Which of the following helps in differentiation of follicular carcinoma from follicular adenoma of thyroid gland?
Robbins clearly write…. ‘Microscopically, most follicular carcinomas are composed of fairly uniform cells forming small follicles. Follicular carcinomas may be grossly infiltrative or minimally invasive. The latter are sharply demarcated lesions that may be impossible to distinguish from follicular adenomas on gross examination. This distinction requires extensive histologic sampling of the tumor-capsule-thyroid interface, to exclude capsular and/or vascular invasion. Extensive invasion of adjacent thyroid parenchyma makes the diagnosis of carcinoma obvious in some cases’.
Ideal answer for a question for diagnosis of follicular cancer is capsular invasion (better than even vascular invasion) but in the given options, vascular invasion is the answer of choice.
Follicular Carcinoma: brushing up key points

It is the 2nd most common form of thyroid cancer
Seen in women of older age (40-50 yrs.)
Vascular invasion is common (less lymphatic spread) to bone, lung, liver etc.
Microscopically, there is presence of cells forming small follicles having colloid with NO Psammoma bodies. Uncommonly, cells have abundant, eosinophilic cytoplasm called as Hurthle cells
Differentiation from follicular adenoma is based on the presence of capsular invasion preferably and not on vascular invasion.
Hurthle cell change
Lining of tall columnar and cuboidal cells
Vascular invasion
Increased mitoses