Comparison of the Direct Power Consumption for Different Linac Treatment Parameters in External Beam Radiation Therapy

Abstract

The negative impact of climate change on the environment has led to increasing concern around the world. Relatively little is known about the contribution of radiation oncology systems to carbon emissions. This study measures the energy utilization of Linac-based EBRT for different treatment parameters and techniques, which was then converted to power use (kWh) and ultimately a carbon footprint of various photon treatment approaches. A Varian TrueBeam was evaluated. The direct power consumption was measured using a Fluke 1736 Power Logger, recorded in one-second intervals for one week. Different photon and electron beam energies, dose rates, treatment techniques (3D, IMRT, gated), and imaging types (kV, MV) were evaluated. Clinical treatment plans were reviewed, and average treatment times used to determine kWh. IRB approval was obtained. The Greenhouse Gas Equivalencies Calculator was used to determine kg of CO2. Power draw for 6 MV, 10 MV, and 18 MV at a rep rate of 600 MU/min was 31.6, 32.0, and 27.5 kW, respectively. The power draw for end of range dose rates for 6 MV (60, 600), 6 FFF (400, 1400), 10 FFF (400, 2400), and 6 MeV (100, 1000) were (22.2, 31.6 kW), (23.7, 31.6 kW), (19.6, 32.1 kW), and (22.6, 23.6 kW), respectively. 6 MV open 2D fields, modulated IMRT fields, and gated/beam hold fields had similar power draw at 31.6, 31.7, and 31.9 kW, respectively. Portal MV imaging with 2.5 MV beam was 32.5 kW, while CBCT and kV/kV imaging was ∼11 kW and not distinguishable from baseline power fluctuations. A total of 191 delivered treatment plans were reviewed. Electron plans (n = 8) treatment time was on average 29 sec (16 sec STDEV) per fraction and 635 sec (185 sec STDEV) per course, 2D/3D plans (n = 63) were 77 sec (32 sec STDEV) and 1004 sec (1002 sec STDEV), and IMRT plans (n = 120) were 182 sec (60 sec STDEV) and 5351 sec (2401 sec STDEV), respectively. The kWh per treatment for electron, 2D/3D, and IMRT plans were 0.19, 0.68, and 1.6 kWh, respectively. The CO2 equivalent for electrons, 2D/3D, and IMRT techniques are 0.08, 0.29, and 0.7 kg. There was some variability in the power draw for different energies and different dose rates but was relatively stable around 32 kW. Power consumption for clinical therapy is a result of kW power draw multiplied by duration of beam delivery, which for our patients varied significantly as a function of technique and number of fractions delivered. Reduction in radiation oncology carbon footprint will likely be driven more by number of fractions and type of treatment technique, and length of patient commute, rather than beam energy and dose rate selection.