Estimates of cardiac implanted electronic device neutron dose for a pencil beam scanning proton therapy system

The presence of a cardiovascular implantable electronic device (CIED) is frequently viewed as a contraindication to proton therapy due to the creation of secondary neutrons that can potentially damage CIED electronics. As a result, photon therapy is typically recommended for patients with CIEDs.

The study aims to provide a method for estimating neutron dose to a CIED by measuring equivalent neutron dose at varying distances from isocenter and field edge. This estimation can be used to guide clinical decisions by balancing the risk of neutron-induced CIED damage against the therapeutic benefits of proton therapy.

Standardized three-dimensional measurement fields of varying dimensions were delivered using the IBA ProteusONE (Ion Beam Applications SA, Walloon Brabant, Belgium) pencil beam scanning proton therapy system, with each field delivering an RBE-weighted dose of 2 Gy. Baseline measurement included one treatment field being delivered with a range shifter. BD-PND detectors (Bubble Technology Industries, Chalk River, ON) were placed at a defined distances from the surface mark of beam isocenter and field edge to record neutron doses. Additionally, clinical treatment plans including two prostate/pelvis plans, two head & neck plans, two brain plans, and one breast plans were delivered, with detectors placed at positions corresponding to the location of a CIED.

For fields without a range shifter, the measured neutron dose ranged from 0.11 µSv/2 Gy for the smallest field at 50 cm from isocenter to 11.0 µSv/2 Gy for the largest field size at 10 cm from isocenter for fields. The addition of a range shifter to the 10 × 10 × 10 cm³ field increased the dose to 0.66 µSv/2 Gy at 50 cm to 14.2 µSv/2 Gy at 10 cm from isocenter. For clinical treatment plans, neutron doses ranging from 0.14 µSv/2 Gy to 9.5 µSv /2 Gy at 24-56 cm from isocenter.

These measurements provide a foundation for estimating neutron doses to CIEDs, enabling physicists and physicians to evaluate the feasibility of proton therapy for patients with CIEDs. The results support informed clinical decision-making by quantifying the risk of neutron-induced damage relative to the therapeutic benefits of proton therapy.