Journal of Otology & RhinologyISSN: 2324-8785

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Research Article, J Otol Rhinol Vol: 3 Issue: 2

Cochlear Implant Obstruction in Radiation Oncology

Michael S Gossman1,2*
1Tri-State Regional Cancer Center, Radiation Oncology Department, Ashland, Kentucky, USA
2Regulation Directive Medical Physics, Forestdale Court, Flatwoods, Kentucky, USA
Corresponding author : Michael S Gossman
Tri-State Regional Cancer Center, Radiation Oncology Department, 706, 23rd Street, Ashland, KY 41101, USA
Tel: 606-329-0060; Fax: 606-325-9366
E-mail: [email protected]
Received: January 15, 2014 Accepted: February 27, 2014 Published: March 10, 2014
Citation: Gossman MS (2014) Cochlear Implant Obstruction in Radiation Oncology. J Otol Rhinol 3:2. doi:10.4172/2324-8785.1000149

Abstract

Cochlear Implant Obstruction in Radiation Oncology

Purpose: Concerns exist as to the impact on therapeutic radiation when dense implantable cochlear implants are kept in the field during treatment. With cochlear implant hearing assist devices increasing in clinical demand, more are discovered present in cancer patients. The effect on the electronics and operation of the cochlear implant was not intended to be addressed here. This research was designed to specifically investigate the magnitude of change exhibited in a radiation beam when a cochlear device remains implanted during delivery. Considered here are typical therapeutic x-ray beams of 6 and 18 MV as well as electron beams of 6, 9, 12, 16, and 20 MeV directly aimed through 3 common cochlear implant Models from MED-EL: Sonata, Concerto and Pulsar.

Methods: It was the aim of this research to determine how the radiation dose is changed around the cochlear device when placed in a phantom environment. Radiation detectors including diode arrays, film and ionization chambers were positioned to measure the change in dose delivery with and without the cochlear implant present, thus giving rise to attenuation and scatter results.

Keywords: Attenuation; Cochlear; Implant; Radiation; Oncology

Keywords

Attenuation; Cochlear; Implant; Radiation; Oncology

PACS

87.56.bd; 41.50.+h; 87.57.Q-; 87.55.Gh; 87.55.K-; 29.40.-n; 87.53. Bn; 87.85.eg; 87.55.-x; 87.55.-x; 87.53.Bn

Introduction

Cochlear implants have been in existence for a half century. It was in 1961 that the first two cochlear patients received their implantable device in a short-term clinical trial [1]. Today, 66,000 are implanted in the United States alone, with three times that implanted worldwide [1]. A principal leader in cochlear implant technology since 1989 is MED-EL (Innsbruck, AUSTRIA) [2]. They report an increase in the number of annual inquiries from physicians treating cancer patients with radiation while having an implanted cochlear device. Clinical radiation research concerns are scarce in this area [3]. This examination is designed to address radiation oncology interests.
There is only a hand full of articles dealing with radiation therapy to patients with intact cochlear devices found in literature. Only a few of them deal with effects to the cochlear device in vitro [4-7]. The investigational research was designed to conversely address the effect on the radiation treatment beam when a cochlear implant device remains within the treatment field. The research presented here is specific for MED-EL currently marketed devices. Similar studies have been published for a variety of implant device types [8-13]. Ventricular assist devices were considered in 2012 for x-ray beams, where up to 84.0% dose obstruction was proven [12]. Neurostimulators were investigated from 2011-2012. Dose blockage was measured to be 11.0% at 6 MV and 5.6% at 18 MV for the vagus nerve stimulator [11]. Finally, implantable heart rhythm devices were studied in 2010. X-ray dose changes were noted up to 9.2% for pacemakers and 8.8% for defibrillators by attenuation, notably less at higher x-ray energies [13]. Cochlear implants were examined for one manufacturer in 2011. A report based on x-ray radiation revealed up to 8.8% at 6 MV and 6.6% at 18 MV were achievable by attenuation [8]. It remains to be seen how radiation interacts with cochlear implant devices with a concluded relative impact from all typical clinically available beam energies and modalities from a linear accelerator, including electrons.
Linear accelerators have been engineered to provide deep radiation treatment in a patient using various bremsstrahlung x-ray energies, generally from 6-18 MV. More superficial treatments are generally prescribed by the radiation oncologist from any of a variety of electron particulate energies, which are typically 6-20 MeV. The aim of this investigation is to directly measure the change in the distribution of dose around MED-EL cochlear implants from x-rays and electrons. It is expected that attenuation will be exhibited when the beam was directed through the cochlear implants. This is observed as a consequential reduction of dose beyond the cochlear device. Scattering processes backward and lateral were also to be directly measured. However, increases in dose for these areas are likely.
There is significant interest in ascertaining attenuation and scatter information for each model, especially for all typically available beam types, since any combination of the two could lead to very different dose distributions. The distribution of dose and physical characteristics of intensity change are different for each beam energy and modality. In the clinic, the prescribing physician will ideally seek to obtain a uniform distribution of dose around the targeted cancerous tumor. Unwanted inhomogeneities in the dose distribution caused by intact electronic devices may compromise local control of the disease [14]. Tabulated results for each cochlear implant device, at each beam type and energy may then provide the desired information needed to clinically judge the distribution of dose around the tumor that is impacted by the cochlear device. This phantom experiment was created to assist in the treatment planning process as an evaluation tool for projected change and prospective treatment guidance.

