Head and Neck Immobilization Masks: Increase in Dose Surface Evaluated by EBT3, TLD‐100 and PBC Method

Abstract Positioning and immobilization tools are considered essential components of radiotherapy treatments to guarantee that the planned dose distribution can be efficiently reached. However, the benefits brought by their use are met by an apparent increase in the patient skin entrance dose. In the current study, we evaluated the dose surface effects provoked by the use of immobilization thermoplastic masks in head and neck radiotherapy treatments, carried out using a 6‐MV linear accelerator beam. The study was carried out using an anthropomorphic head–neck phantom and three dosimetric techniques: (i) thermoluminescent dosimetry (TL); (ii) radiochromic film dosimetry; and (iii) computational simulation using the pencil beam convolution (PBC) method. For calibration purposes, TLD chips and radiochromic (EBT3) small 2.0‐cm 2 strip dosimeters were positioned between two virtual solid water plates, and exposed to absorbed doses ranging from 25 to 200 cGy. The use of an anthropomorphic head–neck phantom allows the dose variation in non‐flat surfaces to be taken into account. TLD chips, positioned on the surface of the supraclavicular fossa anatomical region, covered with a thermoplastic mask, detected an entrance skin dose that was approximately 38.4% higher than that measured without a mask. The EBT3 dosimeters, averaged among all strips used, also detected a medium increase of 58.6%. Both TLD and EBT3 detected increased doses for all measured points, and measured similar averaged surface doses without the use of immobilization masks; that is, 50.5% for EBT3 and 53.7% for TLD‐100. The pencil beam convolution simulation results suggested an increase for most of the measured points; however, no increased, and in some cases even decreased, doses were observed. The surface dose data of three other commercial thermoplastic masks irradiated in a solid water phantom are also provided.