the skull base can be infiltrated by adjacent head and neck tumours such as nasopharyngeal tumours , parotid cancers and malignancies originating in the sinonasal area . it has often been mentioned that mri is the modality of choice in delineating the soft - tissue component of a malignant neoplasm . however , the erosion of the bony structures of the skull base is best achieved by using high - resolution thin section ct . this belief holds for osseous structures that are thin and predominantly composed of cortical bone . however , for the bones forming the central skull base such as the clivus , body of the sphenoid bone , the pterygoid process and the petrous apex , mri often shows early changes more convincingly [ 1 , 2 ] . mri easily detects the replacement of high - signal intensity fat by intermediate signal intensity neoplasm on t1-weighted images . however , it is not always possible to separate tumour involvement from inflammatory reaction induced by the presence of tumour tissue as experience in imaging laryngeal neoplasms has shown [ 3 , 4 ] . mri is increasingly used as the first modality in the assessment of the skull base . when a lesion arising from the skull base is first detected , one should always remember the possibility of localised fibrous dysplasia . unfortunately , the mri characteristics of fibrous dysplasia do not share the distinctive features seen on radiographs and ct . in fact , the mri appearances of fibrous dysplasia often resemble that of tumours . the signal intensity of fibrous dysplasia has been reported to be low on t1-weighted images . however , the signal intensity of fibrous dysplasia on t1-weighted images may be intermediate thus resembling that of a soft - tissue tumour . the signal intensity of fibrous dysplasia on t2-weighted images is often variable ranging from low to high signals in some patients . these high - signal intensities on t2-weighted images correspond to non - mineralised areas and regions of cystic changes demonstrated on ct . when t2-weighted images show high signals the differential diagnosis should include an inflammatory lesion or a neoplastic process . dysplasia is typically a painless anomaly , the presence of pain should also alert the radiologist to the presence of a more sinister process . like destructive osseous lesions elsewhere in the body , the most likely diagnosis of a destructive process in the skull base is a metastatic deposit . one useful method of approaching this problem is to describe the signs and ask ourselves what we would call the lesion if it were located around the knee . in this way it should also be remembered that many of the primary osseous tumours encountered in the head and neck are associated with previous radiotherapy [ 610 ] . before this possibility is entertained , a radiologist should be familiar with the criteria for making a diagnosis of radiation - associated tumours . these criteria include : ( a ) a history of irradiation ; ( b ) the second neoplasm must occur within the field of irradiation ; ( c ) the histology of the second neoplasm must be distinctly different from the primary tumour ; and ( d ) a latency period of many years , arbitrarily taken to be at least 5 years . these openings include the superior orbital fissure , foramen rotundum , foramen ovale , stylomastoid foramen and jugular foramen . various sinonasal squamous cell carcinomas ( scca ) after invading the orbit continue to extend through the superior orbital fissure . alternatively , tumours can spread from the nasal cavity through the sphenopalatine foramen and continue superiorly to the inferior and superior orbital fissures . in addition , tumours such as adenocystic carcinomas have a propensity to spread through the foramen rotundum along the maxillary nerve [ 1315 ] . malignant perineural infiltration of the mandibular nerve is also frequently encountered in head and neck oncologic imaging . scca originating in the oral cavity can spread via the lingual nerve to the foramen ovale . nasopharyngeal carcinomas in particular have a propensity to spread into the intracranial cavity via the mandibular nerve or the foramen ovale [ 16 , 17 ] . it should be noted that the foramen lacerum , although a large opening , is covered with fibrocartilage . intracranial spread through the foramen lacerum does take place but not as frequently as once thought . the vidian canal is located at the junction of the body of the sphenoid bone and the pterygoid process . it transmits the vidian nerve ( formed by the greater superficial petrosal nerve and the deep petrosal nerve ) . imaging plays a central role in the management of cancer patients with skull base involvement . the radiologist , therefore , plays a crucial role in determining disease extent and the choice of appropriate treatment methods .
radiologists are often called upon to solve diagnostic problems involving the skull base in cancer patients . 
 this presentation focuses on three issues : the detection of osseous involvement by infiltrating neoplasms , the diagnostic approach to bone tumours and the demonstration of head and neck tumours showing intracranial spread .