Intramedullary ependymomas are classically described as tumors developing in the center of the spinal cord, whereas intramedullary astrocytomas are more dispersed. This description only remains approximate. The aim of this study was to establish a lateralization index (LI), which is based on radiological measurements and may help in differentiating the two tumors.

Retrospective analysis based on the analysis of MRI of patients operated on for intramedullary astrocytoma or ependymoma in the Neurosurgical Department of Bicêtre Hospital. We consider the distance between the center of the spinal cord and the lateral edge of the tumor, this measurement is called a. Measurement b is the maximal axial diameter of the tumor, measured on the same axis of a. Measurements are made on T1-weighted axial MR images after gadolinium administration. The LI is calculated by the ratio a/b.

Twenty-one patients were included in the study, 11 astrocytomas and 10 ependymomas. The average LI was 0.7425 (0.5611 to 1.1673) for astrocytomas and 0.5462 (0.5276 to 0.5995) for ependymomas. The difference of LI between the two types of tumors was statistically significant (P<0.05).

This study confirms that for ependymomas the LI is close to 0.5, which is the value of a completely centered tumor, unlike astrocytomas. The LI can be an additional tool in helping to differentiate the two types of tumors on the preoperative imagery.