Temporal trends and evolution of photon and proton radiotherapy research.

PURPOSE: Photon therapy has been and remains the predominant external beam radiotherapy treatment modality, but the number of proton therapy facilities and proton research publications has increased considerably in the past decade. We aimed to quantify and compare the growth of research output in photon and proton radiotherapy over time. MATERIALS AND METHODS: A bibliometric analysis was conducted of all peer-reviewed publications reporting on adult patients with cancer who received photon or proton radiotherapy. Each article was classified by radiotherapy type (photon and proton) and study design (commentary/editorial, dosimetry studies, technical/physics studies, case reports/series, cohort studies, phase I/II trials, phase III trials, reviews/meta-analyses, and protocols). Annual publication counts from 1940-2025 were plotted for overall and subgroup trends. RESULTS: A total of 19,961 articles were published from 1940 to 2025. Of these, 53% studied photon radiotherapy, and 47% studied proton radiotherapy. The overall literature has expanded exponentially since the 1990s. Photon publications historically predominated, but proton-related research accelerated rapidly after 2015. Subgroup analyses revealed consistent proton research growth across study types. CONCLUSION: Proton radiotherapy research has expanded rapidly over the past decade. It is important that research in both modalities continues to progress in parallel to ensure that progress in radiation oncology remains clinically meaningful for all patients and that more knowledge is available to optimize patient selection for each treatment approach.