**4.5 Dose constraints of targets and OARs**

In general, there are 3 types of dose constraints settings before the optimization. They are the PTVs, serial OARS, and parallel OARs respectively. For the PTV, it requires the setting of at least one upper objective and one lower objective as shown in

**Figure 6.** *User interface of photon optimizer.*

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

**Figure 7**. The resultant dose-volume histogram (DVH) should show that the majority of the PTV receives the desired dose with little volume receive the higher dose, and the shape should look like a plateau at 100% volume with an extremely steep cliff at the end when it reaches the prescribed dose.

The dose constraints setting for serial OARs only requires an upper objective to limit its maximum dose, as shown in **Figure 8**.

For parallel OARs, since the dose received by the various proportion of volume is the concern for late side effects, setting of upper objectives to limit the maximum dose is not enough. It can be done by setting multiple upper objectives at different dose-volume levels or setting the mean objectives. The purpose is to limit the received dose at all volume levels and to push the DVH to its left end as much as possible. A sample objective setting for a parallel OAR is shown in **Figure 9**.

### **4.6 Practical difficulties of optimizing a radiotherapy plan for head and neck cancers**

Although the planning procedures are driven by treatment planning computer calculations in an inverse planning process, it is not a completely automatic procedure and there are difficulties in the planning. The difficulties in planning are largely related to the number of OARs and the geometric relationship between the PTVs and the OARs. In the optimization process of the inverse planning, it is usually not possible to achieve all the lower objectives for the PTVs while fulfilling all the upper and mean objectives for the OARs because they naturally contradict each other when the PTVs and OARs are in the vicinity [65]. In head and neck cancers, there are many OARs near to the PTVs including but not limited to the brain stem, the spinal cord, the parotid gland, and the optic nerves. Because of this, the treatment planning system optimization usually has no optimal solution that can fulfill all the set objective functions. Therefore, planners need to intervene in the procedure by evaluating the optimized treatment plans using their own experiences, and to balance the trade-off among all the nonoptimal objective functions of the PTVs and OARs.

**Figure 7.** *Dose constraints setting of PTV.*

**Figure 9.** *Dose constraints setting of parallel OARs.*
