Evolution of Flow-Diverter Endothelialization and Thrombus Organization in Giant Fusiform Aneurysms after Flow Diversion: A Histopathologic Study

Giant, partially thrombosed, fusiform aneurysm of the left MCA treated with a construct of 2 Pipeline embolization devices; the specimen was removed 6 months after PED implantation. A, T2-weighted MR image before treatment, showing large mass of mixed signal intensity, associated with significant mass effect and white matter edema, consistent with a giant aneurysm. B, DSA of the same aneurysm. The arrow points to the proximal, normal portion of the M1 section. and the double arrow points to the fusiform aneurysm expending into the M2 sections. C, Follow-up DSA 6 months later showing “angiographic reconstruction” of the distal M1 section (arrow), significant enlargement of the dilated proximal section of the cranial M2 branch (bent arrow), and lack of filling (occlusion) of this branch distal to the dilation. The PED construct can be seen between the 2 dotted arrows. D, Follow-up T2-weighted MR image from 6 months after treatment showing unchanged mass effect, edema, and mixed signal intensity. E, Longitudinal cut of the proximal landing zone. The luminal surface of the PED is covered by a smooth tissue layer. F, Microscopic section of the layer removed from the luminal surface of the PED showing neointimal growth consisting of smooth muscle cells (h-Caldesmon staining). G, The same layer is covered by a single cell layer of endothelium (arrow) (CD34 staining). H, Macroscopic cross-section of the specimen at the level of the fusiform aneurysm. The implanted PED construct (arrow) is uncovered and surrounded by fresh clot (bent arrow).