A comparison of clinical target volumes determined by CT and MRI for the radiotherapy planning of base of skull meningiomas

Abstract

Purpose: To assess the utility of image registration and to compare the localization of clinical target volumes (CTV) using CT and MRI for patients with base of skull meningiomas undergoing radiotherapy. Methods and Materials: Seven patients were imaged using CT and a T1-weighted MR volumetric sequence. Following image registration using a chamfer-matching algorithm, transaxial MR slices were reconstructed to match the planning CT slices. The accuracy of the image fusion was assessed in a preliminary study with matching accuracy better than 1.5 mm. The CTV in each patient was separately segmented by two independent observers for both CT and reconstructed MR image sets. Scalar and vector assessments were made of the difference in radial extent between the two outlines on each transaxial plane for all patients. A positive vector value corresponded to a greater extension of the tumor on MR compared to CT and vice versa. Scalar measurements compared the modulus of the differences between MR and CT, regardless of which volume was more extensive. Qualitative comparisons were also performed. Results: Interobserver difference was small with a mean (± 1SD) volume difference of 1.5 ± 1.5 cm 3 for CT and 0.5 ± 1.0 cm 3 for MRI. The mean CT- and MR- CTVs were 17.6 ±10.8 and 19.6 ±14.2 cm 3 respectively. The mean overlap and composite volumes were 13.8 ±10.1 and 23.3 ±14.8 cm 3 respectively. Average scalar differences in the left, right, anterior, and posterior directions were 6.0 ± 7.0, 3.3 ± 2.5, 4.9 ± 3.9, and 4.5 ± 5.0 mm respectively. The average vector differences were 3.3 ± 8.5, −0.3 ± 3.8, 1.1 ± 5.8, 1.5 ± 6.4 mm (for left, right, anterior, and posterior directions respectively). Qualitatively, MR appeared to discern more tumor involvement in soft tissue regions adjacent to the skull base whereas CT appeared to provide larger target volumes within bony regions. Conclusions: MRI appeared to define CTVs that were larger but not inclusive of CT-defined CTVs. Although the average vector differences were small, the differences on individual borders could be large. In some instances, the CT or MR volumes were vastly different, each providing separate information. Therefore, the use of MRI and CT is complementary. Until accurate histological confirmation of disease extent is available, it is prudent to consider composite CT/MR volumes for the radiotherapy planning of base of skull meningiomas.