Dr. Rena Callahan is a hematologist oncologist who practices in Santa Monica and Santa Clarita. As an associate clinical professor of medicine at the David Geffen school of medicine, she is active in the breast cancer research and treatment program. She is board certified in hematology and medical oncology, and sees a wide variety of malignancies and blood disorders.   Dr. Callahan is dedicated to providing patient-centered care, and including her patients in clinical decision making so that…