Non-coplanar VMAT improves plan quality for locoregional radiotherapy of left-sided breast cancer with internal mammary nodes

Purpose: A planning study was performed to evaluate the feasibility of non-coplanar volume modulated arc therapy (ncVMAT) for locoregional radiotherapy of left-sided breast cancer with internal mammary nodes. Methods and materials Ten patients with left-sided breast cancer after breast conserving surgery were retrospectively studied. The planning target volumes (PTV) were contoured encompassing the whole breast/chestwall, internal mammary nodes (IMN) and supraclavicular nodal (SCN) region. For each patient, ncVMAT plan with 4 partial arcs composing of two coplanar arcs and two non-coplanar arcs with couch rotating to 90˚ was generated. The prescription dose was normalized to cover 95% of PTV with 50 Gy delivering in 25 fractions. For each ncVMAT plan, dosimetric parameters were compared with coplanar VMAT (coVMAT) plan optimizing with identical coplanar beam angle arrangement. Results With ncVMAT, the mean conformity index of the whole PTV increased from 0.82±0.02 to 0.86±0.01 (p=0.005), and there was no significant difference for the homogeneity index. The mean coverage of internal mammary target volume (PTVimn) with ncVMAT increased from 88.77±3.07% to 91.67±3.84% comparing with coVMAT (p=0.005). Meanwhile, the V55 in PTVscn and PTVimn decreased significantly from 5.76±6.03%, 13.73±7.77% to 2.94±3.93%, 3.75±3.36%, separately (p<0.01). For organs at risk, the average V30, V20, V10, V5 and Dmean of heart decreased from 5.13±3.22%, 12.18±7.29%, 33.98±15.73%, 72.03±16.79% and 10.47±2.97 Gy to 4.86±2.54%, 9.35±5.43%, 25.16±12.42%, 62.27±13.18% and 9.08±2.34 Gy, separately (p<0.01). Furthermore, ncVMAT significantly reduced the mean V20, V10, V5, Dmean of left lung, and V10, V5, Dmean of contralateral lung (p<0.01). A better sparing of LAD descending coronary artery and right breast were also shown with ncVMAT (p<0.01). Conclusions Compared to coVMAT, ncVMAT provides better conformity, lower V55 in PTVscn and PTVimn, better coverage of PTVimn, better dose sparing in heart, bilateral lung, LAD and right breast for locoregional radiotherapy of left-sided breast cancer with internal mammary nodes, which potentially increase the local control in IMN and reduce the risk of deleterious effects.