Oncology, the medical specialty focused on the diagnosis and treatment of cancer, has a significant carbon footprint. This is due to the complex infrastructure, supply chains, and care processes involved, which often require substantial travel by both patients and healthcare professionals. In France, the country's total carbon footprint in 2024 is estimated at 563 million tons of carbon dioxide equivalent (MtCO2e), or about 8.2 tons of CO2e per person. The healthcare sector contributes nearly 50 MtCO2e annually, representing about 8% of the nation's greenhouse gas (GHG) emissions. Of this, about 45% is attributed to care delivery, while 55% comes from medicines and medical devices, according to the Shift Project, an environmental think tank. Improving the relevance of care, avoiding unnecessary procedures, and optimizing care pathways can reduce the environmental impact of oncology without compromising the quality of care. Most healthcare professionals are willing to change their practices if it doesn't affect treatment effectiveness. Dr. Matthieu Delaye, a medical oncologist at the Curie Institute, emphasizes that decarbonizing healthcare should not be seen as restrictive but as a way to align with the goals of improving treatment effectiveness, reducing side effects, and enhancing patients' quality of life. The primary sources of CO2 emissions in the healthcare sector are medicines and medical devices, which account for about 29% and 21% of GHG emissions, respectively. Travel, including that of patients, professionals, and equipment, accounts for about 13% of emissions. This highlights that reducing emissions in healthcare involves more than just cutting energy use in facilities. It also requires managing purchases, optimizing care organization, and ensuring the relevance of prescriptions. Medicines, in particular, have a large carbon footprint. The production of active ingredients contributes the most, followed by corporate activities, research and development, packaging, and transport. The carbon footprint of an active ingredient can vary widely, with some oncology drugs emitting significantly more than others. For example, a 28-day treatment with pembrolizumab emits about 235.5 kgCO2, equivalent to driving a car for 1,226 kilometers, while the same treatment with irinotecan emits only 6.2 kgCO2. The production location also plays a role, as manufacturing in France could reduce the carbon footprint by about 50% compared to production in China. In terms of prescriptions and administration, oral forms of medications generally have a lower environmental impact than intravenous (IV) forms. For example, paracetamol in tablet form emits significantly less CO2 than its IV form. Similarly, extended-release forms and higher dosages can reduce environmental impact. In radiotherapy, reducing the number of treatment sessions can significantly lower the carbon footprint, as fewer sessions mean less travel and resource use. This approach, known as hypofractionation, has shown environmental benefits in treating cancers such as breast and prostate cancer. The challenge in oncology is to balance the need for effective treatments with the goal of reducing environmental impact. Some advances in oncology, such as better patient selection, reduced treatment duration, and non-pharmacological interventions, can lead to both improved patient outcomes and lower carbon emissions. Healthcare professionals are generally willing to adapt their practices if it does not compromise care quality, but they often lack the necessary data and tools to make informed decisions. The future of oncology may involve integrating environmental considerations into clinical decisions, ensuring that the right care is provided to the right patients at the right time, with the minimum necessary resources.