**3. Intensity-modulated radiotherapy**

As discussed, IMRT has commonly used for radiotherapy of head and neck cancers The concept of IMRT has been introduced as early as 30 years ago [29], when the method of optimizing the intensity distribution of the incident beams with the purpose to achieve the required dose distribution in the targets was described. The following points summarize the concept of the delivery of IMRT: (1) There are multiple radiation beams with specially decided nonuniform intensity in beamlets, also known as intensity modulation. (2) The multiple radiation beams are applied from different directions, and the region of the convergence of the beams can achieve the desired dose distribution based on the modulated beam intensity. (3) Calculation of the modulated beam intensity usually follows an inverse approach, in which the final dose distribution indicated by planners is used by the computer to calculate the intensity of each beamlets in the treatment field of the IMRT plan.

The delivery of intensity-modulated beams is largely contributed by the dynamic multi-leaf collimator (MLC). The MLC can change the field shape automatically and the summation of numerous sub-fields in different shapes then generate a field with intensity modulation. A simplified rationale of intensity modulation is illustrated in **Figure 1**. Assume there is no OAR surrounding the target, the intensity of the beam should be proportional to the target thickness from the perspective of each beam. Although beam modifying devices such as wedges and compensators have been used in 3DCRT, their flexibility of beam intensity modification is far less than that in the

*Treatment of Head and Neck Cancers Using Radiotherapy DOI: http://dx.doi.org/10.5772/intechopen.103678*

**Figure 1.** *Illustration of the relationship of beam intensity and target thickness.*

IMRT. This is best illustrated by the fact that IMRT can produce concave shape isodose distribution which 3DCRT can hardly generate. The freedom of intensity modulation has a great impact on the dosimetric superiority of IMRT, in which better target coverage and less dose to the OARs can be achieved.

The superiority of IMRT over 3DCRT is illustrated in **Figure 2**, which shows radiotherapy plans for NPC patients. The dose-volume histogram (DVH) and the isodose distribution show that IMRT is more capable of sparing the dose delivered to both parotid glands while delivering an adequate dose to the PTV.
