Non-coplanar volumetric modulated arc therapy for locoregional radiotherapy of left-sided breast cancer including internal mammary nodes

Abstract

Purpose: Non-coplanar volumetric modulated arc therapy (ncVMAT) is proposed to reduce toxicity in heart and lungs for locoregional radiotherapy of left-sided breast cancer, including internal mammary nodes (IMN).

Methods and materials: This retrospective study included 10 patients with left-sided breast cancer who underwent locoregional radiotherapy after breast-conserving surgery. Planning target volumes (PTV) were contoured by encompassing the whole breast, IMN, and supraclavicular nodal (SCN) region. For each patient, the ncVMAT plan was designed with four partial arcs comprising two coplanar arcs and two non-coplanar arcs, with a couch rotating to 90°. The arc angle for non-coplanar arc ranged from 345° to 40° to irradiate the internal mammary region, while sparing the jaw superiorly and the heart inferiorly. The prescribed dose was normalized to cover 95% of PTV, with 50 Gy delivered in 25 fractions. For each ncVMAT plan, dosimetric parameters were compared with the coplanar volumetric modulated arc therapy (coVMAT) plan optimized with the identical coplanar arc angle.

Results: There were no significant differences in mean conformity index, homogeneity index, and coverage of whole PTV between ncVMAT and coVMAT (p > 0.05). Conversely, the proportion of volume receiving 55 Gy (V₅₅) in PTV of IMN (PTVimn) decreased significantly from 2.05% ± 2.61% to 0.32% ± 0.49% (p = 0.005). Among the organs at risk, the average V₃₀, V₂₀, V₁₀, V_5, and mean dose (D_mean) of the heart decreased significantly from 6.76% ± 4.62%, 15.91% ± 10.34%, 38.34% ± 18.32%, 69.02% ± 18.18%, and 11.24 ± 3.83 Gy, respectively, to 5.93% ± 3.89%, 12.14% ± 7.66%, 27.86% ± 14.13%, 55.36% ± 20.36%, and 9.41 ± 3.30 Gy, respectively (p < 0.05). Furthermore, ncVMAT significantly reduced the mean V₂₀, V₁₀, V₅, and D_mean of left lung and the mean V₅ and D_mean of contralateral lung (p < 0.05). An improved sparing of the left anterior descending coronary artery and right breast were also observed with ncVMAT (p < 0.05).

Conclusions: Compared to coVMAT, ncVMAT provides similar conformity and homogeneity of whole PTV, with lower V₅₅ in PTVimn, better dose sparing of the heart, bilateral lungs, LAD, and right breast for locoregional radiotherapy of left-sided breast cancer with IMN, potentially reducing the risk of normal tissue damage.