ABSTRACT
BACKGROUND AND PURPOSE: Cardiac implantable electronic device (CIED) malfunctions can be induced by secondary neutron dose from spot-scanning proton therapy. A recent in-vitro study investigating secondary neutron dose to CIEDs up to 7 mSv per fraction found that exposure of secondary neutrons in this range was clinically manageable. This study presents decision algorithms proposed by a national expert group for selection of patients with breast and head & neck (H&N) cancer with CIEDs adjacent to target for proton therapy based on the 7 mSv threshold. METHODS AND MATERIALS: Ten patients with breast cancer and five with H&N cancer were included in the study. Five patients with breast cancer received photon therapy with CIED and proton plans were retrospectively created. The remaining patients received proton therapy without CIED and a worst-case position of a virtual CIED was retrospectively delineated. Secondary neutron dose was estimated as ambient dose equivalent H*(10) using Monte Carlo simulations. RESULTS: For patients with breast cancer and with contralateral CIED, the secondary neutron dose to the CIED was below 7 mSv per fraction for CTV < 1500 cm3 in 2 Gy fractions and CTV < 1000 cm3 in 2.67 Gy fractions. The secondary neutron dose to the CIED was below 7 mSv per fraction for all patients with H&N cancer. CONCLUSIONS: Simulations of neutron exposure suggest that proton therapy is feasible for most patients with CIED adjacent to target. This forms the basis for decision algorithms for selection of patients with CIED for proton therapy.
ABSTRACT
PURPOSE: Cardiac implantable electronic devices (CIED) are sensitive to scattered secondary neutrons from proton beam irradiation. This experimental in vitro study investigated risk of CIED errors during pencil beam proton therapy. METHODS AND MATERIALS: We used 62 explanted CIEDs from 4 manufacturers; 49 CIEDs were subjected to a simulated clinical protocol with daily 2 Gy relative biological effectiveness fractions prescribed to the phantom. Devices were located at 3 different lateral distances from the spread-out Bragg peak to investigate the risk of permanent or temporary device errors. Additionally, 13 devices with leads connected were monitored live during consecutive irradiations to investigate the risk of noise, over- or undersense, pace inhibition, and inappropriate shock therapy. RESULTS: We detected 61 reset errors in 1728 fractions, and all except 1 CIED were reprogrammed to normal function. All, except 1 reset, occurred in devices from the same manufacturer. These were successfully reprogrammed to normal function. The 1 remaining CIED was locked in permanent safety mode. Secondary neutron dose, as estimated by Monte Carlo simulations, was found to significantly increase the odds of CIED resets by 55% per mSv. Clinically significant battery depletion was observed in 5 devices. We observed no noise, over- or undersense, pace inhibition, or inappropriate shock therapy during 362 fractions of live monitoring. CONCLUSIONS: Reprogrammable CIED reset was the most commonly observed malfunction during proton therapy, and reset risk depended on secondary neutron exposure. The benefits of proton therapy are expected to outweigh the risk of CIED malfunctioning for most patients.