Materials and Methods

The phantom environment construction began first with a setup to enable attenuation measurements. The linear accelerator chosen for use is the Varian Medical Systems (Palo Alto, CA) Model 21EX. Available energies include 6 MV and 18 MV x-rays as well as electron energies of 6 MeV, 9 MeV, 12 MeV, 16 MeV and 20 MeV. First, a Sun Nuclear Corporation (Melbourne, FL) Map Check 2D silicon diode array containing 445 diodes imbedded in 2.0 cm water equivalent plastic was positioned on the treatment couch. A Kodak (Rochester, NY) EDR-2 Ready Pak film sheet was then placed on top of the diode array. A thin plastic container with a radius of 7 cm and height 2.5 cm was then filled with water to 1.0 cm depth. A cochlear implant was positioned in the water phantom for irradiation. The cochlear implant Models donated by MED-EL include the Pulsar (S/N: 126180), Sonata (S/N: 215938), and Concerto (S/N: 525044). The devices are shown in Figure 1.
Figure 1: MED-EL cochlear implant Models from left to right are the Pulsar, Sonata, and Concerto.
Care was taken to insure that the same water level existed for each port as they were interchanged. The linear accelerator was programmed to deliver a down-ward directed beam for an absorbed dose to water of 100 cGy and at a dose-rate of 600 MU/min, all depth of maximum dose specific for each energy and radiation type. A symmetric field size of 10x10 cm2 or alternatively 20x20 cm2 was used for all measurements. A source to implant distance of 96.0 cm was used for these measurements.
The beam was intended to pass through the water phantom containing the cochlear implant, then through the film, and finally through the diode array. Data for the attenuation measurements were thus obtained both at a depth directly below the cochlear implant to film at 0.2 cm away, as well as at the depth of the diode array 2.0 cm further downstream. With the high density cochlear implant in the water phantom, the radiation dose is expected to be less than when it is removed. Film was used to determine the attenuation exhibited in the cochlear implant by the 6 MeV electron beam only. It was known a priori that only 42% of the 6 MeV electron beam intensity remains at a distance of 2.2 cm downstream at the diode array, and under a strong declining depth gradient, based on clinical scanning already done. Film was well within the range of the 6 MeV electrons at only 0.2 cm from the cochlear implant. Therefore, the diode array was used for both 6-18 MV x-ray beams and 9-20 MeV electron beams, whereas film was used for the 6 MeV electron beam attenuation measurements only. The amount of beam attenuation was measured as the quotient of measurements with and without the cochlear implant present [14]. Film data required post-processing following development. Diode array data were collected real-time using Sun Nuclear Patient software Version 6.2.0. The experimental set-up for attenuation is provided in Figure 2.
Figure 2: Experimental setup for x-ray attenuation measurements; Pulsar visible in the phantom.
Back-scatter measurements then followed with a required modification to the treatment couch of the linear accelerator. Since scatter measurements involve an upward directed beam, it is inappropriate to pass the beam through the couch material to retain electron beam intensity. If unaccounted for, a substantial loss of beam intensity from table attenuation would limit dose results to only higher energies of electrons. Instead, the table top was removed and a thin sheet of clear plastic was wrapped around the couch stabilization rails to form a flat surface area. A single sheet of Ready-Pak film was placed on top of the tightly secured clear plastic. Next, the water phantom was placed directly on top of the film and then digitally leveled. With an upward incident beam, the radiation was aligned to pass through the clear plastic sheet, then through the film, and finally through the water phantom containing the cochlear implant. Backscattering is identifiable when there is a difference in the readings in the presence of the cochlear implant versus when it is removed [14]. Post processing of film was required for this data analysis. With the film at a mere 0.2 cm from the base of the cochlear implant, all backscatter measurements were taken at that distance. The same linear accelerator programming and geometry was employed, except for inverting the gantry to beam upward. All x-ray and electron beam intensities were measured on film for back-scatter identically.
Side-scatter data is acquired simultaneously with back-scatter measurements. In order to measure the dose in water alongside the cochlear implant, the detector must be water-resistant and very small. The PTW (Freiburg, Germany) Model TN31014 ionization chamber was ideally suited for this purpose with a sensitive volume of 0.015 cm3 and “pin-point” style. Real-time data acquisition was made possible with a connected Capintec, Inc. (Ramsey, NJ) Model 192 electrometer, which polarized the ion chamber to +300 V at the center pin. The ionization chamber was positioned to a distance of 0.2 cm from the middle side of the cochlear implant. As before, each cochlear implant was consecutively placed in the water phantom and then removed following radiation delivery. All x-ray and electron beam exposures were measured with the ion chamber for side-scatter similarly. The set-up for scatter measurements is depicted in Figure 3.
Figure 3: Experimental setup for electron back-scatter without chamber inserted yet for side-scatter; Pulsar visible in the phantom.
All post-processing of film was conducted using an AFP Imageworks (Elmsford, NY) Mini-Medical Model 90 automated film processor. Optical density was then meticulously measured with an X-Rite (Grand Rapids, MI) Model 301 transmission densitometer. All film analysis was correlated to Hurter & Driffield film calibration curves measured subsequently for each beam energy and radiation type [15,16].

