A, Weinstein-Boriani-Biagini tumor classification system. Vertebra is divided into 12 radiating zones that are numbered clockwise, beginning at one half of spinous process. Concentric layers are lettered sequentially from extraosseous soft tissues to intradural space. B, The Spine Oncology Study Group modified classification by numbering radial zones in counterclockwise fashion, beginning at left half of spinous process to allow for more anatomic orientation of diagram for ease of use. Circled letters: A, extraosseous soft tissues; B, intraosseous (superficial); C, intraosseous (deep); D, extraosseous (extradural); E, extraosseous (intradural).