Head and neck field matching verification using three dimensional PAGAT polymer gel dosimetry

Independent jaw collimation on modern linear accelerators allows matching of non-diverging field edges (laterals and anterior) when using head and neck treatment techniques. It is a common practice to split a head and neck cancer radiotherapy treatment volume into two separate abutting mono-isocenter fields because the treatment volume lies above and below the line of the shoulders. In a standard three field technique parallel-opposed lateral fields are used to treat the primary tumour site, upper and mid-cervical nodes and a single anterior field is used to treat the lower neck and superior mediastinal lymph nodes. If these fields are non-diverging fields, there is a possibility of creating an under dose or over dose region at the junction of the abutting fields. The purpose of this study was to evaluate the suitability of the PAGAT polymer gel dosimeter as a tool for verification of the dosimetric match of non-diverging field junctions of a mono-isocenter setup.