ESTRO 2023 - Abstract Book

S724

Monday 15 May 2023

ESTRO 2023

Conclusion BgRT is a novel radiotherapy modality in which a radiolabeled tumor can act as its own fiducial for radiotherapy targeting. This study is the first human validation of real-time BgRT dosimetry. The investigation demonstrated that 15 mCi FDG provides adequate activity for BgRT planning and delivery. Emulated BgRT dose distributions calculated from continuously acquired real-time PET data were accurate and machine-deliverable. The safety of multiple FDG administrations and the feasibility of the BgRT workflow were also confirmed.

Mini-Oral: Adaptive radiotherapy

MO-0877 Evaluating biologically effective dose in daily adaptive proton therapy for head-and-neck cancers M. Bobi ć 1 , H. Lee 1 , B. Winey 1 , A. Lomax 2 , H. Paganetti 1 1 Massachusetts General Hospital and Harvard Medical School, Department of Radiation Oncology, Boston, USA; 2 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland Purpose or Objective To evaluate the impact of daily adaptive proton therapy on the biologically effective dose (BED) in head-and-neck patients receiving intensity-modulated proton therapy (IMPT). Materials and Methods IMPT plans are created for eight head-and-neck patients. For each patient, virtual CTs are generated from daily cone-beam CTs (median=33 images per patient) using deformable image registration. Dose delivery and dose-averaged linear energy transfer (LETd) are simulated on daily virtual CTs with the Monte Carlo code Moqui (Lee et al. 2022 PMB). Daily adaptation using an in-house developed workflow (Bobi ć et al. 2021 PMB) is compared to non-adaptive treatment by calculating both the LET-weighted and RBE-weighted dose (assuming RBE=1.1). The LET-weighted dose can be interpreted as BED based on the simplified RBE model: BED = (1+0.04*LETd) * dose (Unkelbach et al. 2016 IJROBP). Our online adaptive workflow currently does not consider LET during optimization; instead, RBE=1.1 is applied to all dose calculations. Fraction doses and accumulated doses are evaluated for both adaptive and non-adaptive delivery. Results Our results show an increase in LETd across all structures when using daily adaptation compared to no adaptation, both in targets and organs at risk (OARs). For OARs, this increase had minimal impact on the relative differences between daily adaptation and no adaptation, independent of the dose weighting method. For the target dose, however, we observed considerable differences based on which dose weighting method is used. When moving from RBE-weighted to LET-weighted dose, the dosimetric benefits of daily adaptation decreased, especially regarding the target coverage. Figure 1a shows LETd-volume histograms of all evaluated fractions in one example patient, indicating increased LETd for daily adaptation. Bands represent the minima/maxima across all fractions, while the solid line represents the mean. Figure 1b shows dose-volume histograms (DVHs) of all fraction doses in the same patient, comparing daily adaptation with no adaptation for both dose weighting methods. The narrow bands of the RBE-weighted dose indicate clear benefits in target coverage for daily adaptation compared to no adaptation. However, the evaluation of the LET-weighted dose does not reflect the narrow bands of the target DVHs for daily adaptation.