Findings:
A well-defined, fat-containing, non-enhancing, expansile mass lesion is seen centred in the right mastoid process, extending anteriorly to involve the right occipital condyle and posteriorly to the right occipital bone, measuring approximately 5.8 x 3.5 x 3.2 cm. Consequential remodelling of the right mastoid process with cortical thinning and cortical breach in the right mastoid process and the right occipital bone.
The lesion protrudes into the right tympanic cavity to a bony defect involving the floor of the right tympanic cavity. Further bony dehiscence at the medial aspect of the right external auditory canal.
Possible bony dehiscence of the basal turn of the cochlea, the cochlear aqueduct and the posterior semicircular canal on the right. The ossicular chain is unremarkable. The tegmen tympani and the scutum are intact.
Medially, the lesion compresses the right sigmoid sinus and the right jugular bulb with occlusion of the right sigmoid sinus. The intracranial circulation is otherwise unremarkable. The right internal jugular vein shows reduced enhancement at the skull base with reduced calibre. The carotid and vertebral arteries are patent, as is the left jugular vein.
There is mild mass effect on the adjacent cerebellum with no associated oedema. No hydrocephalus or midline shift.
The soft tissue of the neck is unremarkable with no evidence of sinister pathology or lymphadenopathy.
The lung apices are clear.
The visualised of the spine is unremarkable.
Conclusion:
Known, large, expansile mass lesion in the right mastoid process with bony remodelling, cortical breach and bony dehiscence involving the middle and inner ear structures on the right, as described above, likely in keeping with primary intraosseous dermoid cyst of the right mastoid.
Mass effect with compression and occlusion of the right sigmoid sinus and the right jugular bulb.