ESTRO 38 Abstract book

S961 ESTRO 38

would have been reached for either recalculation or measurement-based method. EP-1777 Improvements in pencil beam algorithm in proton therapy: do we still need Monte Carlo in brain? L. Widesott 1 , S. Lorentini 1 , F. Fracchiolla 1 , P. Farace 1 , M. Schwarz 1 1 APSS, Protonterapia, Trento, Italy Purpose or Objective Dose accuracy in presence of heterogeneities and range shifter (RS) modelling are some of the reasons that have made in the last years Monte Carlo (MC) dose calculation algorithm increasingly present in the clinical routine in proton therapy. On the other hand, MC brings with it significantly higher calculation times, especially if you want to do robust optimization/evaluation and/or frequent re-planning. The purpose of this work is to evaluate the last version of the pencil beam (PB7) present in our TPS comparing it with measurements and observe how it behaves with respect to the previous version of the pencil beam algorithm (PB6) and MC. Material and Methods We compared the new RayStation 7 pencil beam (PB7) algorithm vs. MC dose engine and the previous one (PB6) clinically implemented so far, in critical conditions such as superficial targets (i.e. in need of range shifter), different air gaps and different gantry angles to simulate both orthogonal and tangential beam arrangements. For every plan the PB7, PB6 and MC dose calculation were compared to measurements using a gamma analysis metrics with passing criteria of 3% of maximum dose, 3mm distal-to-agreement, global approach, and dose threshold of 5%. Measurements were performed with a 2D ion chamber array detector (MatriXX PT, IBA Dosimetry GmbH) placed underneath the following targets: 1) anthropomorphic head phantom (with two different thickness) and 2) a biological sample (i.e. half lamb’s head). Results For both the configurations of the head phantom (i.e. one and two slabs) the gamma passing rate (GPR) was almost always better for PB7 compared to PB6 (on average >92% for PB7 vs >85% for PB6) but still below MC GPR (on average >99%). Overall the PB6 algorithm tends to overestimate the dose to the target (up to 25%) and underestimate the dose to the organ at risk (up to 30%). PB7 performed significantly better but an overestimation of the hot spot compared to MC was found (Figure). We found similar results for the two targets of the lamb’s head where only two beam gantry angles were simulated (Table). Each field was optimized with PB6, the final dose calculation time was the same for PB6 and PB7 (about 20 seconds) and ten times greater for MC (about 200 seconds).

Conclusion Our data show very promising results with PB7 and much lower computation time than MC. Questions remain about the use of PB7 for brain tumors only for some specific scenarios as use of RS with very large air gaps and beam directions tangential to the patient surface. On the contrary, speed gained with PB7 is a very important issue in the context of multi-criteria-optimization and/or robust optimization and/or on-line re-planning. EP-1778 Accumulated dose prediction from pre- treatment dosimetric parameters in cervical cancer PotD method S. Heijkoop 1 , J. Godart 1 , E. Novakova 1 , J.W. Mens 1 , B. Heijmen 1 , M. Hoogeman 1 1 Erasmus Medical Center Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective In our institute locally advanced cervical cancer patients are treated with an online adaptive Plan-of-the-Day (PotD) protocol. Daily CBCT scans are used to select a plan from the plan library that best fits the observed anatomy for that day. This radiotherapy technique allows for better sparing of organs at risk (OAR). However, due to organ motion and daily plan selection, accurate a-priori prediction of the total dose received at the end of the treatment course is challenging, which can lead to inaccurate prediction of normal tissue complications. The aim was to investigate whether pre-treatment established treatment plan parameters can predict accumulated OAR doses for the fractionated treatment. Material and Methods Treatment plans and daily CBCTs of 24 patients were included in the study. 14 patients had a tip of uterus displacement of more than 2.5 cm as measured with a full and empty bladder planning CT scan (‘movers’) and 10 patients had a motion less than 2.5 cm (‘non-movers’). The movers had 3 plans in the plan library to cover the range of motion (full and empty-bladder IMRT plans with minimal adequate margins and a backup plan with an enlarged margin), whereas the non-movers had 2 plans in the plan library (1 IMRT plan with a minimal/adequate margin and a backup plan with enlarged margin). In all daily CBCT scans the bowel cavity, bladder and rectum were contoured. To calculate accumulated OAR doses non-rigid registration was used (1).