Surface dose and position consistency comparison of different bolus types in radiotherapy

Purpose To compare the surface dose and setup consistency using different bolus types in various radiotherapy treatment techniques. Methods The bolus types of commercial constant thickness (cont. bolus), 3D-printed of polylactic acid with 20% and 40% infiltration (PLA20% and PLA40%, respectively), and molded plastic sheet of polycaprolactone (PCL) were used for the surface dose increasing. The thickness of the PCL was 4 mm, whereas the thickness of the other bolus types was 5 mm. The percentage depth dose was measured using EBT3 film in a cubic water phantom. A skin involvement of MALT lymphoma was simulated on a phantom as the treatment target. The planning was conducted using three treatment techniques, including a three-dimensional conformal radiotherapy (3D-CRT), intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). The percent dose difference was evaluated for a dosimetric comparison with the surface dose without bolus. The final test was the bolus positioning consistency. The Hausdorff distance (HD) and Dice similarity (DS) were used to evaluate the positioning consistency. Result The surface dose revealed the highest value of 99.2% in PCL. The treatment technique of 3D-CRT, IMRT and VMAT in PCL also showed the highest increasing surface dose value of 82.4%, 42.1% and 47.4% for 3D-CRT, IMRT and VMAT, respectively, from treatment without bolus. Finally, the bolus positioning consistency of PCL and PLA demonstrated a superior value of HD (2.2 mm - 3.0 mm) and DS (90.2% - 92.5%) to the constant bolus value. Conclusion All types of boluses can increase the surface dose. The PCL showed a superior surface dose to other bolus types, although the thickness value was smaller than others. The individual bolus (PCL and PLA) provided a superior positioning consistency value to the constant bolus type.