ESTRO 36 Abstract Book

S159 ESTRO 36 _______________________________________________________________________________________________

simulations were performed for spot or raster scanning (positioning with/without beam pulse between successive spots), for constant and varied beam current, and for cyclotron or synchrotron beam dynamics (e.g. continuous or pulsed beam). The resulting 4D plans were compared using dose-volume metrics (D5-D95, V95) for the CTV, as well as total predicted treatment time (TT).

Results Independent of the delivery scenario and prescribed dose, neither gating alone nor rescanning alone could mitigate motion effects completely, with residual interplay effects (D 5-95 ) of more than 10-20% being observed for GW=5mm w.o. rescanning (shown by purple error-bars). Moreover, the D 5-95 of synchrotron based simulations were found to be >5% higher than for cyclotron scenarios. However, with re-gating (re-scanning + gating, shown by green and blue error-bars), motion mitigation performance was found to be similar effective (close to static reference) for all scanning dynamics and rescan modes, with the main difference being only in treatment efficiency. Without any mitigation, mean TT’s for the 2Gy/12Gy plans were 2x/3x longer for synchrotron than for cyclotron scenarios. For re- gating (GW5+LS5), mean TT’s of synchrotron based plans were on average 2.5x higher when combined with LS and 3.5x higher when combined with VS. Moreover, the advantage of varying beam current has been demonstrated by the approximately constant TT as a function of prescription dose. In addition, for the high dose scenario, variations caused by differences in geometry, motion amplitude, field direction and starting phase, are smaller for varying beam current scenarios in comparison to corresponding constant scenarios.

Conclusion In sum, independently of PBS delivery scenario, the treatment of liver tumours under free-breathing conditions is not recommended for motions over 10mm, even when applying large numbers of rescans. However, re-gating (LS5+GW5) is predicted to be sufficient to achieve acceptable 4D plan quality for all scenarios, even though synchrotron based delivery could have a significant added time cost in comparison to cyclotron based systems.

Symposium: Focus on ART: the clinical difficulties

SP-0307 Multi-parametri c functional PET/MR imaging for RT individualisation D. Thorwarth 1 1 University of Tübingen, Sectio n for Biomedical Physics- Department of Radiation Oncology, Tübingen, Germany Multi-parametric functional imaging using combined positron emission tomograph y and magnetic resonance (PET/MR) imaging may be highly beneficial for the assessment of tumour stage, size, therapy response and functional information before and during radiotherapy (RT) in order to guide not only geometrical but also biological adaptation during the course of RT treatment. This talk will discuss a number of issues, which might compromise the use of multi-parametric functional imaging in a clinical setting. In order to register functional PET/MR image data to the planning CT of an individual patient, tailored patient positioning devices and dedicated algorithms for PET attenuation correction with respect to the hardware components of the positioning aids are required. Furthermore, robust and accurate deformable registration strategies may be required to match PET/MR data acquired during RT to baseline imaging data. In addition, first results of a multi-parametric PET/MR imaging study in head-and-neck cancer (HNC) patients will be discussed. Here, [18F]FDG PET/CT images were acquired in addition to functional PET/MR data consisting of [18F]FMISO PET, anatomical T1- and T2-weighted as