[1] UNIV PENN, MED CTR, DIV NEUROSURG & GEN INTERNAL MED, PHILADELPHIA, PA 19104 USA
[2] UNIV PENN, MED CTR, CTR CLIN EPIDEMIOL & BIOSTAT, PHILADELPHIA, PA 19104 USA
[3] UNIV PENN, LEONARD DAVIS INST HLTH ECON, PHILADELPHIA, PA 19104 USA
[4] FOX CHASE CANC CTR, DIV POPULAT SCI, PHILADELPHIA, PA 19111 USA