These drugs are not always expected to cause an immediate change in the size of a tumour. It is often desirable to use molecular imaging methods, such as PET, to find out:

• if the molecular target is present in the tumour
• whether the drug changes the target

Obtaining this information in individual patients is an important part of personalised medicine.

In theory, PET is an excellent technique for testing the effects of these drugs on molecular targets as it is able to locate and measure tiny amounts of tracer molecules in the body. However, it can take a long time to develop a suitable PET tracer molecule. The radiolabelling and manufacturing process must also be acceptable for something that will be injected into patients. We have a long way to go before this becomes a routine approach.