What does this all mean please? Ct scan 1: "bone window images show a mixed lytic and sclerotic lesion in the body of t9"

Mixed density. This means there is an area in the 9th thoracic body (mid back) which has areas which are lower density (lytic) and higher density (sclerotic) than normal bone at that location. There are many reasons for this appearance. A thoracic spine MRI of this area, probably with and without contrast, may be helpful.
Bone lesion. T9 is the 9th thoracic vertebral body. In ct the image can be altered or "windowed" to depict certain types of tissue in greater detail, such as bone. Lytic means less bone, sclerotic is more dense bone. In the spine the more common mixed lesions would be hemangiomas or fibrous dysplasia.