Abstract Book

ESTRO 37

S510

PO-0939 Dosimetric comparison of four motion adaptation strategies for stereotactic liver radiotherapy S. Nankali 1,2 , E.S. Worm 3 , R. Hansen 3 , B. Weber 1,4 , M. Høyer 4,5 , P.R. Poulsen 1,5 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 2 Nuclear Science and Technology Research Institute, Radiation Application Research school, Tehran, Iran Islamic Republic of 3 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 4 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark 5 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark Purpose or Objective The success of stereotactic body radiotherapy (SBRT) depends highly on the treatment accuracy, which is limited by inter- and intrafraction tumor motion. Many strategies exist to mitigate the effects of motion, but selection of the most suitable strategy requires better understanding of the dosimetric consequences. In this study, we compare the actually delivered tumor doses for 15 liver SBRT patients treated with electromagnetic transponder-guided respiratory gating with simulated treatments with no intrafraction motion adaptation, with baseline drift correction by couch shifts and with MLC tracking. Material and Methods Fifteen patients received liver SBRT in three fractions. A planning target volume (PTV) was formed by expanding the clinical target volume (CTV) with 5mm axially and 7mm cranio-caudally. Seven-field conformal or IMRT plans were designed to cover the CTV with 95% and the PTV with 67% of the prescribed mean CTV dose. The treatments were delivered with Calypso-guided respiratory gating with a ±3-4mm gating window around the full exhale position. The actually delivered target dose with gating was reconstructed for all 45 fractions with a multiple isocenter shift method and compared with the doses of simulated treatments with (1) adaptation to interfraction motion only by a setup CBCT scan, (2) further adaptation to intrafraction baseline drift by couch corrections after each field in case the mean error during the field exceeded 2mm, and (3) real-time adaptation to the full motion by MLC tracking. The motion-induced reduction in CTV D 95 relative to the planned dose (ΔD 95 ) was calculated for all fractions. Results The mean (range) number of couch shifts to compensate for tumor drift was 2.8 (0-7) with gating, 1.4 (0-5) with inter-field couch corrections, and zero for the other strategies. For three patients, systematic intrafration tumor drift resulted in a mismatch between the tumor and high dose volume without intrafraction motion adaptation (Figure 1). The dose was partly restored with inter-field couch corrections and fully restored with gating and MLC tracking (Figures 1-2). Over all treatments the mean (range) of ΔD 95 was 8.2 (0.6-29.4) %-points without intrafraction motion adaptation, 4.0 (0.4-13.3) %-points with inter-field couch corrections, 1.1 (0.3-2.7) %-points with MLC tracking, and 0.8 (0.1-1.8) %- points with gating (Figure 2). All differences were statistically significant (p<0.05).

Conclusion Four motion adaptation strategies were compared for liver SBRT. Inter-field couch correction can mitigate gross dose errors without the requirement of real-time motion monitoring. However, gating and MLC tracking were much more effective strategies that ensured high similarity