Radiotherapy is a critical and inseparable component of comprehensive cancer treatment and care. For many of the most common cancers in low-income and middle-income countries, radiotherapy is essential for effective treatment. In high-income countries, radiotherapy is used in more than half of all cases of cancer to cure localised disease, palliate symptoms, and control disease in incurable cancers. Yet, in planning and building treatment capacity for cancer, radiotherapy is frequently the last resource to be considered. Consequently, worldwide access to radiotherapy is unacceptably low. We present a new body of evidence that quantifies the worldwide coverage of radiotherapy services by country. We show the shortfall in access to radiotherapy by country and globally for 2015-35 based on current and projected need, and show substantial health and economic benefits to investing in radiotherapy. The cost of scaling up radiotherapy in the nominal model in 2015-35 is US$26.6 billion in low-income countries, $62.6 billion in lower-middle-income countries, and $94.8 billion in upper-middle-income countries, which amounts to $184.0 billion across all low-income and middle-income countries. In the efficiency model the costs were lower: $14.1 billion in low-income, $33.3 billion in lower-middle-income, and $49.4 billion in upper-middle-income countries-a total of $96.8 billion. Scale-up of radiotherapy capacity in 2015-35 from current levels could lead to saving of 26.9 million life-years in low-income and middle-income countries over the lifetime of the patients who received treatment. The economic benefits of investment in radiotherapy are very substantial. Using the nominal cost model could produce a net benefit of $278.1 billion in 2015-35 ($265.2 million in low-income countries, $38.5 billion in lower-middle-income countries, and $239.3 billion in upper-middle-income countries). Investment in the efficiency model would produce in the same period an even greater total benefit of $365.4 billion ($12.8 billion in low-income countries, $67.7 billion in lower-middle-income countries, and $284.7 billion in upper-middle-income countries). The returns, by the human-capital approach, are projected to be less with the nominal cost model, amounting to $16.9 billion in 2015-35 (-$14.9 billion in low-income countries; -$18.7 billion in lower-middle-income countries, and $50.5 billion in upper-middle-income countries). The returns with the efficiency model were projected to be greater, however, amounting to $104.2 billion (-$2.4 billion in low-income countries, $10.7 billion in lower-middle-income countries, and $95.9 billion in upper-middle-income countries). Our results provide compelling evidence that investment in radiotherapy not only enables treatment of large numbers of cancer cases to save lives, but also brings positive economic benefits.