Evaluating the performance of a 3D PLA buildup bolus in breast intraoperative radiotherapy.

Introduction: The received dose to the surface of tumor bed is one of the main issues in beast intraopeative radiotherapy (IORT). Totally, the administered dose to the surface is usually lower than that of radiotherapy reference point (distal end of tumor bed). Employing a buildup bolus can effectively increase the surface dose and therefore, eliminate such created cold points during IORT. The aim of this study is to evaluate the performance of a 3D buildup bolus, made of PLA (polylactic acid), for breast IORT.
 
Materials and Methods: A cubic PLA bolus with different infill percentage of 20% to 100%, with 10% increments, was simulated through BEAMnrc Monte Carlo code and irradiated by intraoperative electron energies of 6, 8, 10 and 12 MeV produced from LIAC IORT accelerator. The LIAC head was also modeled using BEAMnrc code. Then, the PDD curves for different infill percentages of studied bolus were calculated by the DOSXYZnrc code and compared with those obtained in water at various energies. All of comparisons were performed by gamma analysis and gamma index calculation. The dose difference and distance to agreement in gamma index calculations were considered as 2% and 2 mm, respectively.
 
Results: The results showed that high infill percentage corresponds to the high density and desirable homogeneity of bolus material. The bolus modeling in BEAMnrc code as a homogeneous material is satisfying for infill percentages greater than 20%. With increasing the bolus infill percentage, the accordance of obtained PDDs with those of water increments too. The PDD curves in the presence of PLA bolus with 100% infill percentage has the best agreement with the obtained PDDs inside water at different energies.
 
Conclusion: Based on the obtained results, it can be concluded that the dosimetric characteristics of high infill percentages of PLA material are compatible with those of water. Therefore, PLA can be considered as an effective buildup bolus for surface dose enhancement in breast IORT. By tuning the infill percentage of PLA material, one can change the dosimetric properties of obtained bolus. The best dosimetric results are obtained when the PLA bolus is made with 100% infill percentage.