**5. Intensity-modulated radiotherapy (IMRT)**

The 3D-CRT technique is based on the delivery of uniform fluence across the beam and involves less complex beam arrangements. Here, the parameters for planning such as beam directions, beam weights, wedges, etc. are based on the trial and error method making it a time-consuming process. To overcome these challenges, a more sophisticated method of conformal planning known as Intensity-modulated radiotherapy (IMRT) is used in HNC. Unlike the 3D-CRT method, here beams of non-uniform intensity are used. IMRT is based on the principle of inverse planning optimization and uses computed-controlled multi-leaf collimators to modulate the intensity of beams across the tumor. Intensity modulation allows the conformal dose coverage to the target and sharp dose fall-off beyond the target thus sparing the critical structures surrounding the target [8].

Due to the complex anatomy of the head and neck, and proximity of various critical structures with the target, IMRT is the better choice of treatment [9]. Multiple beams of non-uniform intensity deliver dose with high conformity to the irregularly shaped target and steep dose gradient at the boundary of target and organs at risk (OARs) minimize the dose to adjacent OARs like spinal cord, parotids.

Head and neck tumor configurations are usually concave in nature. With multiple organs such as the parotid glands, brainstem, spinal cord etc. surrounding such concave-shaped lesions, there is a need for concave-shaped dose distribution to avoid risking dose to these OARs [10]. Head and neck IMRT clearly offers such advantages of better normal tissue sparing, improved dose coverage to the target, giving multiple isodose levels in tumor volume, and dose -escalation to the tumor [11].

IMRT has given better results over 3D-CRT in head and neck cancer with a significant reduction in xerostomia, prevention of acute and late toxicities, and improved quality of life. Parotid-sparing IMRT has superior results over conventional 3D-CRT [12–14]. However, for successful results from IMRT, there is a requirement of precision in the patient setup, immobilization, and correct tumor volume delineation to avoid any marginal miss, proper plan evaluation, strict imaging protocols, and rigorous quality assurance tests. Owing to the tight margins of dose around the target, any negligence during the IMRT treatment can pose a serious risk to the patient and can impact the treatment outcome. Another concern that arises during IMRT treatment is the volumetric and spatial changes in tumor volume and OARs due to shrinking of tumor, weight loss, radiation-induced toxicities as well as variation due to physiological movements like breathing [15–26] (**Table 1**). One such change observed during treatment of HNC is the movement


**Table 1.**

*= ipsilateral; contra = contralateral.*

*Summary of anatomic and dosimetric changes throughout treatment for head and neck radiotherapy.*

of parotid closer to high dose regions during the progress of treatment owing to the shrinkage in tumor volume, inducing high-grade xerostomia (even worse than predicted) [27]. To limit the dose variation due to these factors, there is a need to adapt to the varying treatment volume and OAR volumes changing during the course of treatment (**Figures 1–4**). This could be possible through a technique known as adaptive radiotherapy.

*Volumetric and Dosimetric Inconstancy of Parotid Glands and Tumor in Head and Neck Cancer… DOI: http://dx.doi.org/10.5772/intechopen.104745*

#### **Figure 1.**

*Comparison between pre-treatment (CT1) [left] and per-treatment (CT4) [right] images of a patient. Coronal CT slices of a 57 years old male patient with carcinoma hypopharynx (T4aN2c). The decrease in volume of GTV primary (maroon) [from 30.426 cc in CT 1 to 5.964 cc in CT 4] and GTV nodal (pink) [from 10.005 cc in CT 1 to 4.638 cc in CT 4] can be appreciated along with decrease in volume of both right (brown) and left (light blue) parotid glands.*

#### **Figure 2.**

*Comparative DVH of a patient [(Ca hypopharynx T4N2c)] comparing CT1 (-------) and CT4 (––––). The mean dose to the right parotid (brown) has decreased in CT4 compared to CT1 whereas the mean dose to left parotid (light blue) has increased in CT4 compared to CT1.*

#### **Figure 3.**

*Comparison between pre-treatment (a) and per-treatment (b) images of a Patient undergoing IMRT-SIB. Coronal CT slices of a 55 years old male carcinoma pyriform fossa (T1N3b) patient. We can appreciate the decrease in GTV nodal (pink) volume by end of treatment along with decrease in volume of both right (brown) and left (light blue) parotid glands (b). The low dose isodose curves of 20 Gy (light yellow) and 30 Gy (light green) can be seen covering more areas of right parotid gland in (b) compared to (a).*

**Figure 4.** *DVH comparison between start (A) [--------] and end of treatment (B) [–––––].*
