Avaliação dosimétrica de erros de posicionamento para implementação da técnica de neuroeixo com arcoterapia volumétrica modulada (VMAT)

Abstract

Retrospectively evaluated corrections of planar images of patients already submitted to craniospinal irradiation (CSI) treatments, in order to quantify setup errors magnitude and simulate them in different volumetric modulated arc therapy (VMAT) plans to define the one with the lowest dosimetric impact for implementation in clinical practice.

219 planar images of 15 patients previously treated were selected for statistical evaluation of positional errors. For VMAT plans, four pediatric patients who completed treatment with 3D conformational technique were selected and five VMAT plans were optimized in two different techniques: "overlap" with 3 cm, 6 cm and 9 cm of junction and "gradient" with 6 cm and 9 cm. The plans were evaluated by V_100%, V_95%, D_98%, D_2%, conformity number (CN) and heterogeneity index (HI). In each, setup errors with the magnitude found from the retrospective analysis were used to shift the isocenter in the longitudinal direction, in order to assess the dosimetric impact the errors, as well as to evaluated the difference in dose distribution when considering the treatment couch.

The analysis of the images showed average displacements between 1 and 5 mm and in some cases reaching a maximum of 15 mm. Regarding the quality of the plans, all obtained consistent results based on the metrics without significant differences between them. However, the smooth gradient technique (9 cm of junction) obtained a lower dosimetric impact when compared to others when subjected to positional errors. Finally, when considering the treatment couch in the planning, a reduction of 10% was found in the V_100%.

The implementation of the VMAT for CSI, it’s recommended daily positioning image to minimize setup errors with magnitudes greater than the average of 3 mm, as these have a significant impact on the dose distribution in the modulated plans.