Abstract Book

S1117

ESTRO 37

Results

Conclusion Passing rates for the HNC cases substantially decreased through the course of treatment. Analysis of on- treatment EPID images can be used to detect the inter- fractional anatomical variations in the HNC treated with VMAT. However, passing rates were stable for the PC cases. EP-2043 Assessing The Impact Of Bowel Motion In Pelvic Imrt By Daily MVCTs And Probability Distribution Map L. Perna 1 , M. Mori 1 , C. Cozzarini 2 , G.M. Cattaneo 1 , C. Sini 1 , R. Calandrino 1 , C. Fiorino 1 1 San Raffaele Scientific Institute, Medical Physics, Milano, Italy 2 San Raffaele Scientific Institute, Radiotherapy, Milano, Italy Purpose or Objective Whole pelvic radiotherapy (WPRT) may cause gastro- intestinal (GI) toxicity due to the irradiation of bowel loops (BL). Predictive models of toxicity based on BL DVH show conflicting results, likely due, at least in part, to BL changes during WPRT. The aim of current study was to develop, implement and apply a method to assess inter- fraction BL motion and quantify its impact on the dose received by BL. Material and Methods Ten prostate cancer patients (pts) were enrolled: 5 adjuvant pts treated with a SIB technique delivering 52.5/60Gy to pelvic nodes (PN)/prostatic bed (followed by a boost on the prostatic bed); 5 high risk pts treated with radical intent to 51.8/65.5/74.2Gy in 28 fractions in a SIB approach to PN/distal seminal vescicles(SV)/ prostate+proximal SV respectively. All pts were treated by Helical Tomotherapy (HT) with full bladder. For each pt, BL contours (including sigmoid, cecum, pelvic large and small bowel), were drawn on fifteen MVCTs collected every other day during WPRT. A previously validated semi-automated method for BL segmentation, was applied: BL contours were propagated from the planning kVCT to in-room MVCTs by elastic registration and then 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).

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 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.