Evaluation of Hybrid VMAT Advantages and Robustness Considering Setup Errors Using Surface Guided Dose Accumulation for Internal Lymph Mammary Nodes Irradiation of Postmastectomy Radiotherapy

Abstract

Objectives: Setup error is a key factor affecting postmastectomy radiotherapy (PMRT) and irradiation of the internal mammary lymph nodes is the most investigated aspect for PMRT patients. In this study, we evaluated the robustness, radiobiological, and dosimetric benefits of the hybrid volumetric modulated arc therapy (H-VMAT) planning technique based on the setup error in dose accumulation using a surface-guided system for radiation therapy.

Methods: We retrospectively selected 32 patients treated by a radiation oncologist and evaluated the clinical target volume (CTV), including internal lymph node irradiation (IMNIs), and considered the planning target volume (PTV) margin to be 5 mm. Three different planning techniques were evaluated: tangential-VMAT (T-VMAT), intensity-modulated radiation therapy (IMRT), and H-VMAT. The interfraction and intrafraction setup errors were analyzed in each field and the accumulated dose was evaluated as the patients underwent daily surface-guided monitoring. These parameters were included while evaluating CTV coverage, the dose required for the left anterior descending artery (LAD) and the left ventricle (LV), the normal tissue complication probability (NTCP) for the heart and lungs, and the second cancer complication probability (SCCP) for contralateral breast (CB).

Results: When the setup error was accounted for dose accumulation, T-VMAT (95.51%) and H-VMAT (95.48%) had a higher CTV coverage than IMRT (91.25%). In the NTCP for the heart, H-VMAT (0.04%) was higher than T-VMAT (0.01%) and lower than IMRT (0.2%). However, the SCCP (1.05%) of CB using H-VMAT was lower than that using T-VMAT (2%) as well as delivery efficiency. And T-VMAT (3.72) and IMRT (10.5).had higher plan complexity than H-VMAT (3.71).

Conclusions: In this study, based on the dose accumulation of setup error for patients with left-sided PMRT with IMNI, we found that the H-VMAT technique was superior for achieving an optimum balance between target coverage, OAR dose, complication probability, plan robustness, and complexity.

Keywords: H-VMAT; IMNIs; PMRT; SGRT; biological models; setup error.

Figure 1

Treatment planning design for three techniques: (A) T-VMAT; (B) IMRT; (C) H-VMAT.

Figure 2

The beam eye view of treatment planning design of H-VMAT technique: (A) An arc from 295° to 20°; (B) An arc from 40° to 150°; (C) Tangential field one; (D) Tangential field two; (E) An arc from 150° to 295°.

Figure 3

The workflow used to generate the SGRT-based setup errors and dose accumulation is shown. First, non-rigid algorithm was used for interfraction setup error analysis. Then we subdivided the intrafraction setup error of each field and each technique divided 25 fractions into 5 sets (5 fractions per set). Furthermore, sum up the average setup error for each set. Finally, the setup errors imported to TPS and converted each field into an isocenter group, then accumulated and compared dose. In the DVH comparisons figure, —shown as T-VMAT, —shown as H-VMAT, —shown as IMRT.

Figure 4

The dose distribution of three techniques before and after setup error re-calculation. A, B, C before setup error [T-VMAT (A), IMRT (B), H-VMAT (C)], (D–F) after setup error [T-VMAT (D), IMRT (E), H-VMAT(F)].

Figure 5

Box-whisker plot of dosimetry parameters with error bars:Heart Dmean, Left ventricle Dmean, Left anterior Dmean, Lungs V5, Lungs V20, Contralateral breast Dmean. Each figures show the dose changes of each OARs before and after the recalculation of setup error. Gray bars represent the accumulated dose after the consideration of setup error, and yellow bars represent the dose not considered of setup error. The points in the graph represent outliers, the black horizontal lines in the figures represent the average of each parameter.

Figure 6

Box-whisker plot of mean radiobiological parameters with error bar: CTV coverage, Contralateral breast SCCP, Heart NTCP, Lungs NTCP with error bars. Each figures show the parameters changes of each organs before and after the recalculation of setup error. Gray bars represent the accumulated dose after the consideration of setup error, and yellow bars represent the dose not considered of setup error. The points in the graph represent outliers, the black horizontal lines in the figures represent the average of each parameter.