Session Item

Planning breast cancer targets including lymph nodes with conventional technique is challenging in terms of organ at risk (OAR) sparing. Advanced planning techniques, such as VMAT provides better sparing of OARs at high-intermediate dose levels and better target homogeneity. However, in the absence of toxicity data after long-term follow-up, the spread of low dose to the contralateral side and increased spread to ipsilateral OARs should be optimized. The purpose of this study is to find an optimal planning technique for treating breast cancer with the aim to preserve the benefits of both conventional and VMAT technique and to improve the compromises of the two techniques.

Ten left-sided patients with breast cancer and lymph node irradiation (including IMN) scanned in deep inspiration breath hold (DIBH) were retrospectively selected. Dose plans using short tangential arc VMAT (STA), standard long arc VMAT (sVMAT) and conventional tangential field-in-field (CONV) were generated for each patient in Varian Eclipse planning system. STA plans were compared against sVMAT and CONV. The arc length and range of angles in STA affected the OAR doses the most and showed to yield the lowest OAR doses. Evaluation was performed using dose indices for both lungs, contralateral breast, heart, CTVbreast and CTVnodal.

Coverage criteria of V95%≥ 98% for CTVbreast and V90% ≥ 98% for CTVnodal were satisfied in all three techniques.

STA achieved the lowest Dmean to the heart (2.43 ± 0.99Gy) compared to sVMAT (3.46 ± 0.37Gy; p=0.003) and CONV (3.53 ± 2.07Gy; p=0.037). For ipsilateral lung, STA achieved lower V5Gy compared to sVMAT (p=0.034) and lower V17Gy and Dmean compared to CONV with p=0.001 and p˂0.001 respectively.  STA also achieved lower V5Gy and Dmean to contralateral breast and lung compared to sVMAT. Contralateral breast’s V5Gy reduced to 2.38 ± 2.49% vs 15.45 ± 8.68% (p=0.001) and Dmean  to 0.65 ± 0.42% vs 3.13 ± 0.85% (p˂0.001). Right lung received 1.16 ± 2.03% vs 16.41 ± 8.02% (p˂0.001) and 0.65 ± 0.30% vs 2.98 ± 0.51% (p˂0.001) for V5Gy and Dmean, respectively. The results for contralateral organs were comparable to the results using conventional technique (CONV).

In terms of delivery efficiency, STA achieved lower beam-on time (p=0.016) and total number of MUs (p˂0.001) compared to sVMAT whereas the values were comparable with CONV.

STA technique achieved lower OAR doses at high-intermediate range compared to CONV technique and significantly reduced the spread of low dose bath in the ipsilateral and contralateral OARs without compromising target coverage. Thus, STA may provide an optimal solution in difficult cases where the CONV technique is not sufficient.