This chapter examines the pivotal decade of 1925–1935 in the history of radiation oncology, set against a backdrop of significant global events, including the Great Depression and the rise of totalitarianism. Despite these challenges, the field witnessed a crucial transition from its early, less precise stages toward a more scientifically grounded discipline. The chapter highlights key technological advancements in X-ray tubes and brachytherapy techniques that allowed for improved treatment depth and skin sparing. A major focus is placed on the burgeoning era of international collaboration, marked by the establishment of the International Congresses of Radiology and the subsequent birth of radiation dosimetry, including the adoption of the Roentgen as an international unit of dosage. Innovations in ionization chamber design led to more accurate dose measurements and the standardization of x-ray equipment. The chapter also explores the growing awareness of radiation safety and the formation of international protection committees. Furthermore, early investigations into radiation biology and the exploration of dose fractionation began to shape treatment strategies. Finally, the significant contributions of Edith Quimby to the field of radiation physics and dosimetry during this period are highlighted, underscoring the foundational work that paved the way for modern radiation oncology. This decade represents a transformative phase, laying the groundwork for the future of cancer treatment with ionizing radiation.