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Michael Gossman, MS, DABR, RSO

Recent improvements to the functionality and stability of implantable pacemakers and cardioverter-defibrillators involve changes that include efficient battery power consumption and radiation hardened electrical circuits. Manufacturers have also pursued MRI-compatibility for these devices. While such newer models of pacemakers and cardioverter-defibrillators are similar in construction to previously marketed devices � even for the recent MRI-compatible designs currently in clinical trials � there is increased interest now with regard to radiation therapy dose effects when a device is near or directly in the field of radiation. Specifically, the limitation on dose to the device from therapeutic radiation beams is being investigated for a possible elevation in limiting dose above 200 cGy. We present here the first-ever study that evaluates dosimetric effects from implantable pacemakers and implantable cardioverter-defibrillators in high energy X-ray beams from a medical accelerator. Treatment plan simulations were analyzed for four different pacemakers and five different implantable cardioverter-defibrillators and intercompared with direct measurements from a miniature ionization chamber in water. All defibrillators exhibited the same results and all pacemakers were seen to display the same consequences, within only a � 1.8% deviation for all X-ray energies studied. Attenuation, backscatter, and lateral scatter were determined to be -13.4%, 2.1% and 1.5% at 6 MV, and -6.1%, 3.1% and 5.1% at 18 MV for the defibrillator group. For the pacemaker group, this research showed results of -15.9%, 2.8% and 2.5% at 6 MV, and -9.4%, 3.4% and 5.7% at 18 MV, respectively. Limited results were discovered from scattering processes through computer modeling. Strong verification from measurements was concluded with respect to simulating attenuation characteristics. For IP and ICD leads, measured dose changes were less than 4%, existing as attenuation processes only, and invariant with regard to X-ray energy

I. INTRODUCTION

An external pacemaker was first designed and built by the Canadian electrical engineer John A. Hopps in 1949, utilizing a series of vacuum tubes and an AC wall power supply.(1) This artificial pacemaker, which should not be confused with the heart's natural pacemaker, was used to regulate heart rate declines (bradycardia) through electrical impulses delivered when a generator conducts electricity through electrodes contacting heart muscles. The detection of blood movement and breathing are interpreted, recorded, and processed, where an electrical impulse may then be sent to the electrodes in order to help maintain a normal heart rhythm. Newer devices have multiple circuits (chambers) for connecting leads. This design permits timed electrical stimulation for one or even multiple areas of the heart (i.e. the right ventricle only, both the right atrium and right ventricle, or an atrium and both ventricles).

Twelve years of further research went into the development of a device capable of correcting both for the onset of arrhythmia and ventricular fibrillation. The new device, invented by Nobel Peace Prize recipient Bernard Lown in 1961, was capable of delivering an impulse in less than 20 seconds.(2,3) Initially referred to as the cardioverter, it has since become known as the defibrillator.(4) Operating from DC voltage alone, human implantation became possible for it and the pacemaker. Modern day versions of the defibrillator are now able to detect and correct for ventricular tachycardia.

Within the following eight years, advances in these devices became more political and publicized, yet more complex to achieve. Researchers struggled to create a device capable of sending multiple electrical impulses and providing different levels of electrical shock when required.(5) The aim was to determine if it were possible to design a device that could modulate the impulse sent to increase low heart rates to an acceptable rate (pacing), deliver a mild shock synchronized to an in appropriately fast heart rate (cardioversion), or deliver a much more powerful shock to a heart in ventricular fibrillation (defibrillation).(6) The use of computer algorithms and advancing hardware, pioneered by a team led by Michel Mirowski and Morton Mower, resulted in the first ever implantable cardioverter-defibrillator (ICD) in 1969.(7)

Now, forty years later, the implantable pacemaker (IP) and ICD are extremely valuable and commonly used by cardiovascular physicians. There are still, however, some aspects of the use of pacemakers and ICD's that are not well understood. Within the field of radiation oncology, the aim is to utilize scientific techniques available to treat cancer with radiation. When the material being irradiated is not similar to that of a patient � like a metallic pacemaker, for example � the physics of the radiation beam is altered. This presents a challenge on multiple levels. Problems are known to have occurred during in vitro testing.(8,9) From these observations, clinicians have been made aware of possible interactions with implanted and external devices.(10) Not only is this an issue during computerized tomography (CT) image acquisition initially, but it is an issue for delivering radiation for the treatment of cancer.(11,12) Figure 1 illustrates the size and location of such devices from a chest X-ray radiograph.

. . . Click here for entire article, figures and footnotes (PDF).

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Michael Gossman, MS, DABR, RSO, is a Board Certified Qualified Expert Medical Physicist - Currently the Chief Medical Physicist & RSO of Radiation Oncology in Ashland, KY - a Medical Consultant to the U.S. Nuclear Regulatory Commission (U.S. NRC) - and an Accreditation Site Reviewer for the American College of Radiation Oncology (ACRO). He is the highest ranking scientist in the medical community. His expertise involves the safe, effective and precise delivery of radiation to achieve the therapeutic result prescribed in patient care by radiation oncologists.

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