What test should be done to determine if lump at base of spine is benign or malignant? mri?

Biopsy: is the absolute answer. All the imaging that we do can only give your doctor an idea of what it is. Both CT and MRI are excellent for that. But they do not substitute for tissue diagnosis except in rare occasions. Lipoma made up of fatty tissue or cysts can be diagnosed amongst others. When in doubt biopsy is usually needed.

Spine lump: first you should see a spine specialist since what you think is a lump may be a normal part of anatomy or even muscle spasm. For definitive confirmation if this mass is something abnormal you should get an MRI with gadolinium contrast.