ESTRO 37 Abstract book

ESTRO 37

S478

20 patients presenting early stage breast cancer were selected for planning 40.05 Gy in 15 fractions. Two VMAT arrangements (VMAT_tang and VMAT_full) and two 3DCRT (field-in-field, FinF, and using hard wedges, 3D_Wdg) plans were optimized and calculated with an accurate dose calculation algorithm, Acuros within the Eclipse treatment planning system. VMAT optimization used the Photon Opimizer inverse planning engine (vers. 13.6), while in 3DCRT plans the forward planning strategy was used. Objective was to minimize the dose in lungs, contralateral breast, heart and coronary artery. Particular efforts were paid to the contralateral structures in the frame of second cancer induction risk minimization. EAR was estimated using a model that uses carcinogenesis model and epidemiological data for carcinoma induction, and accounts for cell repopulation/repair during the radiotherapy dose fractionation. EAR was computed for contralateral breast and lung, as well for ipsilateral lung. Normal tissue complication probability (NTCP) was computed to estimate the ipsilateral lung, heart and skin toxicity, to balance with respect to second cancer induction. Results EAR for contralateral breast carcinoma induction was estimated to be 1.7, 2.4, 2.8 and 8.5 [/10’000 patients per year] with FinF, VMAT_tang, 3D_Wdg, and VMAT_full, respectively. For contralateral lung, the corresponding figures were 1.5, 1.6, 2.1 and 7.3 [/10’000 patients per year]. NTCP, for all the analyzed endpoints was significantly higher with FinF relative to both VMAT, with the VMAT_full presenting the lowest toxicity risk. 3D_Wdg presented the highest NTCP for all OARs. Conclusion VMAT, in particular with VMAT_tang setting, could have the same risk of second cancer induction as 3DCRT delivered with FinF on contralateral organs, while reducing acute and late NTCP for ipsilateral organs. VMAT might be considered a safe technique for breast cancer treatment for those aspects. The use of hard wedges with 3DCRT should be minimized. PO-0899 Uncertainties in dose-response relations strongly affect the expected gains of robust dose- painting S. Petit 1 , S. Breedveld 1 , J. Unkelbach 2 , D. Den Hertog 3 , M. Balvert 3 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands 2 University Hospital Zürich, Department of Radiation Oncology, Zurich, Switzerland 3 Tilburg University, Department of Econometrics and Operations Research/Center for Economic Research CentER, Tilburg, The Netherlands Purpose or Objective The first dose-painting trails have been initiated, but the largest challenge of dose-painting has not been solved yet: how much dose should be delivered to each tumor region. Ideally dose-painting plans are optimized based on dose-response relations of the different tumor regions, but these relations are extremely uncertain. The goal of this study is to determine how uncertainties in dose- response relations affect the potential benefit of dose- Poster: Physics track: Treatment plan optimisation: algorithms

Conclusion Prostate volume is estimated before treatment for computing the required number of seeds. The volume is also measured during treatment using manual contouring on rectal ultrasound images. The two obtained values may significantly differ, for example because of oedema caused by needle insertion. Since 80 patients are treated each year at our centre, 20 remaining seeds for each patient would lead to 1600 radioactive seeds. The management of such an amount of radioactive seeds is costly and very time consuming. In this context, the machine-learning-based abacus that we propose in this study aims at estimating the necessary number of seeds for future patients according to our past experience (Fig.2B). This new abacus is well-suited to our centre, can be adapted to any other one, and is a good alternative to non-specific methods that are generally proposed by manufacturers. PO-0898 Second cancer induction risk from breast radiotherapy with VMAT and 3D conformal therapy A. Fogliata 1 , F. De Rose 1 , D. Franceschini 1 , A. Stravato 1 , J. Seppälä 2 , M. Scorsetti 3 , L. Cozzi 3 1 Humanitas Cancer Center, Radiotherapy and Radiosurgery, Milan-Rozzano, Italy 2 Kuopio University Hospital, Cancer Center, Kuopio, Finland 3 Humanitas Cancer Center and Humanitas University, Radiotherapy Radiosurgery and Biomedicine Faculty, Milan-Rozzano, Italy Purpose or Objective The use of intensity modulation for breast cancer radiotherapy, particularly the volumetric modulated arch therapy (VMAT) is often considered with skepticism, due to the increased low-dose bath delivered with respect to the consolidated 3D conformal radiotherapy (3DCRT) delivered with the classical two tangential fields. Aim of this work is to evaluate the excess absolute risk (EAR) comparing VMAT and 3DCRT in breast cancer

radiotherapy treatment. Material and Methods