Answer: Reactive vascular proliferation related to a prior FNA

Histology: The nodule is comprised a cellular proliferation of Hurthle cells. The Hurthle cell proliferation is completely confined by a fibrous capsule without any evidence of extension into the tumor capsule. The capsule and adjacent thyroid parenchyma are involved by a peculiar vascular proliferation. The character of the vascular proliferation ranges from gaping vascular channels with intraluminal protrusions of endothelial-lined papillae to slit-like vascular spaces surrounded by a cellular collar of spindled cells.

Discussion: Iatrogenic manipulation of thyroid nodules including fine needle aspiration can induce histologic alterations of the thyroid. These changes are sometimes referred to as “WHAFFT” effect (worrisome histologic alterations following fine needle aspiration of the thyroid) and include tumor infarction, tissue injury and repair, and epithelial displacement. Of these, epithelial displacement through the tumor capsule is most notorious for the way it mimics invasive tumor growth (i.e. pseudoinvasion). Less well appreciated are some of the reactive vascular changes that can be induced by a traumatic FNA. When well developed, these changes can be mistaken for a vascular neoplasm such as angiosarcoma. The vascular changes related to WHAFFT generally occur in association with other FNA-related tissue alterations (e.g. fibrosis, hemorrhage, hemosiderin deposition), include a component of papillary endothelial hyperplasia, and lack any significant cytologic atypia.