ESTRO 2022 - Abstract Book

S1280

Abstract book

ESTRO 2022

Conclusion The presented clinical guidelines are considered acceptable, given the uncertainties in RBE models. In particular, the McNamara model has been shown to overestimate max doses for high LET values and low α / β (Rørvik et al 2018 Phys. Med. Biol.) . The difference between the D1cc and D1cc,var was larger for the 50.4 Gy patients than expected. Guidelines have been updated to aim at Brainstem D1cc less than the prescribed dose for patients treated to doses below 54 GyRBE, where the distal edge rule is not routinely applied. Further studies are needed to explore the validity of the RBE models and to evaluate other high risk organ doses.

PO-1502 The clinical benefit of range uncertainty reduction in robust optimization for proton therapy

I. Sojat Tarp 1 , V.T. Taasti 2 , M.F. Jensen 1 , A. Vestergaard 1 , K. Jensen 1

1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 2 Department of Radiation Oncology (Maastro), GROW School for Oncology, Maastricht University Medical Centre+, Maastricht, The Netherlands Purpose or Objective For proton treatment planning, one of the main causes of range uncertainty is on the CT-based estimation of stopping power ratio (SPR) relative to water. SPR estimation is usually based on single energy CT (SECT) scans, however, previous studies have shown that dual energy CT (DECT) can result in a more accurate SPR estimation and thereby a reduction of range uncertainty. In robust optimization, typically setup and range uncertainties are accounted for, and lead to an expansion of the irradiated volume to assure target coverage in all error scenarios. This study investigated how large a range uncertainty reduction is needed to obtain a clinically relevant dose sparing to organs at risk (OARs) for patients with brain tumours. Materials and Methods This study included treatment planning CT scan of ten patients with brain tumours acquired in Twin Beam mode at a SOMATOM Definition Edge CT scanner (Siemens Healthineers, Forcheim, Germany). The patients were treated with spot scanned proton therapy and a prescription dose of 50.4Gy in 28 fractions. Based on the field directions and optimization objectives of the original treatment plan, new plans were robustly optimized with reduced range uncertainties using Eclipse treatment planning system (Varian, Palo Alto, CA). Seven plans were created for each patient, with a range uncertainty of 3.5% (original plan), 3.0%, 2.5%, 2.0%, 1.5%, 1.0% and 0.0%, respectively. For all robust optimizations, a setup error of 2mm was used. Each plan was optimized until a clinically acceptable plan (D95%>98% for clinical target volume (CTV)) was obtained for all fourteen setup and range scenarios. The plans were normalized to CTV mean dose. Dosimetric effect of a