Evaluation of Cied Dosimetry Using Oslds for Patients Treated for Lung Cancers Using SBRT

Abstract

For medically inoperable patients with peripheral lung cancers, SBRT is the standard of care according to ASTRO guidelines. The presence of a cardiovascular implantable electronic device (CIED) complicates the planning of SBRT treatments, as excess radiation dose and neutron production could produce an undesirable malfunction in the CIED. Current recommendations based on AAPM TG34 limit the treatment beam energy to ≤ 10MV to minimize neutron dose and limit dose to the CIED to < 2 Gy. At the author’s institution, dosimetric contribution from scatter and leakage are calculated to estimate CIED dose. The scatter dose is determined from the treatment planning system (TPS) based on work from Howell et al. (2010), which scales TPS dose dependent on field edge to CIED distance. Leakage dose is determined by the total number of MU per fraction, and the number of fractions in the treatment. The purpose of this work was to evaluate CIED dosimetry using OSLDs in an anthropomorphic phantom for patients treated with lung SBRT. An anthropomorphic ATOM® phantom developed by CIRS was CT-simulated. Contours were drawn for the PTV, lungs, heart, esophagus, chest wall, skin, spinal cord, and CIED. Two lung SBRT plans were generated using an IMRT static gantry and a VMAT approach (AAA algorithm, 6 MV photons, and 1 mm grid). The IMRT plan included 11 fields and the VMAT plan used four full arcs, with both plans optimized to minimize entrance dose to the CIED. The plans were delivered using a medical linear accelerator on the phantom with OSLDs placed 1 cm below the surface, which was created using bolus. Five OSLDs were placed in an array and were marked using BBs. A singular OSLD was placed inside the CIED contour, and four additional OSLDs were placed 0.5 cm from the CIED in the lateral and superior/inferior directions. Comparisons between the TPS dose, estimated CIED dose, and the OSLD measurement were performed. For both IMRT and VMAT plans, the TPS-calculated dose to out of field CIEDs is underestimated when compared with the measured dose (Table 1). The difference between the TPS-calculated and measured doses were consistently 10 cGy for both plans, an increase of 225%. This discrepancy may be due to the excess dose from the CBCT that was used for phantom alignment. Implementing scatter factors determined the CIED dose to be 14.6 cGy and 37.9 cGy for the IMRT and VMAT plans, respectively. This overestimated the measured CIED dose of 13.4 cGy and 20.8 cGy. Preliminary results showed that the TPS-calculated dose underestimates the out of field dose to a CIED by 10 cGy compared to OSLD measurements. Future work will investigate the effects of the CBCT dose on the CIED dose.Abstract 3808; Table 1OSLDLocationIMRT EclipseIMRT OSLDDifferenceVMAT EclipseVMAT OSLDDifference[cGy][cGy][cGy][cGy][cGy][cGy]1Left6.715.18.412.027.115.12Inferior4.215.311.126.135.79.63CIED6.613.46.89.020.811.84Superior8.718.49.712.125.012.95Right6.817.710.94.89.64.8Average:9.4Average:10.8 Open table in a new tab