Carbon Footprint of Proton Therapy: How Many Trees do We Need to Offset It?

Abstract

<h3>Purpose/Objective(s)</h3> There is an increasing concern about rising carbon dioxide (CO2) levels and its hazardous impact on human health and climate change. Radiation oncology uses high energy machines; however, their carbon footprint is not well understood. This study quantifies the energy utilization of proton therapy, and estimates corresponding carbon footprint. Additionally, the study evaluates possible ways to offset CO2 with planting new trees. <h3>Materials/Methods</h3> Patients treated between 07/2020 and 06/2021 using Mevion S250 proton passive scattering system were evaluated. Current measurements were recorded at 2 second intervals and converted to kilowatt (kW) power consumption. Patients treated were reviewed for disease, site treated, dose, number of fractions, and duration of beam during each fraction. EPA web calculator was used to convert power consumption to tons of CO2, and the number of new trees required to offset it. <h3>Results</h3> There were 185 patients treated (average 19.9 per day); 63% had full course with protons, 31% had photon boost, and 6% had protons as boost. Total of 5,176 fractions were delivered (average 28 fractions per treatment course). Total BeamOn time was 538,546 seconds (149.6 hours), average 104 seconds per fraction. Power consumption was 55.8 kW in standby/night mode and 64.4 kW during BeamOn. Average daily power use was 1,346 kWh, and annual consumption was ∼490,000 kWh. BeamOn consumption accounted for 1.7% of total machine power draw. Breast cancer with RNI required highest power consumption (140 kWh per patient), while prostate cancer lowest (28 kWh), average across all patients was 52 kWh. Highest cumulative power use was for CNS tumors (2,753 kWh), lowest for lymphomas (208 kWh). Power draw of the clinical and administrative areas was 11 kW, and annual consumption was ∼96,000 kWh, for a program total of 586 MWh. The corresponding carbon footprint for BeamOn time was 6.83 metric tons of CO2, or 37 kg per patient course (highest for breast cancer at 99 kg, lowest for prostate cancer 20 kg). Carbon footprint for the annual machine power consumption was 347.5 tons CO2, and for the proton program was 415.5 tons of CO2. Attributed footprint was thus 415.5 tons / 185 = 2,200 kg CO2 per patient treated. The carbon offset required for BeamOn time would be 113 new trees planted (average 0.6 trees per patient courses). The corresponding offset required to operate the Mevion system for a year would be 5,743 new trees planted, and the proton program composite offset would be 6,867 new trees planted. The attributed carbon offset per patient treated would thus be 37 new trees planted. <h3>Conclusion</h3> Carbon footprint varied by disease treated and was on average 37 kg of CO2 per patient, and 415.5 tons of CO2 for the proton program. The number of new trees to be planted to offset the power use would be 6,867 new trees annually, or 37 per patient treated. We hope this study inspires radiation oncology practices to evaluate their own carbon footprint.