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 (CTV0) 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 CTV0 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.