**Abstract**

The treatment of head and neck cancer using external beam radiotherapy is commonly done with three field techniques, which involves bilateral parallel opposed beams and one anterior lower neck field. Conventional treatment is based on 2D fluoroscopic images where there is no facility to shield the organs at risk like parotid. The most common side effect of such conventional radiotherapy treatment is xerostomia. The incidence of radiotherapy-related xerostomia varies depending on the specific radiotherapy technique used and the dose delivered to the parotid glands. Dosimetric variation in the tumor and normal tissue including parotid glands due to volume shrinkage during intensity modulated radiotherapy is the leading challenges in radiotherapy delivery in head and neck malignancy in terms of acute and late radiation related toxicities. Therefore if the planning target volume and normal tissue anatomy are changing with time during intensity modulated radiotherapy, it would be beneficial and acceptable to adapt our treatment delivery to minimize normal tissue toxicities where it really matters.

**Keywords:** volumetric, dosimetric, parotid glands, head and neck cancer, IMRT

## **1. Introduction**

One of the biggest challenges in radiotherapy delivery in head and neck cancer is radiation related acute and late toxicities. Symptoms of acute toxicities can be present for up to 3 months post-radiotherapy, and late toxicities, tend to persist several months or years after the completion of radiotherapy.

Xerostomia is considered to be a major concern in radiation related acute and late toxicity after head and neck radiotherapy.

## **2. Radiobiology of parotid glands**

Salivary gland cells are slow dividing cells even though they are highly radiosensitive, factors attributing to this could be smaller number of cells in the functional subunit of acinar cells, slow recovery of acinar cells, depletion of stem cell population, pattern and rate of terminal differentiation of stem cells, proliferation rate of the stem cells, turn over or life time of acinar cells. If turn over or life time of cell is short there will be early appearance of symptoms of radiation injury and vice versa if the cell turn over or life time of cell is long.

In slowly dividing tissue radiation injury becomes more prominent when dose per fraction is increased, because at higher dose there are fewer division cycles that cells can successfully negotiate before their death. Therefore injury develops more quickly with increase in dose per fraction in late dividing tissue.

Various inter current insults e.g. chemotherapy, surgery, dental or mechanical trauma, hyperthermia, infection are also capable of precipitating the expression of radiation injury in slowly responding tissues.
