnosis between these 2 tumors could help surgeons plan treatment because ameloblastomas require resection and keratocystic odontogenic tumors can be treated with enucleation.Radiologic differentiation between these tumors is not difficult if the ameloblastoma is of the solid and multicystic type and the keratocystic odontogenic tumor is of the unilocular type. However, substantial ameloblastomas are unicystic, and some keratocystic odontogenic tumors are multicystic or multilobular with scalloped margins, thereby resembling ameloblastomas.Several attempts have been investigated to differentiate between ameloblastomas and keratocystic odontogenic tumors with use of CT and MR imaging.^2–5 In this study, we further characterize these 2 aggressive benign tumors in the maxillomandibular regions on the basis of the diffusion-weighted MR imaging features, with particular emphasis on the apparent diffusion coefficients (ADCs) of the nonenhancing lesions of these tumors.