# Development of Patient Specific Conformal 3D-Printed Devices for Dose Verification in Radiotherapy
## 3 Results and Discussion
### In Vitro Dose Verification Using 3D-Printed Devices  
In order to evaluate the influence of 3D boluses on the phantom surface dose distribution, three single-field plans were created: one without a bolus and two others with boluses of different thicknesses (0.5 cm and 1.0 cm) (Figure 7). The results indicated a lack of surface dose coverage without 3D bolus since the isodoses were shifted from the surface inside the phantom by 0.7 cm (for 95% coverage) and by 1.0 cm (for 99% coverage). More detailed information regarding the bolus effectiveness were obtained using dose volume histograms (DVH) for 95%, 98% and 99% surface volume coverage. The DVH results for the virtual target and dosimetric parameters derived from each of the investigated treatment plans are presented in Figure 8 and Table 3, respectively. Figure 6: Marked air gaps between patient-specific phantom and each of the 3D-printed boluses: (**A**) 0.5 cm thick bolus; (**B**) 1.0 cm thick bolus. It was found that when no bolus is used, only 57.64% of the prescribed dose (i.e., 1.15 Gy from 2 Gy/fr) could be delivered to 95% of the target volume, while absorbed doses covering at least 97% and 99% of the investigated volume were 51.24% (1.02 Gy) and 47.96% (0.96 Gy) respectively. This indicates that the prescribed dose cannot be delivered efficiently to the target volume when no bolus is applied and will lead to the tumor underexposure. Dose coverage of the planned treatment volume (PTV) was significantly improved when 3D-printed bolus was attached to the surface of patient-specific phantom (Table 3). In the case of the 0.5 cm thick bolus, 95%, 97% and 99% of the same investigated surface volume were covered with 99.5% (1.99 Gy), 98% (1.96 Gy) and 97% (1.94 Gy) of the prescribed dose, respectively. Application of the 1.0 cm thick 3D bolus yielded results comparable with those observed by application of 0.5 cm thick bolus: the surface volume of (95%, 97% and 99%) was covered with 100% (2.0 Gy), 99.5% (1.99 Gy) and 98.5% (1.97 Gy), respectively).The results show that the surface dose coverage was almost equivalent with both 3D-printed boluses and increased by up to 60% when compared with no bolus indicating that the 3D boluses are good buildup materials. The impact of 3D-printed boluses on surface dose coverage was also investigated in the case of a realistic clinical scenario involving IMRT with five irradiation fields (Figures 9 and 10). Figure 8: Dose volume histograms for the three single-field plans obtained for the patient-specific phantom: without bolus (blue), with 0.5 cm thick 3D-printed bolus attached to the phantom surface (gray) and 1.0 cm thick 3D-printed bolus attached to the phantom surface (red). Figure 7: Isodose distribution in single field treatment plans: (**A**) CT head phantom with attached patient-specific phantom; (**B**) patient-specific phantom with attached 0.5 cm thick bolus; (**C**) patient-specific phantom with attached 1.0 cm thick bolus. The results showed that 1.48 Gy of the prescribed 2 Gy dose was delivered to 99% of the target volume in the case of no bolus (Figure 10). Application of 1 cm boluses on the phantom shifted the dose toward the surface and allowed the delivery of 100% of the prescribe dose to the delineated target. Similar results were obtained for the 0.5 cm bolus (99% of the surface volume was covered with 98% of the prescribed dose) and thus, the 0.5 cm bolus is deemed satisfactory for the treatment of HNC with five intensity modulated beams of 6 MeV FFF. Analysis of single field and IMRT plans with and without 3D boluses indicated that application of patient-specific boluses can significantly improve the treatment outcome as a result of optimization of target dose coverage (Table 4).