Abstract Book

S1237

ESTRO 37

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. EP-2363 Comparison of VMAT and IMRT for post- operative RT in prostate cancer patients with a rectal balloon H. Jung 1 1 Seoul National University Bundang Hospital, radiation oncology, Seongnam-si-, Korea Republic of Purpose or Objective In radiotherapy for prostate cancer, different treatment techniques result in different dose distributions in the major organs and healthy tissues. This study established treatment plans for VMAT and IMRT, which have different numbers of rotations, in post-operative RT in prostate cancer patients with a rectal balloon. The dose distribution characteristics of these treatment plans were compared, and their treatment efficiency was evaluated. Material and Methods The subjects consisted of 10 patients who underwent post-operative RT for prostate cancer. CT images were obtained from patients with a rectal balloon using a slice thickness of 3 mm, and Eclipse (Ver 11.0, Varian, Palo Alto, USA) was used to apply 10 MV from a Truebeam STx (Varian, Palo Alto, USA) fitted with an HD120MLC. 1-arc VMAT, 2-arc VMAT, and 7-field IMRT plans (1A-V, 2A-V, 7F-I) were generated for each patient, using the same dose volume constraint and plan normalization values in all plans. In order to evaluate the 3 plans, we compared the coverage of the PTV, the conformity index (CI), and the homogeneity index (HI); in order to examine the dose delivered to adjacent healthy tissue, we calculated the ratio of 50% isodose volume to the PTV (R 50% ). Among the organs at risk (OAR), D 25% in the rectum and D mean in the bladder were compared. In order to evaluate treatment efficiency, MU and irradiation time were measured; for each evaluated variable, the mean was calculated across the 10 patients and this value was used in the comparative analysis. Portal dosimetry was also performed using an EPID to test the accuracy of dose delivery. Results Although there was no significant difference between treatment plans in PTV coverage or HI, CI was 1.036, 1.035, and 1.230 in 1A-V, 2A-V, and 7F-I, respectively, meaning that it was significantly higher in 7F-I than in VMAT (p=0.00). R 50% was 3.083, 3.054, and 3.991, respectively, meaning that it was lowest in 2A-V and highest in 7F-I (p=0.00). D 25% in the rectum was similar between VMAT strategies, but was approximately 7% higher for 7F-I (p=0.02), while D mean in the bladder did not show any significant differences (p>0.05). Total MU was highest for 7F-I at 757.9 (p=0.00), compared to 494.7 and 479.7 for 1A-V and 2A-V, respectively. Irradiation times were 65.2s, 133.1s, and 145.5s, meaning that 1A-V had by far the shortest irradiation time (p=0.00). In portal dosimetry tests, all treatment plans showed a gamma pass rate (2 mm, 2%) of at least 99.5% (p=0.00). Conclusion This study showed that when post-operative RT was performed in prostate cancer patients with a rectal balloon, although there was little difference in PTV coverage according to treatment technique, 1A-V and 2A- V were effective at reducing the dose delivered to the

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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 NTO7 1.0 105 40 0.05 NTO8 1.0 105 20 0.05 NTO9 1.0 105 60 0.15 NTO10 1.0 105 60 0.40 NTO11 0.2 80 20 1.00 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%).

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