Best seen on axial T1 with contrast is an extra-axial mass located at the expected location of the left endolymphatic sac (posterior aspect of the petrous bone). It is lobulated and vividly enhancing with erosion of the adjacent bone. Inferiorly it involves but does not occlude the sigmoid sinus.
Conclusion: given the location and erosion of bone an endolymphatic sac tumour is favoured.