Evaluation of a Ventricular Assist Device: Stability Under X-Rays and Therapeutic Beam Attenuation

Improved outcomes and quality of life of heart failure patients have been reported with the use of left ventricular assist devices (LVADs). However, little information exists regarding devices in patients undergoing radiation cancer treatment. Two HeartWare Ventricular Assist Device (HVAD) pumps were repeatedly irradiated with high intensity 18 MV x-rays to a dosage range of 64-75 Gy at a rate of 6 Gy/min from a radiation oncology particle accelerator to determine operational stability. Pump parameter data was collected through a data acquisition system. Second, a computerized tomography (CT) scan was taken of the device, and a treatment planning computer estimated characteristics of dose scattering and attenuation. Results were then compared with actual radiation measurements. The devices exhibited no changes in pump operation during the procedure, though the titanium components of the HVAD markedly attenuate the therapy beam. Computer modeling indicated an 11.8% dose change in the absorbed dosage that was distinctly less than the 84% dose change measured with detectors. Simulated and measured scattering processes were negligible. Computer modeling underestimates pretreatment dose to patients when the device is in the field of radiation. Future x-ray radiation dosimetry and treatment planning in HVAD patients should be carefully managed by radiation oncology specialists. ASAIO Journal 2012; 58:212-216.

The implantation of continuous flow left ventricular assist devices (LVADs) for support of heart failure patients has seen a remarkable growth in the last decade. Patients have enjoyed improved outcomes and quality of life from advances in this technology.1-3 Contraindications for implantation of an LVAD in treatment enrollment include patients undergoing cancer treatment. However, discovery of malignant cancers may occur after implantation of the device. Currently, there is minimal information on the effects of radiation exposure on device operation. As this population segment increases, radiation oncologists will have to be prepared to treat potential cancer patients who have an implanted LVAD.

The interest of this study was twofold. First, it sought to address whether any electronic instability in the HeartWare Ventricular Assist Device (HVAD) pump (HeartWare, Inc., Miami Lakes, FL) could be measured when directly irradiated from megavoltage x-rays provided by a radiation oncology particle accelerator. Previous research has seen instability for devices such as implantable pacemakers and cardioverter- defibrillators (ICDs).4,5 Second, a computed tomography (CT) scan was taken of the device and a treatment planning computer6-8 estimated characteristics of dose scattering and attenuation of the device. Radiation measurements were then compared with the results modeled on computer. The results from these studies are aimed at supporting radiation oncology specialists in treating patients implanted with the Heart- Ware HVAD pump. (The HeartWare HVAD is currently an investigational device restricted by US law to investigational use only.)

Materials and Methods

Two HeartWare HVAD pumps were tested for device stability and therapeutic beam attenuation. The HVAD pump is a continuous flow mechanical assist device implanted in heart failure patients as a bridge to transplant or in some cases as long-term destination therapy (DT). The pump is directly inserted into the patient's failing ventricle and provides centrifugal flow through a wide-blade impeller design. The Heart- Ware ventricular assist system comprises the implanted HVAD pump, which is connected through a percutaneous driveline to a system controller powered through two power sources.9 The testing chamber was composed of a CNMC Company, Inc. (Nashville, TN) model WP-3040 tank that was filled to a depth of 20 cm with water. Stacked plates totaling 8 cm of acrylic were then adhesively affixed to the bottom surface of the tank. The HVAD pump was then immersed in the water phantom and strapped to the platform at a depth of 12 cm.

Stability Testing

The testing chamber was placed on the table of the Varian Medical Systems, Inc. (Palo Alto, CA) model 21EX particle accelerator. The gantry arm was rotated to an incident angle of 270°, simulating the typical positioning expected for an implanted patient. Radiation entered through the side of the phantom laterally toward the pump at a distance of 12 cm. The distance from the source to this location was 100 cm, as verified by in-room lasers mounted on the vault walls. Radiation dose was measured at this point. The particle accelerator was programmed to deliver 18 MV pulsed x-ray radiation at a dose rate of 6 Gy/min. A single beam, having a 30 x 30 cm2 aperture, was applied. The HVAD pump was connected to the system controller and powered with two battery sources. The pump was programmed to operate under normal operating guidelines and was set to a speed of 2,400 RPM. Pump parameters including power, speed, and estimated volumetric flow rate were collected and analyzed with a custom clinical data acquisition system at a frequency of 50 Hz.

