What are the symptoms of a middle back/spinal tumor? is it rare?

Yes, quite uncommon: But, what prompts your concerns? A variety of tumors may present over the thoracic vertebrae, some causing nocturnal pain responsive to aspirin, others compressing the spinal cord, resulting in urinary incontinence, leg weakness and numbness. Since often insidious, and slowly progressive, if there is high suspicion, would urge MRI study to pin down.