ESTRO 2021 Abstract Book

S1618

ESTRO 2021

Conclusion NA-APBI on the MR-linac is dosimetrically feasible in both prone and supine position. In this feasibility planning study, we observed that the advantage for either supine or prone position depended largely on the patient anatomy and tumor location in the breast. PO-1898 Trade-off between microscopic disease and OARs in automated VMAT planning for head-and-neck cancer L.P. Kaplan 1,2,3,4 , A.I.S. Holm 1 , J.G. Eriksen 1,2 , B.J.M. Heijmen 4 , S.S. Korreman 1,5,3 , L. Rossi 4 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 2 Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 3 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 4 Erasmus University Medical Center, Department of Radiation Oncology, Rotterdam, The Netherlands; 5 Aarhus University, Department of Clinical Medicine, Aarhus, The Netherlands. Purpose or Objective Patients with cancer of the larynx or pharynx (LPC) treated with curatively intended RT often suffer from long-term side- effects due to high OAR doses. Our national treatment protocol defines three target levels: high, intermediate, and low risk. Previous pathology and clinical studies point at lower risk of subclinical disease in the intermediate risk CTV. Our aim was to investigate the use of automated planning to systematically explore patient-specific trade-offs between 1) coverage of PTV-edges for intermediate and low risk CTVs, and 2) sparing of adjacent OARs. Materials and Methods Using an in-house system for automatic a-priori multi-criteria optimization, six sets of VMAT plans were created for the first 12 included LPC patients: a baseline plan (BP) with PTV coverages constrained to D99% >= 95% of each target’s prescription dose (as clinically used), and five trade-off plans (TPs) in which all other objectives were kept, while the following coverages for the intermediate and low risk PTVs (PTV2 and PTV3) were aimed at: - PTV2 D99% = 90% (TP2-90) - PTV3 D99% = 90% (TP3-90) - PTV2 D99% = 85% (TP2-85) - PTV3 D99% = 85% (TP3-85) - PTV2&3 D99% = 85% (TP23-85) Prescribed doses were 68, 60, and 50Gy (SIB) for PTV1 (high risk), PTV2, and PTV3; PTV margins 5mm. Mean doses to OARs and PTV D99% were compared between each TP and the corresponding BP. The risk of underdosing malignant cells is likely to rise with increasing distance of underdosed PTV voxels from the PTV edge. For each TP, these distances were found for all underdosed voxels and the 95 th percentile value (near-maximum distance) was reported. Results All BPs were visually evaluated by experienced clinicians and found clinically acceptable. For all patients, substantial patient-specific gains in OAR doses were achieved by allowing controlled coverage reductions for PTV2 and/or PTV3 in the five TPs. This is shown in Fig.1 for three example patients. Mean doses were reduced (or not worsened) for all OARs in 42 of 60 TPs (70%), while sparing of a high priority OAR (left in Fig.1) only led to limited dose increases in some lower priority OARs (right in Fig.1) in 18 TPs (30%). Fig.2 (top) shows gains in parotid and submandibular mean doses for all 60 TPs. Maximum reductions in mean doses were 5.3/5.6Gy for ipsi/contralateral parotids, 11.1/9.0Gy for ipsi/contralateral submandibulars, 5.1Gy for oral cavity (all TP23-85), 5.6/7.9/8.0Gy for upper/lower (TP23-85) and mid (TP3-90) pharyngeal constrictors), 7.9/13.7Gy for supraglottic/glottic larynx (TP3-90/85), 7.9Gy for esophagus (TP3-90), and 7.3Gy for thyroid (TP3-85). PTV2/3 D99% was reduced by max. 6.4/7.1Gy; 92% of TPs had cold-voxel-to-edge near-maximum distances < 4mm (Fig.2).