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

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‐cm2 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.


INTRODUCTION
High-energy clinical beams provide an immediate benefit for the treatment of malignant diseases, and consist simply of low-radiation dose distribution along the patient's skin surface. In fact, in the dose distribution versus the tissue depth curves, an electronic disequilibrium region exists, which is usually larger than the average skin thickness.
This results in an advantageous preservation of this sensitive tissue, and an effect that is commonly reported in the literature as skin sparing. [1][2][3] Regardless of this benefit, the use of immobilization tools, such as thermoplastic masks and molds, both of which are used for patient positioning and immobilization, has led to a reduction in the use of skin sparing in radiotherapy procedures. For instance, Lee et al. 4 measured an average 18% increase in the surface dose caused by thermoplastic masks used for intensity-modulated radiotherapy treatments, whereas Higgins et al. 5  neck immobilization on the surface of a solid water phantom. 6 They measured an increase in surface dose by a factor of at least 50% in the best conditions. In contrast, using a 6-MV linear accelerator beam, Ali et al. studied these effects using Gafchromic EBT films over a flat surface phantom. 7 Furthermore, Kelly et al. 8

Treatment planning in the anthropomorphic phantom
An Alderson Rando anthropomorphic phantom was utilized to reproduce radiotherapy treatments in the head and neck regions. The phantom was submitted to rigorous 3-D radiotherapy planning, and the daily dose delivery parameters were obtained through compu-

Dosimetric instruments
The main goal of the current study was to provide a more realistic dose surface evaluation by using a methodology that allows the dose variation in non-flat surfaces to be considered based on dose comparisons among three different dosimetric systems. In this context, radiochromic films have been extensively used for dosimetry in radiation medicine in recent years. 10,11 They have several advantages when F I G U R E 1 Schematic diagram showing (a) the EBT3 calibration setup using virtual solid water, and (b) the corresponding calibration curves for the RGB components. The red curve shows the higher color intensity for the investigated dose range, and a functional form, such as y = a-b * ln (x), was used to fit it compared with other alternative dosimeters for dosimetry of gamma and X-rays fields. The high spatial resolution of radiochromic films is far superior to that of ionization chambers and TLDs, and the ability to be used in water without waterproof encapsulation is an additional advantage. 11 Furthermore, as reported by the manufacturers, the EBT3 series has weak energy dependence (within 3%) when irradiated with 30 kVp, 100 kVp, 150 kVp (all with 2 mm Al), and Co-60 photon fields for doses ranging from 1 to 300 cGy. For calibration purposes, EBT3 radiochromic films were cut into nine small 2.0-cm 2 identified pieces, placed between two virtual solid water plates, and exposed to absorbed doses ranging from 25 to 200 cGy. A schematic diagram of the calibration arrangement is shown in Figure 1a

Dose evaluation using TLD chips
A total of 12 TLD chips were positioned on the surface of the SCF anatomical region to measure the skin dose variation in radiotherapy procedures performed with and without the use of a thermoplastic mask ( Figure 3). The setups were submitted to an anterior/posterior irradiation procedure, in agreement with the approved radiotherapy planning for head and neck radiotherapy treatments.
The measured skin doses after the irradiation procedure, for each chip position in the anthropomorphic phantom, are shown in Figure 3.    for all masks, the last is quite smaller, leading us to conclude the masks with higher holes; that is, with less mesh, might provide reduced doses to the patient's skin.

Dose evaluation using PBC
The results of the PBC dose calculation analytical method dose evaluation, simulating an exposure of 180 cGy, with and without thermoplastic immobilizer are shown in Figure 5. The results suggest an important increase in the superficial dose when using immobilization masks, for the majority of the measured points. Although these results are in agreement with the previous results obtained using TLD chips and EBT3 film dosimeters, the results obtained at position E11 and C16, unlike the other two dosimetric systems, show reduced doses with the use of a thermoplastic mask. Furthermore, the overall entrance skin doses evaluated by the PBC method with immobilization masks were 50% smaller than those measured by TLD-100 and EBT3.

DISCUSSION
Thermoplastic masks used for patient positioning and immobilization have led to a reduction in the use of skin sparing in radiotherapy procedures, although they contribute to a notable increase in the skin radiation dose. 4 The magnitude of this phenomenon, and its consequent clinical influence, will depend on the characteristics of the immobilizer material, such as the thickness, as well as the characteristics of the incident beam, such as energy spectrum and obliquity.  18 as well as for distribution dose evaluation in electronic disequilibrium regions. 19 The dosimetric uncertainties observed in these regions, for instance, in the interface between tissues of different densities, is somehow accepted by the international organizations that publish radiotherapy protocols. One such example is the Task Group 53 American Association of Physicists in Medicine recommendation that has established 40% as the uncertainty limit in the build-up regions. 20 Nevertheless, hospitals and clinics worldwide make use of software that provides TPS based on the PBC method.
Finally, although this study has discussed the increase in skin radiation dose that arises with the use of thermoplastic immobilizers, it is fair to consider that the immobilization instrument is an essential tool for the success of radiotherapy treatment by ensuring the reproducibility of the patient positioning. Without the immobilizer, the skin damage would be significantly worse than the dose superficialization effect.
Thus, it is reasonable to suggest that the development of new immobilization devices, that can maintain the benefits of the immobilization and simultaneously barely contribute to the dose superficialization in the patient, should be encouraged.
TLD-100, EBT3, and the PBC method have been used to evaluate the increase in surface radiation dose provoked by immobilization masks in head and neck radiotherapy procedures. After irradiation with 180 cGy in the Clinac Linear Accelerator model 2100C at 6 MV, TLD chips positioned on the surface of the SCF anatomical region of an anthropomorphic phantom covered with a thermoplastic mask detected an increase in the entrance skin dose of approximately 38.4%. The data from the small EBT3 2.0-cm 2 strips, averaged among all strips used, also detected a medium increase of 58.6%. Both dosimetric systems have measured similar averaged surface doses without the presence of immobilization masks; that is, 50.5% for EBT3 and 53.7% for TLD-100. The well-known high spatial EBT3 film resolution provides additional information regarding the limit between the high dose region (>90 cGy) and the low-dose region (20 cGy), which is attributed to the secondary collimator in the linear accelerator head.
In contrast to PBC evaluation, both TLD and EBT3 detected increased doses for all measured points.