Comparing the robustness of different skin flash approaches using wide tangents, manual flash VMAT, and simulated organ motion robust optimization VMAT in breast and nodal radiotherapy

Abstract

Compare the robustness of wide tangents (WT) and volumetric modulated arc therapy (VMAT) using different skin flash approaches in breast and nodal radiotherapy. Ten patients treated with WT using 2-cm flash were replanned with VMAT using no flash (NF), manual 2-cm flash (MF), and robust optimization (RO). Plan robustness was assessed for target coverage and organs at risk (OAR) by recalculating on 5 deformed CT scans (SOM1-5), daily cone beam (CBCT), and by shifting the isocenter 5 mm. VMAT NF gave poor coverage of CTVp with its smallest change of -3.2% for V_38Gy on CBCT. VMAT RO plans showed the least variations in target coverage loss compared to WT and VMAT MF which dropped as anatomical swelling increased. CTVp D_0.5cc decreased on CBCT and increased most for VMAT MF plans (case max increase +3.3 Gy), whereas VMAT RO plans were relatively stable (case max increase +1.2 Gy). OAR dose changed little with anatomical changes (isocenter shifts more important with medial, posterior, and inferior increasing dose). Nodal coverage was superior for VMAT which led to the WT being less robust for coverage toward both geometric and anatomical uncertainties. All techniques except NF plans gave high levels of coverage under minor uncertainties. VMAT RO was highly robust for target coverage for anatomical changes. Manually editing control points on VMAT plans was time-consuming and less predictable. CBCT anatomical changes were modest resulting in small delivered dose changes. OAR dose changes were small with no significant differences between techniques.

Keywords: Breast cancer; Dosimetric comparison; Robust optimization; VMAT.

Fig. 1

Example beams eye view of the 4 techniques (WT, VMAT NF, VMAT MF, and VMAT RO). PTVp is red (WT field based), body is green and PTVn_LN orange. (Color version of figure is available online.)

Fig. 2

Mean CTV dose differences for the 4 techniques (WT, VMAT NF, VMAT MF, and VMAT RO) for isocenter shifts and anatomical changes. (Color version of figure is available online.)

Fig. 3

Mean OAR dose differences for the 4 techniques (WT, VMAT NF, VMAT MF, and VMAT RO) for isocenter shifts and anatomical changes. (Color version of figure is available online.)