ESTRO 36 Abstract Book

S508 ESTRO 36 _______________________________________________________________________________________________

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

Conclusion It is feasible to generate conventionally fractionated treatment plans for LA NSCLC patients on a 1.5 T MR-linac with minor differences in dose-volume metrics, which are unlikely to be clinically meaningful. When using standard PTV margins, isotoxic dose escalation was limited on the MR-linac. However, reducing margins alleviates these observed effects. This study only represents an early indicator of the treatment implications of MRI-guided radiotherapy. It is conceivable that the availability of MRI- guidance will result in further benefits through inter- and intrafractional treatment adaptation. PO-0917 Nationwide audit of small fields output calculations in Poland W. Bulski 1 , K. Chelminski 1 1 The Maria Sklodowska-Curie Memorial Cancer Center, Medical Physics Department, Warsaw, Poland Purpose or Objective The delivery of accurate intensity-modulated radiation therapy (IMRT) or stereotactic radiotherapy depends on a multitude of steps in the treatment delivery process. Within the treatment planning system’s (TPS) dose calculation algorithm, various unique small field dosimetry parameters are essential, such as multileaf collimator modeling and field size dependence of the output. One of the most considerable challenges in this process is to determine accurate small field size output factors. Modern radiotherapy routinely involve s the use of small radiation fields as components of IMRT. Because of the difficulties in commissioning small field data, a set of field size dependent output factors could prove to be an invaluable tool to confirm the validity of an individual institution’s dosimetry parameters. Such a set of data has been prepared by the Radiological Physics Center (RPC), M. D. Anderson Cancer Center, Houston. The RPC has gathered multiple small field size output factor datasets for X-ray beam qualities, ranging from 6 to 18 MV, from Varian, Siemens and Elekta linear accelerators. These datasets were measured at 10 cm depth and ranged from 10×10 cm 2 to 2×2 cm 2 . Within the framework of the IAEA CRP E2.40.16 project "Development of Quality Audits for Radiotherapy Dosimetry for Complex Treatment Techniques, a methodology of the audit of small field output performance was established. Material and Methods The participants had to calculate t he output factors for the beams formed by the multi-leaf collim ator (MLC). The results of their calculations were compared with the reference RPC data. 32 Polish radiotherapy departments took part in the audit. In total, 65 beams were audited. The participants of the audit were asked to calculate the number of monitor units (MU) for the delivery of a prescribed dose to water with square fields of different sizes. A dose of 10 Gy was prescribed to a reference point at 10 cm depth on the central axis, at 100 cm source-to- phantom distance (SFD). The output factors for five field sizes, 10×10, 6×6, 4×4, 3×3 and 2×2 cm 2 , shaped by a multileaf collimator (MLC), were calculated. Results

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