Paranasopharyngeal space involvement in nasopharyngeal cancer: detection by CT and MRI

Paranasopharyngeal tumour extension (PTE) from nasopharyngeal carcinoma (NPC) is staged in its own subgroup in the American Joint Committee on Cancer classification. Most large clinical trials use computed tomography (CT) to stage PTE, but diagnosis relies on indirect signs of tumour invasion such as asymmetry of the parapharyngeal fat. Magnetic resonance imaging (MRI) has the advantage of directly revealing PTE because of its ability to depict the complex anatomical structures that form the boundary of the nasopharynx. The aim of this study was to compare CT and MRI in the identification of PTE and to determine whether the imaging modality used influenced staging of the disease. The MRI and CT scans of 78 patients (156 parapharyngeal regions) with NPC were assessed for PTE. On MRI, PTE was considered to be positive when there was tumour invasion through the complex anatomical structures of the nasopharyngeal wall. When using CT, it was considered positive when there was: (1) distortion of the parapharyngeal fat plane; or (2) extension beyond a line drawn from the medial pterygoid plate to the lateral aspect of the carotid artery. CT scanning and MRI were compared. PTE was judged to be present in 28 of 78 (36%) patients by MRI and in 41 of 78 (53%) scanning by CT when using criterion 1 or 2. An analysis of the discordant findings revealed that MRI was positive in three sides of the nasopharynx in early tumour extension through the pharyngobasilar fascia but not identified with CT by using criterion 1 or 2. MRI was negative in 20 and 21 sides of the nasopharynx that were judged to be positive on CT by using criterion 1 and 2 respectively. In these patients MRI revealed that the positive CT scan was caused by a large tumour compressing but not invading the parapharyngeal fat space, a metastatic lateral retropharyngeal node, or a combination of the two. The imaging modality used for staging NPC has an impact on the staging of PTE. CT scanning suggested the presence of PTE more frequently than MRI because of its inability to distinguish the primary tumour from lateral retropharyngeal nodes, and direct tumour invasion of the parapharyngeal region from tumour compression. The imaging modality and criteria used for staging PTE should be taken into consideration when assessing the results of clinical studies.