**4.7 Plan evaluation**

In the evaluation of radiotherapy plan dosimetric quality, there are four main parameters to be evaluated: (1) PTV coverage, (2) OAR dose, (3) PTV homogeneity, and (4) PTV conformity [66]. PTV coverage refers to the minimum proportion of PTV covered by the prescribed dose. OAR dose is to see whether it is within the organ tolerance. PTV homogeneity is used to assess the dose uniformity within the PTV whereas PTV conformity is to evaluate whether the prescribed dose level encompasses and follows the shape of the PTV. Examples of different PTV coverage, homogeneity, and conformity situations are illustrated in **Figure 10**.

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

#### **Figure 10.**

*Examples of different PTV coverage, homogeneity, and conformity situations. The PTV is in blue solid lines and the body is in black solid lines. The purple dashed lines are the prescribed isodose and the red dashed lines are the hot spots isodose. Their respective dose-volume histograms are shown above.*

The evaluation of PTV coverage and OAR dose is conducted using the dosevolume histogram (DVH). PTV homogeneity and conformity are assessed by indices known as the homogeneity index [67] and conformity index respectively [68].

### **5. Current challenges and promises in head and neck cancer radiotherapy**

As illustrated, IMRT offers the opportunity for better treatment outcome and less side effects in radiotherapy of head and neck cancers when compared with 3DCRT. A positive aspect of IMRT is that it can increase the dose conformity and homogeneity to the PTV while better sparing of the OARs [69, 70]. The following challenges are needed to be addressed for further development of the advantages of IMRT.

#### **5.1 Organs at risk (OARs) dose estimation**

In the treatment planning of IMRT, the inverse planning process requires planners to define the dose limits of various PTVs and OARs for the optimization of the beam intensity modulation. This process is regarded as the setting of the objective function, which includes the dose constraints and priority of the PTVs and OARs as discussed in Section 4.5. In general, the setting of PTVs objective functions are guided by the prescription whereas those for the OARs are set according to their dose tolerance [71]. In practice, however, the objectives for OARs sparing are often in conflict with the objectives to achieve PTV dose coverage [72]. This is because OARs and PTVs are often in close proximity and sometimes may even overlap one another. In this condition, we may have to deliver OARs doses that are close to or even higher than their dose tolerance in order to achieve PTV adequate dose coverage. On the contrary, when the OARs are far from the PTV, the actual OARs dose would be well below their tolerance. It is logical to deduce that the OARs dose is related to their anatomical relationship with PTVs, and this relationship varies greatly among different patients.

### *5.1.1 Knowledge-based radiotherapy and 4pi VMAT*

Knowledge-based radiotherapy planning has recently emerged as rapidly developing area with the aim to improve the IMRT planning process [73]. Knowledgebased planning refers to the strategy to incorporate past plans data (known as knowledge) into the treatment planning process. Six different categories of purpose in knowledge-based planning have been summarized in a review article, which includes (1) the determination of DVH, (2) specific dose metrics, (3) voxel-level doses, (4) objective function weights, (5) beam parameters and (6) quality assurance metrics [73]. The development of knowledge-based radiotherapy planning enables planners to determine the setting of objective functions in a more systematic approach, less dependent on personal experience, and therefore higher consistency of plan qualities.

The technology of delivering 4pi VMAT is emerging. 4pi radiotherapy refers to the incorporation of beams distributed on the imaginary isotropically expanded spherical surface around the iso-center during plan optimization [74]. The 4pi VMAT can be delivered by non-coplanar arc beams using a static couch or synchronizing the arc rotation of the gantry with a rotating couch [75, 76]. It has been shown that 4pi VMAT has the potential to further decrease the dose to OARs compared with coplanar VMAT. For example, a study on head and neck cancers reported that the mean Dmax to the brain stem and spinal were decreased by 6 Gy and 3.8 Gy respectively using 4pi VMAT [77]. In addition, the method of delivering 4pi VMAT with synchronized gantry and couch rotation enabled more sophisticated arc trajectories compared with the static couch method. It was expected to deliver a highly conformed dose to the PTV with a reduction of OARs dose and 50% isodose volume in the patient body [76]. Although the treatment time will increase by 30% in current linear accelerators compared with coplanar VMAT [75], the potential of 4pi VMAT can be unleashed with the advancement of the future linear accelerators with automatic couch and gantry motion capabilities for faster 4pi VMAT delivery [78].

#### **5.2 Tumor dose escalation**

IMRT offers the possibility to escalate the dose to the tumor because of its better ability to spare the OARs. In fact, dose-escalation has already been implemented in IMRT in the treatment of NPC when the gross tumor dose was raised from 66 Gy in conventional radiotherapy to about 70 Gy [79]. NPC is known for its radiosensitivity and the existence of dose-tumor-control relationship beyond routine cancericidal dose [80], hence increasing the dose to the tumor volume is able to increase the local control rate. It has been reported that in the group of predominantly locally advanced NPC (T3-4 N0-1), 61.8% of the failure was caused by local relapse [81]. Another study also revealed that 80% of the recurrent cases had the relapse sites at the region delivered with the median dose of 70.4 Gy in the previous treatment [82]. Clinical investigations on the dose escalation in the treatment of NPC using external beam radiotherapy [83] and brachytherapy have been reported [84]. Although it has shown good local control and survival in both reports, treatment side effects were the concern. For example, grade 3 mucositis was observed in about 80% of the cases [83]. Also, by assessing the acute toxicity, it has been suggested that the maximal tolerable dose in IMRT of head and neck cancers was 2.36 Gy per fraction to a total of 70.8 Gy [85].
