Abstract
Background: The use of thermoplastic immobilization devices in radiotherapy increases the positional repeatability of irradiation. However, these devices act as boluses and increase surface dose. In this study, we evaluated the increase in surface dose using seven types of thermoplastic immobilization products.
Methods: Seven types of products were cut to a size of 5×5 cm2 and stretched by 5 cm in two directions. The surface dose was measured using Gafchromic EBT3 films placed above a 20 cm stacked water equivalent phantom, and the source surface distance was 100 cm. Subsequently, the shells were placed on a film. The films were then irradiated with 200 MU of 6 MV X-rays. Theirradiated films were scanned after 24 h and converted to the absorbed dose using a density-dose conversion table.
Results: The surface dose with no stretch shells was 2.9–fold higher than that without shells. Among the seven products, the maximum and the minimum increase were 3.2– and 2.6–fold, respectively. Meanwhile, the 5 cm stretched shells increased the surface dose by 2.3–fold (max: 2.7–, min: 1.9–fold). The increase in the surface dose decreased with the degree of shell stretch (2.5, 5.0, and 7.5 cm). In addition, the pore size and pore area per cm2 showed a negative correlation with the relative dose increase from without shells (r = -0.66 and -0.69, respectively).
Conclusions: Product dif ferences in dose increase were small, suggesting that pore size and pore area per cm2 are important. Meanwhile, stretch effectively reduce the surface dose increase; however, care must be taken to balance the extent of stretch with the stiffness.
