Carbon Footprint of Clinical Photon Therapy: Initial Estimates

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 estimates the energy utilization of photon 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 a technology company's medical linear accelerator system at one of our satellite locations were evaluated. A technology company data sheet was used to estimate power draw of the system. BeamOn mode was specified as 39 kVA, Ready mode as 15 kVA, On mode as 11 kVA, and Standby mode as 7 kVA; these were converted to kW. This is likely the worst-case power use scenario, with power factor typically 0.75 – 1.0. Patients treated were reviewed for dose, number of fractions, number of fields, and duration of beam during each fraction. Field delivery durations were obtained from the EMR, and kWh power consumption was estimated per fraction and per treatment course. EPA web calculator was used to convert power consumption to tons of CO2, and the number of new trees required to potentially offset it. <h3>Results</h3> There were 176 patients treated for 191 treatment courses. Total of 4,517 fractions were delivered (average 23.6 fractions per treatment course). Total BeamOn time was 710,606 seconds (197.4 hours), average 157 seconds per fraction. Total BeamOn power consumption would be 7,698 kWh, or ∼1.7 kWh per fraction and ∼40 kWh per treatment course. Annual power consumption including standby mode would be ∼81,250 kWh; BeamOn consumption would account for 9.5% of total machine power usage. The corresponding carbon footprint for BeamOn time would be 5.46 metric tonnes of CO2, or 29 kg of CO2 per patient course. Carbon footprint for the annual machine power consumption would be 57.6 tonnes of CO2. Attributed footprint would thus be 57.6 / 191 = 301 kg CO2 per patient treatment course. The carbon offset required for patient BeamOn time would be 90 new trees planted (average 0.5 trees per patient course). The corresponding offset required to operate the medical linear accelerator system for a year would be 952 new trees planted. The attributed carbon offset per treatment course would thus be 5 new trees planted. <h3>Conclusion</h3> Carbon footprint of the technology company's medical linear accelerator system is likely to be ∼30 kg of CO2 equivalent for direct BeamOn treatment course energy used, ∼300 kg of CO2 equivalent per overall patient energy expenditure, and ∼5.5 metric tonnes of CO2 in total to operate per year. The corresponding number of new trees to be planted to offset the power use would be 0.5 new trees, 5 new trees, and 952 new trees respectively. We are working on measuring the actual power draw during various treatment modes, which could be 5-15% lower than the kVA specification, and calculating the actual carbon footprint of clinical radiation therapy for our next study.