The robustness of dual isocenter VMAT radiation therapy for bilateral lymph node positive breast cancer

Purpose: To investigate the use of dual isocenters for VMAT planning in patients with lymph node positive synchronous bilateral breast cancer (BBC) compared to a single isocenter option. Methods: Treatment plans of 11 patients with lymph node positive BBC were retrospectively analyzed using two different VMAT planning techniques: dual-isocenter split-arc VMAT plans (Iso2) were compared with monoisocenter VMAT plans (Iso1). For Iso2 plans, PTV dose was investigated after introducing +/- 2 and +/- 5 mm couch shift errors between the two isocenters in the lateral, longitudinal and vertical direction. Results: For both techniques the planning aims for PTV coverage and OARs were met. The mean dose for the bilateral lungs and heart was reduced from 11.3 Gy and 3.8 Gy to 10.9 Gy (p < .05) and 3.6 Gy (p < .05), respectively, for Iso2 plans when compared to Iso1 plans. Positive statistically significant correlation (rho = 0.76, p = .006) was found between PTV volume and D2ccPTV for Iso1 plans. No clinically significant change was seen in the D98(CTV) or D2cc(PTV) after the 2 and 5 mm errors were introduced between isocenters for Iso2 plans. Conclusions: The split arc method was shown to be a feasible treatment technique in the case of synchronous BBC for both mono and dual isocenter techniques. The dose parameters were slightly favoring dual-isocenter option instead of mono-isocenter. The dual-isocenter method was shown to be a robust treatment option in the presence of <= 5 mm errors in the shifts between the two isocenters.