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Deep brain stimulation (DBS) is used to treat movement disorders, such as Parkinson disease, essential tremor, and dystonia. The system consists of 1 or more electrode leads that are implanted in the brain and a wire or extension that is connected to an implanted pulse generator (IPG). The IPG is a battery-powered neurostimulator placed subcutaneously, typically just inferior to the clavicle. Unlike pacemakers and defibrillators, there are few publications in the literature reporting the effects of irradiating DBS systems.
reported a case in which a patient received treatment to a target in the left lung that was within 3 mm of the device. Other authors have reported cases in which the electrode leads received full dose and the IPG received low dose due to scatter.
In this report, we present a case of a patient undergoing radiation therapy to her left chest wall and supraclavicular region with an implanted DBS within the target volume.
A 69-year-old female patient presented with node-positive left breast cancer treated with mastectomy and sentinel lymph node biopsy in early 2019. She subsequently was treated with adjuvant chemotherapy and planning proceeded for locoregional radiation to the left chest wall and regional lymph nodes. However, she had an IPG in her left chest wall, connected to an electrode in her left brain, to control an essential tremor (the device was implanted in January 2018).
The IPG was directly overlaying the treatment area. Relocation of the device involves surgical procedures and their associated risks.
Alternatively, exposing the device to direct radiation may result in patient injury as a result of electromagnetic interference (heating of components resulting in tissue damage) or device operational damage, according to manufacturer Medtronic’s DBS Information for Prescribers. After discussion with the patient regarding the risks and benefits, radiation planning proceeded with a prescription dose of 42.56 Gy and 40 Gy in 16 fractions to her chest wall and the supraclavicular region, respectively. The neurostimulator device was turned off during computed tomography simulation and radiation treatment using the device’s handheld remote control as per the manufacturer’s recommendations.
The patient’s chest wall and supraclavicular region were treated using a 4-field technique. The IPG was located in the superior portion of the tangent fields. Our institution has been using a hybrid intensity modulated radiation therapy technique, developed by Mayo et al,
for the tangent fields as a standard. Typically, the tangent fields consist of 2 opposing open fields, delivering 50% to 70% of the prescription dose, and 2 parallel IMRT fields, delivering the rest of the dose. In this case, approximately 50% of the dose was delivered with open fields in which the IPG was not shielded (Fig 1), and 6 fields were used for the IMRT portion of the treatment (2 of the 6 IMRT fields were parallel to the open tangent beams). The IPG (with a 1-cm margin) was completely shielded in all IMRT fields. This resulted in a plan with compromised dose coverage of the target in the vicinity of the IPG (Fig 2).
We restricted the energy to 6 MV to avoid neutron production, which has been shown to negatively affect the performance of implanted cardiac devices such as pacemakers.
Bolus was applied to the treatment area of the tangent fields.
To assess the dose delivered to the IPG, it was delineated in the radiation treatment planning system (TPS). Furthermore, optically stimulated luminescence dosimeters (OSLDs; Landauer, Inc) were used to perform in vivo dose measurements at various regions around the IPG during treatment (Fig 3). The measurements on top of the device (under the bolus) were in a region of low-dose gradient with respect to the IPG (locations C1 and C2 in Fig 3). These measurements were performed daily and showed excellent agreement with the TPS dose statistics of the IPG contoured structure (Table 1). Therefore, we were confident that the dose received by the IPG was approximately 27 Gy.
Table 1Cumulative dose to the OSLDs over 16 fractions
It should also be noted that 4 cm of the wire extension proximal to the IPG passed directly through the supraclavicular fields, at a depth of approximately 1.2 cm. This section of the wire extension would have received a dose of approximately 40 Gy based on the TPS estimates.
It was observed that the IPG was mobile under the skin. This underscores the importance of verifying the position of the device during daily treatment via setup imaging or other methods. In our case, in addition to checking the position of the device with pretreatment kV imaging, we also placed 4 OSLDs (S1-S4 in Fig 3) at the periphery of the device to ensure the device had not moved into the high-dose region during treatment. This was done by palpating the edge of the IPG during setup. The total dose of S1-S4 ranged from 28.3 Gy to 37.5 Gy. Because the OSLDs were at least 5 mm away from the device, this confirmed that the IPG was never in the high dose of the 42.56 Gy region.
At fraction #10, the patient reported that she had noted an increase in tremors that she related to situational stress around her cancer diagnosis and treatment. However, this did not significantly affect her daily routine and she did not feel any intervention was needed. Based on the in vivo measurement, the device would have received approximately 17 Gy at this time.
Her radiation treatment was completed in September 2019. She was evaluated in January 2020 by her neurologist and she reported her hand tremor was quite bothersome. The device voltage was increased from 3.1 V to 3.8 V to help with her tremor and improvement was seen immediately during that visit. The electrode and therapy impedances were assessed and were within normal limits. A subjective assessment by the patient compared with her last visit in November 2018 was unchanged.
Currently, there are no consensus guidelines on radiation exposure in patients with implanted DBS devices. Clinicians and patients are faced with balancing the need of irradiating the target volume and sparing the device. The American Association of Physicists in Medicine 1994 report set a threshold of 2 Gy, above which the pacemaker could be at an elevated risk of damage.
However, a recent report on cardiac implantable electronic devices (CIEDs) points to the stochastic nature of the CIED malfunction, unrelated to the cumulative dose. The predominant malfunction was recoverable resets to the device parameters that are not likely to cause permanent damage to the device.
a patient underwent treatment to the left lung and the mean dose to the device was determined to be 19.70 Gy. In both cases, the device was found to be fully functional during treatment and the setting was unchanged post radiation therapy. In our case, the mean dose to the neurostimulator device was 27 Gy, and the device’s voltage was increased posttreatment. However, it is unclear whether the need for device voltage adjustment for symptom control was a result of the IPG receiving a high dose. For example, in a report by Page et al,
the dose to the device was estimated to be low at 2.9 Gy, and the device voltage required a slight increase in the last 3 of 30 fractions to improve the patient’s Parkinson-related symptoms that worsened during his radiation therapy treatment.
In the face of a lack of evidence-based guidelines, clinicians will need to monitor the neurostimulator device behavior during and after radiation treatment. It is challenging to provide general clinical guidance based on limited data, but we hope that this and previous case reports highlight the need for a thorough systematic study of neurostimulator implants to help better understand the risks associated with irradiating these devices.
2017 HRS expert consensus statement on magnetic resonance imaging and radiation exposure in patients with cardiovascular implantable electronic devices.