ESTRO 37 Abstract book

S160

ESTRO 37

Results The positional correlation between humeral head and node level is high in the medio-lateral (ML) direction for all node levels (0.67

Conclusion Our protocol was adequate for selecting patients needing adaptive re-planning, but also led to many false positives. Criteria related to changes in lung density, tumor volume and shift outside the PTV were appropriate and clinically relevant indicators for possible adaptation. The optimized protocol leads to a reduction of the workload of 28% (in 47% of patients) while remaining adequate for detecting treatments requiring adaptation. OC-0308 Error in node level positioning after 3D CBCT match in loco-regional breast cancer radiotherapy V. Althof 1 , E. Hoogeboom 1 , A. Moes 1 , P. Jeene 1 1 Radiotherapiegroep, Deventer, Deventer, The Netherlands Purpose or Objective Measurement of set up differences between the matches that focus on node levels I to IV and humeral head, and the clinical Cone Beam CT (CBCT) match respectively. Material and Methods Delineation of lymph node levels I - IV in 3D CBCT (XVI Elekta) was performed, based on the ESTRO Consensus Guideline on Target Volume Delineation for Breast Cancer 2015, making use of surrogate structures. Between June 2016 and April 2017, 34 patients received post-operative loco-regional radiotherapy in our institution. In 20 patients (59%) the CBCT image quality was sufficient to discern the surrogate structures. During treatment 6 to 7 CBCT’s were made. Per CBCT 6 matches were performed, that is: the clinical match, using bony structures and lung- and outer contours and 5 non-clinical matches that focused on humeral head and the four node levels. To calculate the PTV margin necessary to cope with the set up variations of the clinical match, combined with the residual set up variations of the node levels, both systematic and random variations of both sources of variation are added quadratically and the result is used in the PTV margin recipe M=2∑+0.7σ .

Conclusion There is a rather strong correlation between the position of the humeral head and the node levels. This correlation is strongest in the medio lateral direction. In cranio caudal and ventro dorsal direction the correlation is strongest for level I, and less prominent for levels II – IV. For 95% of all set ups, the displacement of a node level is within 5 mm of the displacement of the humeral head. For accurate dose coverage of node levels I to IV, an additional margin of 1 to 2 mm is required on top of the margin needed for the clinical match. OC-0309 Impact of environmental factors on IGRT image interpretation: RTTs’ opinions and perceptions L. Fitzpartick 1 , G. Menezes 1 , E. Forde 1 , L. Mullaney 1 1 Trinity College Dublin, Discipline of Radiation Therapy- School of Medicine, Dublin, Ireland