ESTRO 36 Abstract Book

S888 ESTRO 36 _______________________________________________________________________________________________

Regarding the patients study, mean acquisition time was 3.2±1.2min (range 1.6-11.5min); absolute median shifts were ≤3mm (SD≤4mm) in the three directions, while the vector shift was 8.5±4.4mm (range: 0.0-24.2mm), revealing the necessity of performing a daily CBCT. Differences between the planned PTV and the recalculated PTV coverage (V95%) were within 3.5% proving the feasibility of VMAT in breast cancer treatment (figure 2).

treatment position in which better coverage of PTV is performed. Possible treatments positions may be clip-box, mask or intermediate values of these. Descriptive analysis and Student t test was performed. Translational shifts calculated from dual automatic registrations were compared with the different standard registrations chosen by the physician, and the possible impact of the different location of the tumours on the precision was studied.

Figure 2: Dose recalculation on a DIBH-CBCT. Similar results were obtained in all cases. Conclusion DIBH-CBCT was shown to be feasible with adequate image quality, making possible to evaluate the efficacy of VMAT skin flash tool approach also in DIBH condition. Considering the relative short image acquisition time required, a protocol for DIBH breast SIB treatments in 15 fractions with DIBH-CBCT daily position is starting in our department. EP-1638 Comparison dual image registrations for SBRT treatment in central and peripheral tumour lung cancer D. Esteban 1 , M. Rincón 2 , J. Luna 1 , A. Sánchez- Ballesteros 2 , A. Ilundain 1 , L. Guzmán 1 , D. Gonsálves 1 , W. Vásquez 1 , J. Olivera 1 , I. Prieto 1 , J. Vara 1 1 Hospital Universitario Fundación Jiménez Díaz, Radiation Oncology, Madrid, Spain 2 Hospital Universitario Fundación Jiménez Díaz, Physical Radiation Oncology, Madrid, Spain Purpose or Objective Four-dimensional (4D) Cone Beam CT (CBCT) became available in clinics and has been used to quantify localization precision and intrafraction variability of lung tumour. Dual image registration is an option for the 4D- CBCT image registration: a clip-box and mask registration. The clip-box registration is the same as those in 3D CBCT, while the mask registration is a new feature of Elekta XVI CBCT system, which is a soft-tissue registration using a soft-tissue volume called mask. Published study of 3D CBCT using clip-box with different landmarks led to different registration precisions for peripheral and central lung tumours. The study aims to compare if the different location of the tumour (peripheral and central) has an impact on the precision of the Elekta 4D-CBCT based automatic dual image registrations using clip-box and mask for SBRT treatment of lung cancer. Material and Methods A sample of 29 patients with diagnosed lung cancer and treated with SBRT from 2014 to 2016 was obtained by purposive sampling. Tumours were classified depending upon his location. They were scanned on a Philips 4D CT scanner and 8-phase images were reconstructed. GTV was contoured on each phase and ITV was generated by including the GTVs. PTV was created from ITV with a 5mm margin. Then a clip-box (place on the spine) and mask (margin of 0.5 cm on the PTV) were created allowing translations. Elekta XVI CBCT system using a 4D module was used to acquire 4D-CBCT scans of the patients in treatment position prior to radiation delivery and a total of 101 were obtained. These images were registered with reference 4D CT images using dual image registration and a "balance clip-box mask" was generated (Imagen 1). The registrations were reviewed by physicians, choosing the

Results There are forty 4D-CBCT from central and sixty from peripheral tumours. Differences of translational shifts between automatic dual registrations and the standard (difference=automatic registration-standard) was obtained. The Table 1 lists the mean and standard deviation of translational shifts directions.

Conclusion Although the clip-box is created from a localized midline structure, the mask is more accurate for central tumours. The outcomes showed that using different dual image registrations resulted in different registration precisions for peripheral and central lung tumours, being favourable in all cases for the mask with statistically significant difference. This difference may be explained because clip- box registration made gross alignments whilst the mask made fine adjustments. EP-1639 Does intrafraction motion between vmDIBHs during breast cancer treatment impact on delivered dose? M. Kusters 1 , F. Dankers 1 , R. Monshouwer 1 1 Radboud university medical center, Academic Department of Radiation Oncology, Nijmegen, The Netherlands Purpose or Objective Intrafraction motion simulations were performed in the treatment planning system (TPS) to determine the robustness of the VMAT comprehensive locoregional breast planning technique for residual breathing motion. The simulations are used to determine a threshold for the warning signal of the in-room monitoring system for excessive intrafraction motion deviations. Material and Methods Our current treatment plan technique consists of 6 opposing 24 o VMAT beams in latero-medial and medio- lateral direction. A CTV-PTV margin of 7 mm is used and plans with a PTV coverage of V95% > 95% are clinically accepted. IGRT is based on online CBCT.