Towards estimating the carbon footprint of external beam radiotherapy

•Initial carbon footprint estimate of EBRT workflow.•Carbon footprint studies necessary to ensure route to net zero is evidence-based.•Patient travel forms the largest proportion (70–80%) of the carbon footprint of RT.•Linac idle power was the 2nd largest proportion of the carbon footprint (8% − 19%)•Average leakage rate of SF6 from the linacs was 0.14 kg CO2e /yr/linac. The National Health Service (NHS) in the United Kingdom (UK) is aiming to be carbon net zero by 2040 to help limit the dangerous effects of climate change. Radiotherapy contributes to this with potential sources quantified here. Activity data for 42 patients from within the breast IMRT and prostate VMAT pathways were collected. Data for 20 prostate patients was also collected from 3 other centres to enable cross centre comparison. A process-based, bottom-up approach was used to calculate the carbon footprint. Additionally, patients were split into pre-COVID and COVID groups to assess the impact of protocol changes due to the pandemic. The calculated carbon footprint for prostate and breast pre-COVID were 148 kgCO2e and 101 kgCO2e respectively, and 226 kgCO2e and 75 kgCO2e respectively during COVID. The energy usage by the linac during treatment for a total course of radiotherapy for prostate treatments was 2–3 kWh and about 1 kWh for breast treatments. Patient travel made up the largest proportion (70–80%) of the calculated carbon footprint, with linac idle power second with ∼ 10% and PPE and SF6 leakage were both between 2 and 4%. These initial findings highlight that the biggest contributor to the external beam radiotherapy carbon footprint was patient travel, which may motivate increased used of hypofractionation. Many assumptions and boundaries have been set on the data gathered, which limit the wider application of these results. However, they provide a useful foundation for future more comprehensive life cycle assessments.