Aortic Dynamics after Endovascular Aorta Repair

Thoracic endovascular aortic repair (TEVAR) is the mainly used treatment for
thoracic aorta pathologies (TAP) due to its less invasiveness compared to the
conventional open surgical aneurysm repair. *Regular* TEVAR is performed in
pathologies of the descending thoracic aorta, but when the aortic arch is
included in the pathology hybrid-TEVAR with a frozen elephant trunk (FET) or
branched-TEVAR (BTEVAR) may be performed as well. Different types of endografts
are developed for different procedures, including personalization options to
fit best to the anatomy of each patient. Endografts of interest in the present
study are the Relay(Branch) and Thoraflex Hybrid. Once implanted, the aorta
dynamics and the device affect each other in ways that are currently not
understood. Pre and post-operative imaging of aortic aneurysm is routinely
performed using computerised tomographic angiography (CTA). However, these
static techniques do not consider the aorta dynamics. Consequently, our
understanding of the dynamic behaviour of the stent graft and stented target
vessels is limited. Electrocardiogram (ECG)-gated CTA is a technique that takes
the patient*s heart cycle into account, enabling studies to the motion of aorta
and implanted devices.

Gaining information on the dynamics and shape of the stent graft and stented
target vessels, and how these change over time will improve our understanding
about the fixation and/or sealing of the stent graft, which may help in stent
graft selection and in designing stent grafts that are more durable.

Explorative observational cohort study with patients with an thoracic aorta
pathology undergoing repair
with the Thoraflex Hybrid graft (Hybrid-TEVAR) or the Relay(Branch) graft
((B)TEVAR).
Included patients will undergo ECG-gated CTA preoperatively and at set points
during follow-up to investigate geometry and dynamic changes over time and
during the cardiac cycle.

The main risks of patients during follow-up after a TEVAR procedure are, with
or without inclusion in this study, the increased radiation exposure due to
repeated (ECG-gated) CT scans and the applied contrast fluid. The
administration of contrast fluid can be nefrotoxic and will for these ECG-gated
CTA's not be applied in case of kidney dysfunction (eGFR <30 ml/min) or
allergy.

Clinical guidelines today advice CT scanning for follow-up of patients that
underwent (hybrid/B)TEVAR. Therefore, in the next calculations we only consider
the only the extra radiation exposure that an ECG-gated yields in comparison to
a regular CT scan. In some institutions ECG-gated CT is even used for default
follow-up after hybrid or endovascular treatment of aortic pathologies.

The scan protocol for the ECG-gated CTA scans of the present study is
specifically designed for this study with input of the PI, the coordinating
investigator, CT technologists and a medical physicist. The radiation dose
exposure following an ECG-gated CTA according to this scan protocol was
estimated during a phantom test on the SIEMENS Somatom Definition Flash CT
scanner at the Medisch Spectrum Twente hospital. An Alderson Phantom (Radiology
Support Devices Inc.) was used to estimate the radiation dose for a regular
patient. From a practical point of view we maintained the mammae on the
phantom, realizing an overestimation of the effective dose (E) for male
patients, while the majority of the patients are expected to be male. Two scan
trajectories were scanned: the maximal preoperative scan trajectory (vertebrae
C3 to vertebrae L2), i.e. an overestimation of the expected average radiation
exposure and an average scan trajectory by diaphragmation. The E then varies
between 8.9 resp. 10.6 mSv per scan (DLP: 636-758 mGy.cm), resulting in a total
extra dose for the 4 scans (1 preoperative, 3 postoperative) of 39 mSv.
Pre-operatively the ECG-gated scan will in most cases be extra; postoperative
(before discharge, 6-8 weeks postoperative and 12 months postoperative) will be
instead of a regular CTA. The regular CTA has an E of ~2.0 mSv (simulated with
same test set-up as ECG-gated CTA). In total, a patient would then obtain a
(maximal) E of 39 - 3*2.0 = 33.0 mSv extra when participating in this study.

Taking into account the ALARA ("as low as reasonably achievable") principles,
several points were made: first, the exact scan trajectory will be set for each
individual patient (in practice this would be approximately preoperatively
C3-T7 for stent grafts in the aortic arch and C6/7-T12/L1 for stent grafts in
the descending aorta; postoperatively the scan trajectory will be 3 cm above
and 3 cm below the stent graft). Consequently, the maximal scan trajectory in
the calculations above would not be reached. Also, the scan protocol is in a
way that the dose will be optimized for each patient: a skinny patient will
receive a smaller dose than an obese patient.

Furthermore, the included patients are 65 years and older. Based on the ICRP
62, a correction factor can be used of 1/5 to 1/10 for patients older than 50
when categorizing them into a risk/benefit category. Based on age and average
life expectancy of this patient population, a correction factor of 1/7.5 was
chosen. The corrected total extra radiation exposure for the patients for this
study then would be 33.0/7.5 = 4.4 mSv. This corresponds to risk/benefit
category IIb, i.e. the chance of developing a radiation induced cancer for
these patients would be of magnitude 1 in 10,000. The level of social benefit
schoul be "intermediate to moderate". As was also stated elsewhere, previous
studies with ECG-gated CT scans in several endovascular aorta reperations has
led to multiple recommendations in the placement of the devices and even
alterations in the instructions for use, and thereby improved the treatment
type(s). With the present study we expect this as well.