ESTRO 36 Abstract Book

S253 ESTRO 36 2017 _______________________________________________________________________________________________

tamoxifen treatment. Since high expression levels of ISGs are associated with a worse outcome in patients treated with tamoxifen, this pathway could be a valuable new target in patients, possibly also those treated with radiotherapy.

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Proffered Papers: Inter-fraction motion management

OC-0483 clinical application of an adaptive radiotherapy approach to baseline shifts in lung cancer H. De Boer 1 , C.A. Van Es 1 , J.G. Bijzet-Marsman 1 , M.E. Kamphorst 1 , M.E. Bosman 1 , G.J. Meijer 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective In lung cancer radiotherapy, substantial baseline shifts (shifts in the mid-ventilation position relative to surrounding anatomy) occur. When lymph nodes (LN) are involved, baseline shifts of such nodes relative to the primary tumor can range up to cm’s. Therefore, setup is often based on bony anatomy using generous planning margins for the primary tumor and LN. We present an adaptive strategy to reduce these margins for the primary tumor. Material and Methods In a previous retrospective study in 20 stage III NSCLC patients we found that a separation into ‘movers’ and ‘non-movers’ is useful. Patients with an average baseline shift < 3 mm in the first 3 fractions were deemed ‘non- movers’. Dosimetric analysis of tumor coverage over the entire treatment (evaluated on 8 CBCTs per patient) showed that a 6 mm ITV-PTV margin for the primary tumor was adequate for these non-movers (in contrast to the 1 cm margin applied clinically). In the present study, we prospectively applied this selective margin reduction method. Two plans were prepared with 6 respectively 10 mm ITV-PTV margin. All patients started with the 10 mm plan. Baseline shifts of the primary tumor were determined with CBCT dual registration (XVI, Elekta) using a clipbox match on nearby bony anatomy (often vertebrae) and a simultaneous mask match on the tumor region. This registration was performed by RTTs in routine clinical practice. The results of the clipbox matches were used in an eNAL offline setup correction protocol. If, after 3 fractions, a patient was classified as a non-mover, the 6 mm plan was applied in subsequent fractions. Next, each 3 rd or 5 th fraction (for 25 respectively 33 fractions) was imaged to monitor the average baseline shift over the last 3 imaged fractions. If the latter average baseline shift exceeded 3 mm, a switch back to the 1 cm plan would be made. Results 21 stage III NSCLC patients, treated with curative intent, were included to date. 14 patients (67%, consistent with a prediction of 70%) were found to be non-movers and switched to a plan with 6 mm margin for the remainder of the treatment. Follow-up imaging showed that all these patients remained non-movers: their average baseline shift over the entire treatment remained < 3 mm (3D vectorlength) in all cases. Although highly patient dependent, the margin reduction decreased OAR dose significantly. For instance, the average V20Gy was reduced from 15.0 to 13.4 Gy and the mean heart dose

Conclusion We have developed and clinically applied a practical adaptive method for planning margin reduction in non- stereotactic treatment of lung cancer. This method allows for smaller margins in approximately 70% of patients. OC-0484 Variability of breathing-induced tumour motion: 4DCT – a source of misguiding information? J. Dhont 1 , D. Verellen 2 , M. Burghelea 1 , R. Van Den Begin 1 , K. Tournel 1 , T. Gevaert 1 , B. Engels 1 , C. Collen 1 , C. Jaudet 1 , M. Boussaer 1 , T. Reynders 1 , G. Storme 1 , M. De Ridder 1 1 Universitair Ziekenhuis Brussel, Radiotherapy Medical Physics, Brussels, Belgium 2 GZA Ziekenhuizen- Sint Augustinus - Iridium Kankernetwerk Antwerpen, Radiotherapy Medical Physics, Antwerpen, Belgium Purpose or Objective The purpose of this study was to evaluate both the short and long-term variability of breathing-induced tumour motion. In addition, it was investigated whether 4DCT is a reliable source to represent the tumour motion during the entire course of treatment. Material and Methods 3D tumour motion was evaluated for 22 patients treated with SBRT for either primary NSCLC (6/22, 4x12Gy, 2 weeks), metastatic lung lesions (9/22, 10x5Gy, 2 weeks) or metastatic liver lesions (7/22, 10x5Gy, 2 weeks). Treatment was delivered with dynamic tracking (DT) on the Vero SBRT system, requiring a gold fiducial implanted near the target. With DT, a 20 s orthogonal fast fluoroscopy (FF) sequence is acquired before each fraction. Additional sequences are taken if the breathing motion changes. As such, for each patient at least one set of X-ray images is available per fraction from which the 3D tumour motion can be extracted using the implanted marker. If multiple FF sequences were available per fraction, the tumour motion was obtained for each sequence independently. To evaluate the short-term intra-fractional variability, the amplitude, tumour position at maximum exhale (r0) and hysteresis (ie. 3D distance between the tumour position at mid-inhale and -exhale) were compared between different FF sequences from the same fraction. To assess long-term variability, amplitude and hysteresis were compared between fractions and with the 3D tumour motion registered by the pre-treatment 4D planning CT (free-breathing, using RPM (Varian) and amplitude-based