Development of a transparent and flexible patient-specific bolus for total scalp irradiation.

A commercially available flat bolus (commercial bolus) would not fully fit the irregular surfaces of the scalp. We developed a transparent and flexible material with good fitting properties, analyzed its physical characteristics, and evaluated the clinical feasibility of the bolus fabricated using a three-dimensional (3D) printer (3D bolus). To evaluate the physical characteristics of the new material, treatment plans with virtual, 3D, and commercial boluses were created for water-equivalent phantoms using a radiation treatment planning system (RTPS). Using a head phantom and the dose volume histogram calculated with RTPS, dose distributions for total scalp irradiation were compared between the three treatment plans. To evaluate the clinical feasibility, the fitness and reproducibility of the 3D bolus were compared with the head phantom and clinical cases using dice similarity coefficient (DSC) measurements. A good agreement was observed between the percentage depth dose (PDD) curves for the virtual, 3D, and commercial boluses. The homogeneity indexes of the planning target volume (PTV) for the 3D and commercial boluses were 0.083 and 0.153, respectively, proving that the former achieved a better dose uniformity of PTV than the latter. Good fitness and reproducibility with the 3D bolus were observed in both the head phantom and two clinical cases, with mean DSC values of 0.854, 0.829, and 0.843, respectively. These results successfully demonstrated and verified the utility of the 3D bolus for total scalp irradiation.