A tumour capsule is a layer of tissue that separates a tumour from the surrounding, normal tissue. A capsule may be present around both benign (non-cancerous) and malignant (cancerous) tumours. However, not all tumours will be separated from the surrounding normal tissue by a capsule. Tumours that do not have a capsule are sometimes described as non-encapsulated.
Capsule invasion is the term that pathologists use to describe the movement of tumour cells through the capsule into the surrounding, normal tissue. In some types of tumours, capsule invasion increases the risk that the tumour will behave like cancer and that tumour cells will spread to other parts of the body. This happens because tumour cells that spread into normal tissue can then enter blood vessels and specialized channels called lymphatics. This process is called lymphovascular invasion and it increases the risk of developing metastatic cancer.
In some tumours that start in the thyroid gland, pathologists look for a capsule invasion to decide if a tumour is benign (non-cancerous) or malignant (cancerous). For example, a follicular adenoma is a benign (non-cancerous) thyroid tumour that is surrounded by a capsule. However, if capsule invasion is seen, the diagnosis changes to follicular thyroid carcinoma. Other types of thyroid tumours that frequently have a capsule and do not show capsule invasion include Hurthle cell adenoma and non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP).