Project Summary/Abstract Residual cancer cells left behind following surgery increase the chance of cancer returning in almost every cancer subtype. The current inability to identify these tumor cells during surgery hinders cancer care across the spectrum, including breast and prostate cancers, as 20-40% of these patients suffer from positive margins, which doubles the risk of cancer returning. This proposal solves this problem through an original approach for ultrasensitive optical imaging of cancer cells in live tissue and during surgery. Current intraoperative imaging methods are unable to achieve high sensitivity both on the tissue surface and at depth due to inherent physical limits of both current optical probes and their requisite imagers. They are also too bulky to be integrated onto modern surgical tools, which could guide precision surgery with far greater accuracy than achievable today. Here, we address these dual challenges by introducing a wholly new imaging strategy integrating nanotechnology, protein engineering, and advanced imager design with the goal of real-time highly sensitive intraoperative imaging of cancer cells, both on the surface and at depth. We propose major advances in nanotechnology to redesign upconverting nanoparticles as optical probes that can be safely imaged in tissue, protein engineering to produce antibodies that selectively target the probes to tumor, and detector engineering to build an ultrathin imaging chip, directly integrated into surgical instrumentation. The combination of these novel technologies transforms instruments themselves into imagers to dramatically increase the sensitivity in identifying cancer cells, with the ultimate goal of being able to identify, in real time, all residual disease.