ESTRO 38 Abstract book

S505 ESTRO 38

in deep inspiration breath hold (VMAT-DIBH). Conformity and Homogeneity index were calculated. Statistical analysis was performed. Data were expressed as mean ± standard deviation. Unpaired Student’s t-test was used to compare inter-group means. A P-value equal to or less than 0.05 was considered significant. Results Conformity and Homogeneity index were significantly better for VMAT compared to 3D plans. 4 partial arcs in VMAT-DIBH were appropriate for patients in breath hold, preserving efficacy of PTV coverage. Dose comparison showed a statistically significant reduction in heart and ventricle mean dose respectively from FB 3.8±1.9Gy 5.9±3.4Gy, to VMAT 3.0±0.8Gy 4.1±1.3Gy and DIBH 1.6±0.8Gy 2.0±1.3Gy. VMAT-DIBH was comparable with DIBH, heart 1.7±0.5Gy and ventricle 2.0±0.7Gy, but significantly reduced maximal dose in LAD (43%) and D2 DIBH reduced dose compared with FB in left breast cancer radiotherapy. VMAT showed to be the second choice in case of ineligible patient. VMAT-DIBH was feasible and comfortable for patients using small partial arcs in butterfly configuration, with a maximum delivery time of 20 sec per arc. VMAT-DIBH was efficacy adding LAD dose reduction (Dmean, Dmax and D2) to the DIBH advantages, at the expense of a small increase in the dose to the contralateral breast. Butterfly VMAT-DIBH improve heart protection in left breast cancer radiotherapy. PO-0938 Spine SBRT plan comparison for Cyberknife and VMAT delivery incorporating intrafraction PTV margin T. Oshea 1 , C. Jones 1 , C. Meehan 1 1 The Royal Marsden NHS Foundation Trust, Radiotherapy Physics, London, United Kingdom Purpose or Objective Stereotactic body radiotherapy is an effective method for treatment of spine metastases. At our institute the standard of care is treatment on the CyberKnife (CK) system with kV x-ray based intrafraction motion tracking. We investigate the ability to plan and verify the delivery of treatments using a Varian TrueBeam (TB) Linac with intrafraction motion accounted for in the PTV margin calculation. Material and Methods Intrafraction motion log data from nine previously treated CK patients (T1 – L2 vertebrae) was analysed and used to calculate PTV margins to account for delineation, phantom transfer and patient motion errors. Two-arc VMAT treatments were planned using the Varian Eclipse treatment planning system (with Acuros version 13.7.14 dose calculation) for 27 to 30 Gy in three fractions with 6FFF beams and 1400 MU/min dose-rate. Plans were compared with clinical CK plans which used a nominal 2.0 mm PTV margin. The deliverability of three plans (T1, T7 and L1) was verified by point dose measurement with a semiflex ionisation chamber in a solid water phantom. Results Mean (±SD) intrafraction motion using all CK log data was 0.3±0.8mm (SI), 0.2±1.3mm (RL) and 0.7±0.7mm (AP) resulting in PTV margins of 4.0, 4.3, 3.7mm (which also accounted for delineation, phantom transfer and set-up error estimates). The spinal cord PRV margin was 2.0 mm. The mean PTV coverage for VMAT plans with either 2 mm (87.6±2.7%) or 4 mm (84.3±2.2%) PTV margins was not significantly less than CK plans (85.9±4.8%). CTV coverage was significantly (p<0.05) better for VMAT plans. Risk organ doses were below UK NHS CtE guideline limits in all cases. The three verification plans were delivered to the solid water phantom on a TB Linac with an agreement between measured and calculated dose of 3.5±0.1%. LAD (42%). Conclusion

Figure 1. L1 vertebra: Comparison of CyberKnife (upper left) and TrueBeam (upper right) planning, example of log-file reconstructed intrafraction motion data used in PTV margin calculation (lower left) and solid water phantom plan verification (lower right). Conclusion A 4 mm PTV margin can be used to account for translational motion (and other error sources) in the absence of intrafraction motion compensation. PTV coverage for VMAT plans (with 4 mm PTV margin) was not significantly different to CK plans (with 2 mm PTV margin). 2-arc VMAT treatment fractions maintaining maximum dose-rate can be delivered in a much faster time (mean < 3.5 minutes vs. CK ~ 40 minutes) with deliverability verified within our institutes tolerance (<5%). PO-0939 Suspected impact of linear energy transfer on treatment related toxicities from proton therapy J. Ödén 1,2 , E. Traneus 2 , P. Witt Nyström 3,4 , I. Toma- Dasu 1,5 , A. Dasu 3 1 Stockholm University, Department of Physics, Stockholm, Sweden ; 2 RaySearch Laboratories AB, Department of Research, Stockholm, Sweden ; 3 The Skandion Clinic, The Skandion Clinic, Uppsala, Sweden ; 4 The Danish Centre for Particle Therapy, The Danish Centre for Particle Therapy, Århus, Denmark ; 5 Karolinska Institutet, Department of Oncology- Pathology, Stockholm, Sweden Purpose or Objective To analyse dose-averaged linear energy transfer (LET d ) and relative biological effectiveness (RBE) distributions for three patients with suspected treatment related toxicity following intracranial proton therapy. Various planning strategies to reduce LET d , RBE and normal tissue complication probabilities (NTCP) were also investigated. Material and Methods Plans for three patients treated with single-field uniform dose plans with fractionation doses of 1.8 Gy (RBE) to the target in 28 or 30 fractions (assuming RBE=1.1) were recalculated in an independent treatment planning system (TPS) with a Monte Carlo dose engine using the original CT- data, CT-calibration and beam data. The resulting physical dose distribution and LET d were used as input to calculate the RBE-weighted dose (D RBE ) using two LET d - and α/β- dependent variable RBE-models. For the targets, α/β values of 3 or 10 Gy were used, whereas 2 Gy were assumed for the normal tissues. Resulting distributions of D RBE and LET d were analysed together with NTCP estimations. Following this, four intensity modulated proton therapy (IMPT) plans were generated in order to investigate the potential of LET d , RBE and NTCP reductions in the critical structures: (1) IMPT with the clinical beam arrangements and only dose objectives, (2) alternative beam arrangements and only dose objectives, (3) clinical beam arrangements with dose and track-end objectives (penalized protons stopping in critical structures), (4) alternative beam arrangements with dose and track-end objectives. Results