Central venous cath. Implanted catheters are of several types. They can be completely beneath the skin, with a reservoir that is accessed through the skin with a needle. The reservoir is usually on the chest. Other catheters can be tunneled, so that the end part of the catheter is outside the skin, and the catheter is tunneled to its insertion site into a large vein.
Under the skin IV's. Unfortunately, cancer treatment requires drawing many blood tests, giving fluids and chemotherapy and at times giving antibiotics by vein. If your regular veins were used, they would quickly wear out or worse, toxic drugs could leak out into the skin around them. Placing a thin tube (catheter) under the skin that leads to a large vein simplifies this process. There are different types of catheters.