Target delineation in post-operative radiotherapy of brain gliomas: Interobserver variability and impact of image registration of MR (pre-operative) images on treatment planning CT scans

Background and purpose: To investigate the interobserver variability of intracranial tumour delineation on computed tomography (CT) scans using pre-operative MR hardcopies (CT+MR(conv)) or CT-MR (pre-operative) registered images (CT+MR(matched)). Patients and methods: Five physicians outlined the 'initial' clinical tumour volume (CTVO) of seven patients affected by HGG and candidates for radiotherapy (RT) after radical resection. The observers performed on screen-tumour delineation using post-operative CT images of the patients in the treatment position and pre-operative MR radiographs (CT+MR(conv)); they also outlined CTVO with both CT and corresponding MR axial image on screen (CT+MR(matched)). The accuracy of the image fusion was quantitatively assessed. An analysis was conducted to assess the variability among the five observers in CT+MR(conv) and CT+MR(matched) modality. Results: The registration accuracy in 3D space is always less than 3.7 mm. The concordance index was significantly better in CT+MR(matched) (47.4 +/- 12.4%) than in CT+MR(conv) (14.1 +/- 12.7%) modality (P < 0.02). The intersecting volumes represent 67 +/- 15 and 24 +/- 18% of the patient mean volume for CT+MR(matched) and CT+MR(conv), respectively (P < 0.02). Conclusions: The use of CT and MR registered imaging reduces interobserver variability in target volume delineation for post-operative irradiation of HGG; smaller margins around target volume could be adopted in defining irradiation technique. (c) 2005 Elsevier Ireland Ltd. All rights reserved.