Computed Tomography versus Magnetic Resonance Image in Detection of Thyroid Cartilage Invasion in Laryngeal Carcinoma.

Abstract

Laryngeal cartilage invasion is of great importance in the staging of laryngeal carcinoma according to TNM classification. Tumor invasion through the inner cortex of the thyroid is considered a T3 tumor, whereas tumor invasion through the outer cortex of the thyroid or cricoid is a T4 tumor. The presence or absence of cartilaginous invasion is an important data point for therapeutic decision making, and the presence of cartilaginous invasion often justifies extensive surgery that might not be undertaken otherwise. Because the cartilaginous structures are largely inaccessible to clinical examination, assessment of the possibility of cartilaginous invasion before treatment is the responsibility of the radiologist. Unfortunately, the imaging characteristics of hyaline cartilage are variable. Ossification of the cartilaginous structures of the larynx increases with age but varies and may be asymmetric. Whereas ossified cartilage demonstrates high attenuation on CT at the outer and inner cortices with relatively lower attenuation, medullary space nonossified cartilage demonstrates attenuation similar to that of other soft tissue structures. On MRI, the cortical signal depends on the presence or absence of calcification, and the fatty medullary signal demonstrates T1 hyperintensity with intermediate signal on T2-weighted images. This variability can complicate imaging evaluation because of corresponding inhomogeneity of density and signal on CT and MRI, respectively, presenting a challenge to noninvasive evaluation. Further complicating matters, reactive changes within cartilage may occur without the presence of neoplastic invasion, resulting in overestimation of disease extent, particularly with MRI. Regardless, contrast enhancement is not a normal feature of cartilage on MRI or CT. Either technique should be able to aid in the differentiation of inner cortical invasion (T3) from involvement of inner and outer cortices and extralaryngeal spread (T4). Apparently CT findings cause an underestimation as its sensitivity is low, whereas MRI findings produce an overestimation of the actual presence of cartilage invasion as its specificity is low. The choice between CT and MRI may partly be settled by the clinical question. If it is important to exclude cartilage invasion, as in considerations regarding partial laryngectomy, MR imaging may be indicated. If cartilage invasion should be shown with more confidence, CT may be more appropriate.