ESTRO 37 Abstract book

S1120

ESTRO 37

Purpose or Objective Comprehensive post-mastectomy radiation therapy is technically difficult given the complexity and extension of the target volume. High conformal techniques as volumetric modulated arc therapy (VMAT) can be a valid solution for its ability to shape dose distributions around target in a short delivery time. Nevertheless, the close proximity of the target to critical structures including the heart is a big concern. The purpose of this work is to evaluate the feasibility of estimating radiation dose delivered to the heart by integrating kilovoltage-cone beam computed tomography acquired during the course of the treatment. 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

manually corrected. Coverage probability maps (CPM) were generated: the volumes encompassed by 8/15 (BL50%) and 12/15 (BL80%) daily contours, i.e about 50% and 80% probability of BL position during HT, were assessed: BL80% was considered representative of the most stable BL regions. Daily BL DVH were recalculated on the planning kVCT: average DVH during RT (DVHm) and DVH of BL50%/BL80% were compared against planning DVH (DVHpl). Results

BL50% roughly follows the planning BL contours in all pts, excepting a more caudal position of descending and sigmoid colon in 2 adjuvant pts due to a different average bladder filling during HT. BL80% volumes were much smaller, particularly for adjuvant pts, and mostly located next to the right (cecum) and left external iliac lymph node chains. Small bowel was always associated with random motion, resulting in an almost absent BL80% in the central part of the intestinal cavity. Regarding the impact on DVHs, a summary is shown in Figures 1 and 2: the differences between planning and BL50% values of V45-V50 were < 5cc/10cc for adjuvant/radical pts. V45- V50 of BL80% were much smaller, (3-5 times less than DVHpl), with higher values in radical pts compared to adjuvant pts. Conclusion Daily MVCTs were able to quantify bowel CPM and assess the most likely location of stable BL during pelvic RT. BL next to the external iliac lymph node chains were quite stable while small bowel was associated to large random motion. A systematic impact of different bladder filling was evident in 2/10 pts. Absolute V45-V50 of the stable BL were 3-5 times smaller than the planned ones: this result claims the need of new planning strategies for bowel sparing and suggest to explore BL dose-volume effect by focusing on the anatomical segments more likely to stay stable during RT. EP-2044 Inter-fraction variation of the heart dose for VMAT post-mastectomy irradiation of chest wall B. Tang 1 , S. Kang 1 , P. WANG 1 , J. Li 1 , X. Xin 1 , X. Liao 1 , J. Ma 1 , L.C. Orlandini 1 1 Sichuan Cancer Hospital & Institute, Radiation Oncology Department, Chengdu, China