Results

Diode array measurements revealed a distinct change in the dose uniformity when each cochlear implant was in the way of the beam. Without any implant present, the square beam should have a relatively uniform dose distribution in the center. With the inclusion of the dense cochlear implant, Compton interaction processes between the beam and the implant reduce the intensity. When plotted, a large spike is present in the center of the field. This represents the maximum change exhibited when the radiation field intensity for an open beam is compared to the beam intensity when the cochlear implant is present. Data from the diode array with the cochlear implant present were taken in ratio to the data with it absent to arrive at the attenuation result. The usable reading was taken as the raw reading less the level of background present for each diode independently. While this was true for 6-18 MV x-rays and 9-20 MeV electrons, film was processed for the attenuation observed in the 6 MeV electron beam. The usable film reading from the point densitometer was first taken as the raw reading less the level of base/fog seen in the film. A manual scan was conducted to determine the maximum result by dragging the point along a plane of greatest dose change. The net optical density was then converted to dose using the Hurter & Driffield curve found prior for doses in the range of 50-400 cGy for the 6 MeV beam. All dose results were taken in quotient with the results for films at the same spatial point, when no cochlear implant was present in the field [16]. As illustrated for the Model Sonata in Figure 4 6 MV x-rays and in Figure 5 9 MeV electrons, the uniformity of the field intensity is significantly altered. The profiles depicts the measured -6.9% x-ray attenuation and the -47.1% electron beam attenuation by the diode array, which was further downstream. This is depicted in Figures 4 and 5 for the isodose distribution in 2D as well as in line-dose profiles along the maximum gradient of the y-axis in Figures 6 and 7. The thick red line present along the Y-axis of Figures 4 and 5 represents the data for the line-dose profiles plotted in Figures 6 and 7.
Figure 4: MED-EL cochlear implant Model Sonata attenuation measurements for 6 MV x-rays shown in 2D with device overlay.
Figure 5: MED-EL cochlear implant Model Sonata attenuation measurements for 9 MeV electrons shown in 2D with device overlay.
Figure 6: MED-EL cochlear implant Model Sonata attenuation measurements for 6 MV x-rays shown on a line-dose profile.
Figure 7: MED-EL cochlear implant Model Sonata attenuation measurements for 9 MeV electrons shown on a line-dose profile.
Back-scattering was calculated identically for all beams as was discussed before. Then, with each beam having a different energy, a complete set of independent Hurter & Driffield curves was used for each x-ray and electron beam having a different energy. The process was similarly conducted as mentioned with a densitometer to determine the maximum change exhibited. Side-scattering involved the minimal ratio of raw readings from the ionization chamber with and without the cochlear implant present. With readings taken in short intervals of time, no attention was made necessary to ambient atmospheric temperature, pressure and humidity corrections. The resulting data are presented in Table 1.
Table 1: MED-EL cochlear implant attenuation and scattering results.

