ESTRO 36 Abstract Book

S240 ESTRO 36 2017 _______________________________________________________________________________________________

Three hollow plexiglass cubes filled with VIPAR polymer gel were produced and used in this study. Planning CT scans of each one of the gel filled cubes and arbitrary RStructures have been used for treatment planning. Cube- 1 was planned to be irradiated with mono-energetic proton beams (90MeV & 115MeV) avoiding overlapping of the irradiated gel areas (Max Dose : ~ 15 Gy). Cube-2 was planned to be irradiated with a multi-energetic beam forming a spread-out Bragg peak (SOBP) (Max Dose : ~ 13 Gy). Cube-3 was planned to be irradiated with two opposing beams (Max Dose : ~ 13 Gy) each delivering an overlapping and uniform SOBP. Set-up and irradiation of each cube followed. One day post-irradiation each cube was MRI scanned in order to derive high spatial resolution 3D-T2 maps that were subsequently co-registered to the corresponding planning-CT scans and DICOM-RT Dose and Structure data. Assuming a linear gel dose response, 1D, 2D and 3D dose measurements were derived and compared against corresponding TPS data. Results VIPAR gel response seem to be non-dependent on LET for LET values < ~6 keV/µm implying that their use for most clinical cases is acceptable. No matter their LET dependence, the protons range can be well verified. Even if uncertainties related to imaging, set-up, beam delivery, dose calculations, co-registration, gels LET dependence were incorporated, the range measured by the proposed method was within ~ 1 mm to that calculated by TPS. Moreover, the corresponding ranges at the 80% value of the maximum dose point for both TPS and polymer gels derived percentage depth dose profiles (pdds) were equal within ~1 mm. Additionally, for the opposed beams experiment (cube-3), the proposed methodology results in even more accurate dosimetry due to the reduced LET values inside the SOBP compared to the high LET values present in the irradiated schemes of cubes 1 and 2. Conclusion The proposed End-to-End Quality Assurance method based on polymer gel dosimetry, provides valuable outcomes for proton range verification and 3D proton dosimetry.

The objective was to study the differences in target coverage and dose-volume parameters for heart and lung between Deep Inspiration Breath Hold (DIBH) 3D Conformal Radiation Therapy (3D-CRT), DIBH Volumetric Modulated Arc Therapy (VMAT) and free breathing Intensity Modulated Radiation Therapy (IMRT) in patients treated with synchronous bilateral breast cancer. Material and Methods This planning comparative study was conducted in nine patients previously treated for synchronous bilateral breast cancer. These patients were treated with either DIBH 3D-CRT or IMRT in free breathing. All patients were treated with whole breast irradiation and those requiring a boost were given a simultaneously integrated boost (SIB). Three treatment plans were constructed for each patient individually; a DIBH 3D-CRT plan, a DIBH VMAT plan and an IMRT plan in free breathing. DIBH IMRT is clinically not feasible due to the extended duration of treatment. Three patients were treated without a boost, three were treated with unilateral SIB and the remaining patients were treated with double sided SIB. DIBH 3D-CRT plans were created using tangential fields for both breasts and up to three boost fields for each breast, if a boost was required. IMRT plans were created using 14 fields around the patient, 24° apart, covering both breasts and simultaneously covering the boost target in one or both breasts. DIBH VMAT plans without boost targets were created using eight 30° arcs, four on each side, oriented in a tangential design. Four 60° arcs, in a tangential design, were used in patients with boost targets, two for each breast, with an additional semi-circle arc on either side covering the boost targets. The parameters reviewed were V95% (percentage of volume receiving 95% of the prescription dose) PTV1 and PTV2 coverage, with PTV1 being the elective target and PTV2 the boost target, the mean heart dose and heart left ventricle V5 (percentage of volume receiving 5 Gy), mean lung dose, lung V5 and lung V20. The parameters were compared using the paired T-test for normally distributed data and the Wilcoxon signed rank-test for not normally distributed data. Three statistical analyses were performed on each parameter, therefore the Bonferroni correction was applied. P ≤0.016 was considered statistically significant in this study. Results Target coverage of PTV1 and PTV2 were comparable between the three techniques (table 1), except the V95% PTV1 left. All dose volume parameters of the heart and lung were lower for the DIBH VMAT technique (table1) in comparison with the DIBH 3D-CRT and free breathing IMRT technique.

A. T2-map of the irradiated polymer gel cubic phantom, co-registered to the corresponding planning-CT scans and TPS calculated dose. B. Pdd measurements C. Isodoses in an arbitrary 2D plane D. GI (5%dose/ 2mm criteria) calculated by the data presented in C First row: SOBP irradiation. Second row: Mono-energetic 115 MeV irradiation

Conclusion DIBH VMAT is the most optimal radiation technique in the treatment for patients with synchronous bilateral breast cancer. Both PTV coverage and the sparing of the organs at risk give better results for DIBH VMAT in comparison with DIBH 3D-CRT and IMRT in free breathing. PV-0457 Delay between planning and stereotactic radiotherapy for brain metastases: margins still accurate? C. Bonnet 1 , A. Dr Huchet 1 , E. Blais 1 , J. Dr Benech-Faure 1 , R. Dr Trouette 1 , V. Dr Vendrely 1 1 Hopital Haut Leveque, Radiotherapy, Pessac, France

Poster Viewing : Session 10: RTT

PV-0456 Volumetric Modulated Arc Therapy for patients with bilateral breast cancer S. Lutjeboer 1 , J.W.A. Rook 1 , G. Stiekema 1 , A.P.G. Crijns 1 , N.M. Sijtsema 1 , E. Blokzijl 1 , J. Hietkamp 1 , J.A. Langendijk 1 , A.J. Borden van der 1 , J.H. Maduro 1 1 UMCG University Medical Center Groningen, Radiation Oncology, Groningen, The Netherlands

Purpose or Objective