Stability testing was conducted at doses deemed clinically relevant in application to lung cancer patients, as this would likely present a larger challenge in treatment planning than would other sites of disease occurrence. Lung cancer treatment prescriptions differ depending on the classification and staging of the disease. A typical non-small-cell lung cancer patient staged 1b (T2aN0M0) may be given 66 Gy, whereas one with the same disease type staged X (T2N0M0) may be prescribed 74 Gy. Common dosages delivered to lung cancer patients in x-ray radiation treatments are 60-74 Gy. The setup for the stability testing is presented in Figure 1.

Computer Modeling and Treatment Planning

The same testing chamber was used for treatment planning. The HVAD pump was disconnected from the system controller and was not powered for this phase of testing. The pump was scanned and radiation measurements were taken as described. Computer modeling results from the treatment planning system were compared with actual measurements.

CT scan acquisition. Computed tomography scan acquisition was conducted using a General Electric (Fairfield, CT) LightSpeed RT scanner. The helical mode stereotactic radiosurgery technique included 120 kVp x-rays at 340 mA for 86 seconds. A total of 254 scan slices defined the image set sent to the treatment planning system. Each slice was spaced 1.25 mm/slice and corrected for extended Hounsfield unit range.10

Dose computation. Varian model Eclipse treatment planning system, version 8.6 (HeartWare, Inc., Miami Lakes, FL) with the anisotropic analytical algorithm was used for dose modeling. This software is used clinically as a means of estimating dose delivery before irradiation through measurement from the 21EX particle accelerator. Approximations to the dose delivery are illustrated within the software for a patient's CT anatomy. Similarly, the water phantom CT scans containing the HVAD pump model an implanted patient. Image artifacts were removed using Boolean operation as described in the literature. A single 18 MV x-ray beam having gantry angle 270° was applied as used previously in electronic stability testing. The aperture was 30 x 30 cm2 with a dose rate of 600 MU/min. The isocenter chosen for dose calculation was the location where the device driveline enters the pump. Dose calculation points were dispersed around the device at distances varying from 1 to 3 cm away. Two types of dose computations were performed. First, a 3D calculation was made assuming all media to be composed only of water. Then, the computer was set to provide dose results with density correction applied so as to account for the interaction of the submerged HVAD pump with the incident x-rays. The ratio of absorbed dose at each point in the two plan types gives rise to the overall effect of having the pump in the beam. Dose data from points interiorly more near to the beam represent backscattering, whereas lateral points represent side scattering. Finally, point doses determined beyond the HVAD pump detail attenuated intensity.

Radiation measurement. Measurements were performed for scatter dose using a similar phantom setup. With the HVAD pump submerged, point measurements were taken using a Wellhofer-Scanditronix (Bartlett, TN) Farmer-type ionization chamber model TN31014. The thimble chamber was connected to a CNMC Company model 206-110 electrometer with charge range element that equilibrated the chamber to nominally +300 V at the center-pin. Having a sensitive volume 0.015 cm3, the water-resistant "pin-point" detector was placed at each of the planned point locations near the HVAD pump at scattering locations upstream and laterally. The Sun Nuclear (Melbourne, FL) model MapCheck provided attenuation measurements beyond the pump. The MapCheck 2D diode array contains 445 N-type diodes, all embedded in 2 cm of water-equivalent plastic and variably spaced 22 x 22 cm2 in area. As the diode array cannot be placed in water, a setup change was necessary. The acrylic plates were removed from the water phantom entirely. Then the HVAD pump was set on the bottom of the tank. The tank was raised and leveled immediately on top of the MapCheck diode array. The HVAD pump was reoriented in the direction of a beam set to be 180°. All attenuation measurements were taken at 3 cm distance from the HVAD pump. Pulsed radiation was given at a dose rate of 6 Gy/min identically.

Results

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