Delineation of the target is one of the main remaining error sources in conformal radiation therapy [1, 2]. By the nature of the procedure, delineation errors are systematic in external beam radiotherapy. Any deviation remains the same throughout the radiation course, which results in reproducible dose differences. Earlier reports on ethmoidal and maxillary sinus and nasopharyngeal tumors demonstrated a dose dependency of both observer variation and irradiation technique. Despite improvements in the latter, the impact of delineation variation remains large with regard to impact on dose to the target and to the other organs at risk [2, 3].
Several efforts have been undertaken to decrease observer variation. Two main topics can be distinguished:
Guidelines for delineation
Early guidelines for delineation of the neck levels were published by Som et al. Shortly thereafter, Robbins, Nowak and Gregoire et al published guidelines on the same topic. Currently, more than five different guidelines for delineation of the neck have been published in the international literature [1, 4–9].
In an effort to reach consensus, Gregoire et al published consensus guidelines for neck delineation on behalf of the Radiation Therapy Oncology Group (RTOG) and European Organization for Research and Treatment of Cancer (EORTC) . Although validation of these guidelines is still to be performed, they more than likely improve delineation agreement of the elective neck nodes. However, guidelines for the delineation of primary tumors of head and neck cancers are scarce.
Computer Tomography (CT) based delineation is the current standard of practice for conformal radiotherapy, although other imaging modalities like Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) have also proven their value in several tumor sites [6, 11–13]. Notably, a study comparing CT, MRI, PET and pathological specimen based Gross tumor Volume (GTV) determination for larynx carcinomas demonstrated that MRI was more accurate than CT and PET and was even closer to the pathological specimen measurements that are regarded as the "gold standard" . The addition of MRI to CT decreases observer variation and leads to smaller Gross Tumor Volumes, as seen in this study and others concerning this topic [2, 6, 11, 14]. For example, the addition of PET to lung cancer observation considerably decreased the delineation variation [13, 15–17]. This was demonstrated in a multiobserver study performed by Steenbakkers et al , in which the addition of PET to CT-based delineation particularly decreased observer variation at the interfaces towards mediastinum and hilum and in the case of atelectasis. Furthermore, the use of sagittal or coronal reconstructions during the delineation led to more agreement .
18 Fludeoxyglucose-positron Emission Tomograpy (FDG-PET) Imaging for Head and Neck provides functional information on the extent of the tumor [18–21]. However, its main strength lies in the detection of involved regions or (lymph node) metastasis, with an overall sensitivity of 79% [18, 22]. For precise delineation of the tumor extent itself, however, it is less suitable. This is due to poorer spatial resolution, the lack of a universal threshold uptake value, and a large uptake in the brain tissue close to the primary tumor when invasion towards bone or parapharyngeal regions is suspected (i.e., when the delineation becomes difficult) [18, 23]. MRI was superior to FDG-PET for showing the extent of the primary tumor in 54 nasopharyngeal cases.
The addition of MRI to CT-based delineation has proven its value in the delineation of the Head and Neck region and has resulted in smaller target volumes [2, 6, 11, 14, 25, 26]. Especially when posterior invasion is suspected, MRI has proven to be superior to CT based staging . However, the effect on observer variation is limited [6, 11].
The above-mentioned studies for the Head and Neck have defined the variation in various modalities, but have not attempted to determine measures for decreasing this variation within the study, or measure the impact of any measures taken. It is the aim of the present study to determine the extent of baseline variation, to analyze the results, and then to take measures including improved delineation guidelines, multi-modality imaging, and delineation tools targeted at the specific variations found to reduce this variation. The impact of these measures will then be assessed by re-delineation.