Abstract
The purpose of this article is to share the excitement of the science of proton therapy, told by two physicists, who started their career in this area at different times. The authors' journey spans the evolution of proton therapy over the past 30 years, taking the reader from the time when it was an extremely exotic treatment modality until its more common use today. Over this time period, the authors' research and development aimed at an improved understanding of the physical benefits of intensity-modulated proton therapy and arc therapy, treatment planning and optimization to take proton-specific uncertainties into account, and imaging to measure the proton range in the patient. The final section focuses on emerging themes to democratize proton therapy by substantially reducing its size and price, for much greater affordability and global availability of this treatment modality.