ESTRO 36 Abstract Book

S504 ESTRO 36 2017 _______________________________________________________________________________________________

For Elekta accelerators, all the calculation results show a deviation from the reference values lower than 3%. For Siemens and Varian accelerators, the resulting calculations for fields larger than 2×2 cm 2 differ less than 4%. For 2×2 cm 2 large fields formed by Siemens and Varian MLC, the differences between the calculated and measured output factors often exceed 5%, but still are The RPC measured values provide a consistent dataset for small field output factors that can be used as a redundant QA check of a treatment planning system dosimetry data for small-field treatments. The results of the audit are very useful for the participants who should carefully investigate any detected discrepancies between the standard dataset and calculated values, with attention to the specific beam model. PO-0918 Radiotherapy and Her2 targeting agents: synergism and antagonism in clonogenic and confluence assays N. Suchowerska 1 , J. Toohey 2 , S. Carroll 2 , L. Rogers 2 , G. Lyons 3 , J. Beith 4 , A. Dos Santos Esteves 2 , D.R. McKenzie 5 1 Chris O'Brien Lifehouse, Radiation Oncology, Camperdown- Sydney, Australia 2 Chris O'Brien Lifehouse, Radiation Oncology, Camperdown Sydney, Australia 3 Royal Prince Alfred Hospital, Dermatology Cancer, Sydney, Australia 4 Chris O'Brien Lifehouse, Medical Oncology, Camperdown Sydney, Australia 5 University of Sydney, School of Physics, Sydney, Australia Purpose or Objective Her2 amplified cancers, comprising 15-20% of patients presenting with breast cancer, are now routinely prescribed Trastuzumab (Herceptin), a monoclonal antibody targeting Her2 receptors, leading to a significant improvement in outcomes in this previously high risk breast cancer subtype. Such targeting agents are rapidly being introduced into the clinic, based on trials showing a survival advantage. Now combination therapies with drug conjugates have emerged. The biological interactions of combined targeting agents, when given concurrently with radiation, are not well described. Our aim is to identify whether there is a synergistic or antagonistic interaction between targeting agents and ionising radiation for two Two molecular subtypes of HER2+ breast cancer cell lines were used: HCC-1954, which is ER and PR hormone negative and BT-474, a luminal B which is ER negative and PR positive. Both cell lines were treated to Her2 targeting agents (Trastuzumab and T-DM1) and radiation (6MV photons, 0 to 4Gy), individually and in combination to identify whether the response was synergistic, additive or antagonistic. The alpha/beta ratio was experimentally determined for each cell line. Synergy ( S ) is defined as the fractional difference between the observed ( S o ) and the predicted survival for each treatment given alone ( S 1 x S 2 ): below 10%. Conclusion distinct Her2+ subtypes. Material and Methods

showed a 20% decrease in survival when exposed to Trastuzumab alone, but a combined treatment with radiation did not yield the expected decrease in survival, indicating an antagonistic interaction. Conclusion Our results show that before starting clinical trials, the combination of radiation therapy and combined targeting agents needs to be closely examined for each sub-type under consideration. The assumption that a combination of treatments will result in a synergistic response is clearly not always true. Acknowledgements We acknowledge funding from the Sydney Breast Cancer Foundation PO-0919 Stereotactic radiotherapy for brain metastases : Cyberknife versus VersaHD / ExacTrac M. Perdrieux 1 , M. Celeste 1 , I. Lecouillard 1 , E. Nouhaud 1 , C. Blay 1 , F. Jouyaux 1 , N. Delaby 1 , J. Bellec 1 , C. Lafond 1 1 Centre Eugène Marquis, Radiotherapy, Rennes CEDEX, France Purpose or Objective The aim of this study was to compare dosimetric and geometric performances of the CyberKnife (Accuray) and VersaHD (Elekta) with the ExacTrac system (BrainLab) in stereotactic radiotherapy for brain metastases. Material and Methods This study was conducted on 10 patients for Cyberknife M6 v10.6 with Iris collimator and VersaHD equipped with ExacTrac v6.1 and the Frameless system (BrainLab). The prescribed dose was 27 Gy in 3 fractions with 1mm margin between CTV and PTV for both modalities. The dosimetric study was also conducted with 2 mm margin for VersaHD plans in accordance to our clinical practices. Plans have been computed for CyberKnife with non- isocentric non-coplanar beams generated by inverse optimization on Multiplan v5.3 (Accuray) with the RayTracing dose calculation algorithm. For VersaHD, 4 non-coplanar arcs (VMAT) have been generated b y inverse optimization on Pinnacle v9.10 (Philips ) with the Adaptative Convolution algorithm. For each case, plans were normalized to obtain the same PTV co verage at +/- 0.2 %. The comparison was based on the brain volume outside PTV receiving 23.1 Gy. The volume of isodoses 6 Gy, 2.7 Gy and 1 Gy have been reported as well as the Paddick’s Gradient Index to characterize the dose gradient around PTV and the spread of low doses. Quality controls have been performed with Gafchromic EBT3 films (Ashland) and with an ionization chamber (Pinpoint 31014 /PTW) in an anthropomorphic phantom (STEEV/CIRS). The measured dose with film has been compared to the calculated dose according to the gamma index method with a 3% (local) / 2 mm criteria (analytical threshold : 30% of the maximum dose). The geometric shift between the measured and calculated dose distribution has been also reported. Results Table 1 shows that dosimetric criteria for plan validation were reached for both modalities and both margins. Compared to VersaHD, dose gradients obtained with Cyberknife were greater and lower volumes of healthy tissue received doses below 6 Gy. Ionization chamber measurements showed mean differences with the calculated dose of 2.53% and 0.03% for Cyberknife and VersaHD respectively. The mean value of the gamma index was 0.42 for the Cyberknife and 0.38 for the VersaHD. The mean geometric shifts between the measured and calculated dose distributions were 0.87 mm and 0.84 mm for Cyberknife and VersaHD respectively.

The observed response was determined using two assays: the clonogenic assay and the confluence assay. Results The alpha/beta ratio for HCC-1954 (ER-/PR-/Her2+) and BT474 (Luminal B ER-/PR+/Her2+) are found to be 35 Gy and 5 Gy respectively, highlighting a heterogeneous treatment response. The survival of HCC-1954 was not affected by Trastuzumab alone, but when combined with radiation, a synergistic interaction was observed. BT-474