Various embodiments disclose systems and methods for tracking regions (e.g., tumor locations) within living organisms. Some embodiments provide real-time, highly accurate, low latency measurements of tumor location even as the tumor moves with internal body motions. Such measurements may be suitable for closed-loop radiation delivery applications where radiation therapy may be continuously guided to the tumor site even as the tumor moves. Tumor motion may be associated with periodic motion (e.g., respiratory, cardiac) or aperiodic motion (e.g., gross patient motion, internal bowel motion). Various embodiments facilitate accurate radiation delivery to tumor sites exhibiting significant motion, e.g., lung, breast, and liver tumors.