Discussion

Maintaining a cochlear implant or speech processor in the field of radiation is not recommended by MED-EL. However, user guidance documents indicate that the MED EL Pulsar, Sonata, Concert (and Concert PIN) cochlear implants are robust against a 6 MV pulsed photon beam from a linear accelerator up to an absorbed dose of 240 Gy [17]. As a simple solution to patients in need of diagnostic imaging involving x-rays or cancer therapy from a particle accelerator from all sorts of beam modalities, the recommendation is to have the patient switch off and remove their speech processor. Possible interference with cochlear device electronics have only been theoretically predicted based on published research on other implantable electronic devices [11].
X-ray attenuation changes exhibited by these cochlear implant models resemble the values published for implant models of other manufacturers as discussed in the Introduction earlier [8]. In contrast, results were less than those seen for neurostimulators, heart rhythm devices and ventricular assist devices, which are in order of least to most attenuation in literature [11-13]. However, electron beam attenuation results reveal the significance of the thickness and density of the object in relation to the incident radiation type. Electron beams were shown to penetrate the cochlear implant better, by contrast, than x-ray beams through a ventricular assist device. In either scenario, the results are significant and bar caution when clinically relevant to patient care.
The research presented here was designed to assist the radiation oncologist and medical physicist in determining dosimetric consequences when megavoltage x-ray and electron beams are directed through devices. The significance of the research is first to identify attenuation and scatter information that does not exist in literature elsewhere. The investigation addresses concerns from in-calling clinicians, where there have been reported times when the patient or radiation worker has forgotten to remove the device prior to delivery. The dosimetry data provided enables guidance to the cancer treatment team about what effects the device had on treatment field doses retrospectively. As such, these results apply to all cancer patients where the location of the cochlear implant device is either directly in the field of radiation or immediately adjacent to it within 3-5 cm.
This research also offers a unique perspective for the purposes of future product research and development. The advancement of cochlear implant electronics and engineering mechanisms may be instrumental in producing a device model with minimal characteristics of attenuation and scatter. This aim too may yield cochlear devices that exhibit a tolerance to withstand certain doses, dose-rates, energies, and types of ionizing radiation.

Conclusion

Considered here were typical therapeutic x-ray beams of 6 and 18 MV as well as electron beams of 6, 9, 12, 16, and 20 MeV directly aimed through 3 common cochlear implants Models from MED-EL: Sonata, Concerto and Pulsar. The effect on the electronics and operation of the cochlear implant was not addressed. Rather, this research investigates the magnitude of change exhibited in a radiation beam when a cochlear device remains intact during delivery. The inclusion of a cochlear implant in a therapeutic beam is not recommended due to the high clinical impact in reducing beam intensity and altering dose distributions. Attenuation and scatter for photon beams should be considered just as important as for particulate radiation, even if physical interaction processes are generally substantially greater for incident electrons than for x-rays. Dose distribution effects should be considered clinically significant at all typical linear accelerator energies.
Here, the MED-EL Model Sonata exhibited the greatest dosimetric change, with the Model Pulsar showing the least. Significantly high attenuation was observed in all cochlear implant models for electron beams along with notable scattering processes. X-ray beam attenuation ranged from -1.9% to -6.9% while electron beam attenuation ranged from -14.9% to -50.9%. Greater attenuation was exhibited at lower energies for both modalities. Back-scattering varied between 2.5% to 7.6% for x-rays and 1.9% to 12.5% for electrons. Side-scattering was measured with less magnitude at 0.3% to 1.5% for x-rays and 0.6% to 3.1% for electrons.
During radiation therapy treatment planning, Radiation Oncologists should weigh the risk of potential and unknown damage to the cochlear implant as well as the substantial dose change that results for all devices and beam types presented here. Dose nonuniformity is to be expected if a cochlear device is in the direct field of radiation or in its penumbra. Computerized treatment planning systems with heterogeneity correction should be used to simulate the direction of such consequences prospectively. Unknown effects to the device and dose non-uniformity can be substantially mitigated by redirecting beams to alternative beam angles around the cochlear implant with a 3-5 cm margin. Unaccounted for non-uniformity in dose delivery may result in a clinical under-dose of the targeted area. Under-dosing can compromise tumor control.

Acknowledgments

The opportunity to study each of the cochlear implants was supported by the issuance of donated devices from MED-EL Elektromedizinische Geraete GMBH.

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