ESTRO 37 Abstract book

NTO7 1.0 NTO8 1.0 NTO9 1.0 NTO10 1.0 NTO11 0.2

105 105 105 105

40 20 60 60 20

0.05 0.05 0.15 0.40 1.00

80

Results Eight of 16 flagged plans achieved lower normal brain dose and increased dose to central GTV on replan based on geometry, six of the eight improved plans had static MLCs. Repeated measures one-way ANOVA showed no significant difference for OAR Dmax for any optimization strategy. Brain-GTV V10Gy was significantly higher with manual NTO strategies two through ten compared to the most aggressive NTO (NTO11) or manual strategies (p<0.01) (Figure 1). GTV Dmax was significantly lower with most NTO strategies compared to NTO11 or manual strategies (p<0.01). NTO11 lead to increased Dmax to GTV and doses above 120% being placed at the GTV periphery in some cases which is highly undesirable. Total plan MU did not change significantly for any of the optimization strategies. Reoptimizing plans with automatic NTO did not reduce brain dose but did increase the average number of MU with each iteration (3-5%).

Conclusion A breath hold technique combined with a VMAT plan reduces the mean heart dose significantly for a local recurrence of left sided breast cancer patient in previously irradiated area, compared to a free breathing VMAT technique. EP-2362 Optimization strategies for stereotactic radiosurgery plans in Eclipse S. Ehlert Tvile 1 , N.K. Jensen 2 , L. Ohlhues 2 , L.S. Fog 2 1 Rigshospitalet, Finsenscenteret- Dep. of Oncology, Roedovre, Denmark 2 Rigshospitalet, Finsenscenteret- Dep. of Oncology, Copenhagen, Denmark Purpose or Objective Determine which optimization strategy in Eclipse provides optimal normal tissue sparing without compromising target coverage for stereotactic radiosurgery (SRS) plans for single and multiple brain metastases. Material and Methods 18 patients with 21 plans treated for one or multiple brain metastases after February 1 st , 2017 were reviewed by two medical physicists and two dosimetrists. Each reviewer independently evaluated each plan and noted if they agreed with the chosen gantry, collimator and couch angles. Any plan flagged by at least one reviewer was replanned with a different choice of arc span, collimator angle and/or couch angle to better avoid nearby OARs. All planning was done in Eclipse version 13.7 with the Photon Optimizer and AcurosXB on a 1x1x1 mm 3 resolution grid. All flagged plans originally planned with VMAT (Ten plans from eight patients) were replanned with 13 different normal tissue optimization strategies: DVH objective only on brain-GTV, DVH objective only on ring support structure (4 mm from GTV, 30 mm wide), and 11 normal tissue objective (NTO) parameter sets (Table). Each plan was normalized to 100% isodose to cover GTV. The following doses were extracted from each plan: GTV Dmax, brain–GTV V10Gy, Dmax to optic chiasm, brainstem, and both optic nerves, eyes, hippocampi, cochlea and lenses. V10Gy for brain was chosen as it has been shown to predict radionecrosis (Minniti G. et al., Radiat Oncol. 2011). All plans generated with automatic NTO were reoptimized four times iteratively to determine if the possible benefit of reoptimization. NTO1 Automatic Automatic Automatic Automatic NTO2 1.0 105 60 0.05 NTO3 0.3 105 60 0.05 NTO4 0.6 105 60 0.05 NTO5 1.0 95 60 0.05 NTO6 1.0 85 60 0.05 Name Distance [cm] Start dose [%] End dose [%] Fall-off

Figure 1

Figure 2

Conclusion We recommend manual optimization objectives on either normal brain minus GTV or ring support structure as well as adjacent OARs instead of NTO for SRS planning. Automatic and default values of NTO appear unsuited for SRS treatment planning.