The dose deposited in a volume behaves like a piece of clay, or a balloon. Initially, the dose required to irradiate the tumour should be kept to a minimum. After that, there is large freedom of shaping the (high) dose in various shapes, with almost constant volume. This is called dose-shaping.
The main problem here is: what should the dose look like? As demonstrated in the following example, there are many possible alternatives.
Trick question: with which plan should the patient be treated? This one is not an easy one to answer. Radiotherapy is a multi-criteria problem. In the presented example, the main criterion is the tumour, but the 7 healthy organs are also part of the optimization problem (aside from several others, like conformality and homogeneity). It is obvious that it is impossible to achieve the best for each individual criterion (all dose in the tumour, no dose at all to the surrounding organs). So, the main question is: how to treat the patient and thereby minimizing the development of radiation-induced complications?
Answer: there is no easy answer. Those decisions are usually handled by the treating physician, based on historical data, experience and sometimes measurements. For us non-physicians, there are 2 challenges: