A thickened endometrium on a post menopausal lady mean always cancer?

Not always: Not always, but certainly would be suspicious for cancer-and further evaluation- i.e. Biopsy should be done to rule in/rule out. Biopsy is the only thing that can tell you for sure whether it is malignancy or not.

Endometrium: Thickening of endometrium can have bengin or malignant causes and we have to evaluate the clinical history, diagnostic imaging such as ultrasound and do a physical +/- biopsy to reach a diagnosis.