Abstract Book

S1118

ESTRO 37

Material and Methods Twenty-seven patients admitted in our hospital for post left mastectomy irradiation were included in this retrospective study. Patients underwent a CT scan with 3-mm slice thickness in the supine position on a commercial breast board. The patients received 50Gy in 25 fractions delivered with a 6MV photons beam of an Elekta Axesse linear accelerator equipped with xVi cone beam computed tomography (CBCT). VMAT treatments were performed using Pinnacle 3TM treatment planning systems (TPS). Targets and ROI were defined by an experienced radiation oncologist following the Radiation Therapy Oncology Group guidelines for breast cancer. Weekly CBCT were acquired along the course of the treatment. The volume of the heart defined on the planning CT was compared with the volumes contoured in the different CBCT series acquired. The DVH for the Heart_CBi was recorded. The heart DVH obtained with the CBCT of the first session (CBCT 0 ) was considered as baseline for further comparisons. The percentage volume of the heart receiving 5Gy(V5), 10Gy(V10) and 25Gy(V25) and D mean extracted from dose-volume histogram (DVH)of the planning CT were collected for all the ROIs of the heart along the radiotherapy course and compared. Results A total of 135 CBCT scans of 27 patients were retrospectively analysed. The heart volume from planning CT images was smaller than the average volume from the subsequent CBCT scans (535 ± 67 cm 3 vs. 643 ± 85 cm 3 ; p < 0.05); moreover, for each patient the volumes based on the different series of CBCT images were comparable (p > 0.05). All relative and absolute dose-volume indices obtained from CBCT images were significantly larger than those from planning CT scans (p < 0.05), with the exception of V5 which remained the same. The comparison between CBCT 0 and the subsequent CBCTi shown a variation of the heart V10, V25 which resulted out the tolerance level for four patients out of 27. Conclusion The use of the CBCT acquired during the clinical routine can be a useful tool to monitor the dose delivered to the heart. An added workload due to the heart contouring, images registration and dose evaluation should be considered for this procedure. EP-2045 Breast morphology changes during irradiation: a measurement method based on optical surface scanning Y. Pin 1 , H. Seo 2 , M. De Mathelin 3 , G. Noël 1 1 Centre de lutte contre le cancer Paul Strauss - Strasbourg- France, Radiation Oncology, Strasbourg, France 2 Icube UMR, Équipe Informatique- Géométrique et Graphique, Illkirch-graffenstaden, France 3 Icube UMR, Équipe Automatique- Vision et Robotique, Illkirch-graffenstaden, France Purpose or Objective Breast radiotherapy, after conservative breast surgery, is a systematic step in the treatment of women with breast cancer. Being a mobile external organ, mammary gland is subject to several uncertainties regarding the daily breast positioning. To the best of our knowledge, morphological variations of the breast and their impact on the treatment quality remain unknown. We propose here an original method based on optical 3D surface scanning to get objective measures of the irradiated breast volume evolution and its local deformations. Material and Methods We developed a software using the dosimetric CT structures and patient surface scans ( S3D ) acquired during the treatment. The hand-held, optical, non ionizing surface scanner Artec (R) Eva was chosen for its quick acquisition and infra millimeter precision and resolution characteristics. When realizing the dosimetric

CT, a metal wire was placed on the patient’s skin to circumscribe the palpated gland. Surfaces were aligned with manual and automated registration. Then a surface correspondence algorithm was applied. The wire delimiting the targeted gland on the dosimetric CT was then virtually identified on a S3D selected as reference ( refS3D ) thanks to correspondence data from CT to S3Dref , then identified on other S3Ds (correspondence data from S3Dref to each S3D ) to recognize breast position and perform volume evaluation. Deformations were evaluated by 325 measurements (each 10 degrees on a half-sphere) from the center of the breast base to the skin. To evaluate our software we performed a dosimetric CT, contoured the skin, the targeted breast and wire, then performed a S3D . For all tests, the S3Dre f was exactly the same. Volume and deformation measurement reproducibility was tested on 5 strictly identical S3Ds (and identical to the S3Dref ). Robustness to extra breast misposition was tested on 5 S3Ds progressively deformed. Then, the ability to measure targeted breast morphology was tested on 5 S3Ds with known volume variation. Results Reproducibility tests - We found a global mean volume deviation of +2.9% ([1.2% ; 3.3%], σ = 0.82). A mean error of +0.04mm (σ = 3.71) was observed in deformation measurements. Robustness to mispositions - We described a mean volume deviation of -1.8% ([- 11.1% ; 4.2%], σ = 5.37). A mean error of -0.36 mm (σ = 3.01) was observed in deformation measurements. Ability to follow breast volume changes - The software had a mean volume deviation of +3.37% ([0.04% ; 5.84%], σ = 2.25).

Figure: Volume variations measurement Conclusion

We propose a reproducible and clinically relevant method to follow irradiated breast volume and morphology during radiation therapy after con-servative surgery for breast cancer. By the use of non ionizing surface scanner, we could propose, in the future, a safe and easy to implement clinical trial to study breast deformations and volume changes and their impact on the treatment quality. EP-2046 Robust optimization (4D) IMPT strategies using full and empty bladder CT scans for cervical cancer C.L. Brouwer 1 , W.P. Matysiak 1 , P. De Boer 2 , J.A. Langendijk 1 , J.C. Beukema 1 , S. Both 1 1 University of Groningen- University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 2 Academic Medical Center, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective External beam radiotherapy of cervical cancer is challenged by large inter- and intra-fraction motion,