ESTRO 36 Abstract Book

S440 ESTRO 36 _______________________________________________________________________________________________

3 University of Santo Tomas, Department of Radiation Oncology, Manila, Philippines 4 MD Anderson Cancer Center, Department of Radiation Oncology, Houston, USA Purpose or Objective To fully automate radiotherapy planning for cervical cancer (4-field box treatments) and head/neck cancer (VMAT/IMRT). Material and Methods We are using a combination of in-house software, Eclipse Treatment Planning System, and Mobius 3D to create and validate radiotherapy plans. Most planning tasks have been automated using a primary algorithm for the treatment plan, and a secondary independent algorithm to verify the primary algorithm. The first step is to automatically determine the external body surface and isocenter (based on radiopaque markers in a 3-point setup) using two independent techniques. For H/N cases, the radiation oncologist manually delineates the GTV. Normal tissues (parotids, cord, brainstem, lung, eyes, mandible, cochlea, brain), cervical neck nodes (levels II-IV, IB-V or IA-V) and retropharyngeal nodes are automatically delineated using an in-house multi-atlas segmentation tool. The RapidPlan tool (Eclipse) is used to create a VMAT plan. For 4-field box cervical cancer treatments, the field apertures (jaw and MLC positions) are automatically calculated based on bony anatomy using two techniques: The primary technique uses atlas-based segmentation of bony anatomy, and then calculates apertures based on the projection of these bones to each beam’s-eye-view. The secondary technique deformably registers atlas DRRs to the patient’s DRR for each beam, then uses the deformation matrix to deform atlas blocks (MLC positions) to the patient’s DRR. Relative beam weighting is determined based on a least-squares fit, minimizing heterogeneity in the treatment volume. Final dose distributions are automatically sent to Mobius for secondary dose calculation. Results Primary and secondary techniques for identifying the body surface agreed within 1.0mm/0.99 (mean distance to agreement/average DICE coefficient). Primary and secondary techniques for determining isocenter agreed within 3mm. H/N normal tissue and lymph node segmentation was evaluated by a radiation oncologist (128 patients), and found to be acceptable for all structures, except for esophagus and cochlea and in situations where the head position was non-standard. The figure below shows a fully automated plan including contours and optimized doses.

A radiation oncologist found 96% of cervical cancer beam apertures were clinically acceptable, with all failures caused by a slight error in the position of the superior border. The primary and secondary aperture calculations agreed with average DICE and mean absolute distance of 0.93 and 5.5mm, respectively. An example is shown below. Automated beam weighting reduced hotspots by 1.5% on average.

Conclusion Normal tissue segmentation for head/neck cancer patients and determination of the jaw/MLC for cervical cancer patients are very successful. Both have been introduced into use in our clinic. Next steps include full evaluation of the resulting dose distributions, and assessing the use of these techniques for a prototype linac with flattening- filter-free beam and novel MLC design. PO-0821 Automatic re-planning of VMAT plans in prostate and HN patients using constrained optimization L. Künzel 1 , O. Dohm 2 , M. Alber 3 , D. Thorwarth 1 1 University Hospital Tübingen Eberhard Karls University Tübingen, Section for Biomedical Physics, Tübingen, Germany 2 University Hospital Tübingen Eberhard Karls University Tübingen, Radiation Oncology Division of Medical Physics, Tübingen, Germany