ESTRO 2022 - Abstract Book

S1534

Abstract book

ESTRO 2022

absorbed dose and equivalent neutron dose (with range shifter), respectively. We observe an increase in foetal neutron dose caused by the addition of the range shifter, up to 19 % higher in the two-field plan compared to the single field setup.

Conclusion Our phantom study suggests that breast IMPT remains well within recommended dose constraints of 100 mGy (ICRP 84). The use of a range shifter considerably increased the foetal dose in all plans; therefore, clinics may consider a material with lower neutron yield to follow the ALARA principle. Compared to conventional x-ray therapy, IMPT of the breast exposes the foetus to 5 orders of magnitude lower stray dose, roughly 120 mGy (Fenig et al., 2000). Future work will investigate later stages of gestation with larger foetal volumes closer to the treatment volume.

Poster (digital): Optimisation and algorithms for photon and electron treatment planning

PO-1732 Advantages of low tesla MR-Linac advanced cost function for online adaptive dose optimization

L. Placidi 1 , M. Nardini 2 , R. Moretti 1 , A. Capotosti 1 , D. Cusumano 1 , L. Boldrini 1 , M.A. Gambacorta 1 , V. Valentini 1 , L. Indovina 1

1 Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Radiation Oncology, Rome, Italy; 2 Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Radiation Oncology, Rome, Italy Purpose or Objective Magnetic resonance guided radiotherapy (MRgRT) allows online dose adaptation based on the daily patient anatomy variation. Treatment planning approach has to be thought to maximise the advantages of online MRgRT, both in terms of dose conformality, treatment time and flexibility to easily adapt the dose distribution to the daily anatomy. The advanced cost function (ACF) for online adaptive dose optimization available in the low tesla MRI-Linac treatment planning system has been evaluated to further boost the advantages of the MRgRT. Materials and Methods The last eleven treatment plans delivered in our MRI-Linac system have been optimized employing both the simple cost function (SCF) (employed till now for the dose optimization) and the ACF. To the difference of SCF, ACF allows the possibility to set multiple penalties for the same structure. Additionally, user can avoid of specifying the structures which are considered as targets: instead, this is determined by those structures that have increased the dose constraints. Additionally, a new approach of online adaptive treatment plan (fewer dummy structures and more generalize workflow) has been implemented to better optimize the process. Patients’ cohort was composed by eleven SBRT treatments (five fractions) located in abdomen (3), thorax (4) and pelvis (4). Planning (treatment time - respectively divided in gantry and MLC, beam on and total time - monitor units (M.U.), number of dummy structures, number of online adaptive optimization) and dosimetric (target (PTV) coverage and OARs constraints based on our institutional guideline) parameters have been compared between dose distribution obtained with the SCF and ACF for dose optimization. Results All treatment plans generated employing the ACF have been approved by radiation oncologists and clinically delivered. Results of planning parameters are summarized in figure 1.