Medical physics 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 Giovanni Mauro Cattaneo a, * , Michele Reni b , Giovanna Rizzo c , Pietro Castellone d , Giovanni Luca Ceresoli b , Cesare Cozzarini b , Andre ´s Jose ´ Maria Ferreri b , Paolo Passoni b , Riccardo Calandrino a a Medical Physics, Scientific Institute, H.S. Raffaele, Milano, Italy, b Radiochemotherapy, Scientific Institute, H.S. Raffaele, Milano, Italy, c IBFM-CNR, Nuclear Medicine, Scientific Institute, H.S. Raffaele, Milano, Italy, d Physics Department, University of Napoli Federico II8, Napoli, Italy Abstract Background and purpose: To investigate the interobserver variability of intracranial tumour delineation on computed tomography (CT) scans using pre-operative MR hardcopies (CTCMR(conv)) or CT-MR (pre-operative) registered images (CTCMR(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 (CTCMR(conv)); they also outlined CTV0 with both CT and corresponding MR axial image on screen (CTCMR(matched)). The accuracy of the image fusion was quantitatively assessed. An analysis was conducted to assess the variability among the five observers in CTCMR(conv) and CTCMR(matched) modality. Results: The registration accuracy in 3D space is always less than 3.7 mm. The concordance index was significantly better in CTCMR(matched) (47.4G12.4%) than in CTCMR(conv) (14.1G12.7%) modality (P!0.02). The intersecting volumes represent 67G15 and 24G18% of the patient mean volume for CTCMR(matched) and CTCMR(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. q 2005 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 75 (2005) 217–223. Keywords: Interobserver variability; Image fusion; High-grade glioma; Post-operative radiotherapy After the introduction of the ICRU terminology [ICRU 50 and 62; 12,13], attention to the evaluation of uncertainties related to volume delineation, patient set-up on treatment machine and organ motion during radiation treatment planning and delivery increased greatly. Many studies have analysed geometrical errors in patient repositioning and the impact of organ motion, but only in recent years greater attention has been focused on the target volume delineation (gross tumour volume (GTV) and clinical target volume (CTV)) which in many tumours is considered the most important factor contributing to the global uncertainty in planning definition. Several works have reported on observer variability when prostate/seminal vesicles are delineated [6,8,15,26,36]; several authors have analysed observer variability in case of lung [4,7,9,22,32,33,38,39] and brain tumours [19], and a very few works have dealt with other diseases such as breast, head and neck, bladder and cervix neoplasms [11,21,23,27, 31,34,37,40]. Larger variability in CTV is also expected in patient candidates for post-operative radiotherapy (RT), even if limited literature is available [10,35]. In the case of high grade gliomas (HGG), many authors have indicated that MR may be necessary for volume delineation during RT planning; as a review see Jansen et al. [14]. The MR-based volume was usually larger than the CT-based one, while inter-observer variability remains large; the ability of co-registered CT–MR images to reduce observer variability has not yet been fully demonstrated [1,16,24,27,35,41]. 0167-8140/$ - see front matter q 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2005.03.012 Radiotherapy and Oncology 75 (2005) 217–223 www.elsevier.com/locate/radonline