Depending on the size and location of the tumor, it is possible that the treatments being used may not be able to gain access to the target cells. In large tumors, the central portions may be hard to reach due to limited blood supplies in the tumor. A different problem is encountered in the treatment of cancers located in the brain cavity. The brain is supplied with nutrients by a network of blood vessels. These vessels are constructed in a manner slightly different from the majority of the circulatory system. The changes in these vessels make it difficult for many different types of molecules to cross into the space surrounding the brain. The restricted movement of molecules across these vessels is termed the blood-brain barrier. For this reason, certain drugs are ineffective against brain tumors. The process of selective movement across the blood:brain barrier is shown below.(1)
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