Merck Manual

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Pet Owner Version

Spread of Cancer


Manuals Staff

Reviewed/Revised May 2020 | Modified Nov 2022

Once a cell has turned cancerous (malignant), it starts growing and spreading. Because there is no control on the growth of these cells, the cancer will often take advantage of any available path to spread and find new places to grow. This process is called metastasizing. The 2 most common paths for cancerous cell spread are the lymphatic system and the bloodstream. The lymphatic system is made up of small vessels that collect the fluid surrounding cells and return that fluid to the bloodstream. Carcinomas are the malignant tumors formed from the epithelial cells lining the internal and external surfaces of the body. Carcinomas usually spread through the lymphatic system. Sarcomas are malignant tumors formed from bone, cartilage, fat, or other connective tissues. Sarcomas usually spread through the bloodstream.

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