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Abstract. Object: Where no society-based or manufacturer guidance on radiation limits to neuromodulation devices is available, this research provides the groundwork for neurosurgeons and radiation oncologists who rely on the computerized treatment plan clinically for cancer patients. The focus of the article is to characterize radiation parameters of attenuation and scatter when an incident therapeutic x-ray beam is directed upon them.

Methods: Ten neuromodulation models were chosen to represent the finite class of devices marketed by Medtronic before 2011. CT simulations permitted computer treatment modeling for dose distribution analysis as used routinely in radiation oncology for patients. Phantom case results were directly compared to actual clinical patient cases. Radiation detection measurements were then correlated to computational results. Where the x-ray beam passes through the device and is attenuated, dose reduction was identified with Varian Eclipse computer modeling for these posterior locations. Results: Although the computer algorithm did not identify physical processes of side-scatter and back-scatter, these phenomena were proven by radiation measurement to occur. In general, the computer results underestimated the level of change seen by measurement.

Conclusions: For these implantable neurostimulators, the spread in dose changes were found to be -6.2% to -12.5% by attenuation, +1.7% to +3.8% by side-scatter, and +1.1% to +3.1% by back-scatter at 6 MV. At 18 MV, these findings were observed to be -1.4% to -7.0% by attenuation, +1.8% to 5.7% by side-scatter, and 0.8% to 2.7% by back-scatter. No pattern for the behavior of these phenomena was deduced to be a direct consequence of device size. At the time of this writing, manufacturers of Neuromodulation products do not recommend direct exposure of the device in the beam nor provide guidance for the maximum dose for these devices.

1. Introduction

Neuromodulation is becoming an increasingly popular clinical tool to alleviate symptoms of various neurological challenges, including chronic pain, urinary incontinence, and movement disorders. The target structure for Neuromodulation within the central or peripheral nervous system is determined by the particular application. A neuromodulation systemtypically consists of an implantable neurostimulator or pump, and at least one electrode array or catheter. The broad scope and acceptance for neuromodulation may create issues for the radiation oncologist and the neurosurgeon when their patient is diagnosed with a cancer requiring radiation therapy after a device has already been implanted. The surgical removal and re-implantantation increases the likelihood of infection, increases the medical cost due new device purchase, and creates a considerable loss of therapy time [1-4]. Consequences of device removal may include a return of the patient's symptom for which it was originally implanted [3]. Specialized surgeries such as these also come with a risk in detrimental effects, such as infection, hematomas or seromas in the area where the device is implanted. Repeating surgeries are cost burdening and time inefficient as well. Even to the FDA, these are valid concerns even for relocating the device or minimizing exposure to it [5].

It is likewise a quandary for the radiation oncologist specifically to consider the effects of having the device in the field of radiation. Unlike some heart rhythm devices, the maximum allowed dose for these neuromodulation devices is not available from any manufacturer nor have test results been published by researchers. Therefore, a recommendation to "not place the device directly in the field of radiation" is provided, which is expected to be considered when planning radiotherapy procedures [6]. This creates difficulty in targeting the disease of concern. Examples of such situations are when the tumor is in lungs or near the abdomen with the device implanted in the sub-clavicular region or abdomen or when the tumor is in the brain or spinal cord with the electrode implanted in close proximity. Although a single deep brain stimulator is routinely implanted in the recipient, at times a bilateral pectoral implant may be required in the rare case. Radiation therapy delivery for breast and lung then become particularly problematic, since there is not just one device obstacle, but bilateral obstacles. Particle accelerator beam angle adjustments can compensate to some degree to avoid the device and still target the tumor in most cases, however, the optimum angle may not be entirely or acceptably achievable. An unsatisfactory conclusion may be that either the tumor will be inadequately covered by the dose distribution intended or the unknown beam blocking effect will reduce dose significantly at the tumor [7].

At the root of these issues is the concern over the inaccuracy of the computerized modeling depended upon by the radiation oncologist to make these decisions. Medical physicists have published on such modeling inaccuracies for mandibular bridging plates, hip prosthetics, vascular access ports, and most recently pacemakers and cardioverter-defibrillators with remarkable variances of well over 10% in contrast to radiation measurements [8-11]. This research aims to assist the neurosurgeon, radiation oncologist and medical physicist involved in the consideration of such an impasse during the external beam radiation therapy treatment planning process. Ten marketed neuromodulation devices for analysis have been acquired. Computerized tomography scans were conducted on the devices and leads in a liquid water phantom setting. Then each scan was sent to a treatment planning computer for simulation. Radiation measurements followed and compared to the modeling results. Case studies are presented additionally, with dosimetry local to where the device or electrode leads were implanted. From this research, we promulgate this American Association of Physicists in Medicine (AAPM) requested and required information associated with effect of device on radiation beam lacking in literature for the radiation therapy team to consider [12].

2. Materials and methods

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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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