Abstract Book

S1030

ESTRO 37

Material and Methods Eleven left sided breast cancer patients with lymph node involvement were retrospectively analysed with split and non-split arc designs using RapidArc optimiser (Varian Medical Systems, Palo Alto, CA). The patient selection criteria were high lung and heart dose in conventional planning. The traditional non-split arc techniques consisted of two 190° arcs. In the split arc technique, the 190° arcs were split into half at the gantry angle 65° and refocusing the fields to the PTV shielding the lungs and heart by the medial jaw (see Figure 1). Same optimisation criteria were used in both plans.

mm to H, because of the difficulty to spare hippocampus without compromising target coverage. After re-planning D 40% constraint value (7.3 Gy) was met by all the 9 NHS plans. These results are reported in Table 1 . The DVH and 3D structure view of an HS plan for one representative patient (Patient # VI) is shown in Figure 1(a) and (b), respectively. For this plan, both NHS and HS plan had the same CI (0.83) and H values (1.04). D max and D 40% changed from 15.5 Gy to 12.8 Gy and 10.3 Gy to 5.4 Gy, respectively, for H and from 23.8 Gy to 20.0 Gy and 9.9 Gy to 6.4 Gy, respectively, for HZA.

Results Results are shown in Table 1. No significant difference could be seen in PTV coverage and contralateral lung mean dose between the different VMAT designs. Ipsilateral lung dose and heart V5Gy were significantly lower than those in non-split VMAT design. The contralateral breast dose V5Gy was significantly larger for split VMAT plans than in non-split VMAT designs.

Conclusion In conclusion, this study suggests that an hippocampal- sparing approach to FSBRT is feasible resulting in a decrease of the dose to the hippocampus without any loss in conformity or increase in treatment time. HS plans maintain the same target conformity and homogeneity, the same mean dose to surrounding healthy tissues and the same treatment time of the original plans. Although safe threshold doses for the hippocampus have not been defined yet, it is strongly advisable to delineate the hippocampus and put in practice all the necessary strategies to reduce doses especially in patients with a reasonable life expectancy. EP-1901 Split arc VMAT technique for radiotherapy of lymph node positive breast cancer E. Boman 1 , M. Rossi 1 , M. Haltamo 2 , T. Skytta 2 , M. Kapanen 2 1 Tampere University Hospital, Department of Oncology- Department of Medical Physics, Tampere, Finland 2 Tampere University Hospital, Department of Oncology, Tampere, Finland Purpose or Objective Radiotherapy (RT) of breast cancer becomes more challenging when axillary lymph nodes and/or internal mammary nodes are included in the planning target volume (PTV) possibly resulting increased lung and/or heart dose. Volume modulated arc therapy (VMAT) with or without deep inspiration breath hold (DIBH) technique is proven to reduce the dose to organs at risk (OARs). The aim of this study was to investigate the dosimetric advantages in OAR sparing and PTV coverage using a novel VMAT arc design for lymph node positive breast cancer patients when compared to standard VMAT designs.

Conclusion The proposed split VMAT technique was shown to be superior to previously published non-split arc technique