ESTRO 2020 Abstract book

S1111 ESTRO 2020

position during the repeat CT procedure and therefore improve reliability of the treatment delivery evaluation. Material and Methods The average and standard deviation registration between repeat CTs and planning CT for 20 patients undergoing proton therapy for the breast cancer were studied. Each patient received a weekly repeat CT. 10 patients were positioned at CT with lasers and tattoos and 13 patients with surface imaging (AlignRT TN ). Three patients received a repeat CT during the transition period and therefore were positioned with both techniques. The impact of the arm position on nodes coverage for 5 mm robust optimized treatment plans was evaluated by comparing dose coverage 98% of the nodes volume (D98) for treatment plan recomputed on repeat CTs. Results Patients positioned with surface imaging during their repeat CT showed better registrations. Average translations of less than 3 mm and rotations less than 0.5° compared to 5 mm and 0.7° without surface imaging (figure 2) were observed. The reproducibility was less than 1.6 mm in translations and less than 0.5° in rotations (except for ROLL with 0.45°), vs 2.6 mm and 0.5° without surface imaging. The analysis of the dose distributions on repeat CTs showed differences depending on the arm position. For 19 patients, L1 to L4 nodes D98% was 95% of the prescribed dose and differences in coverage were less than 1% compared to the nominal plan. For one patient with a sub-optimal arm position (figure 1), L1 was underdosed and covered by only 90% of the prescribed dose. Conclusion The introduction of surface imaging at repeat CT allows to achieve more reproducible patient positioning, in particular the arm position, and therefore more accurate evaluation of the delivered treatment for the supraclavicular and axillary lymphnodes can be achieved. Large differences in arm position resulted in significant impact on axillary/supraclavicular nodes coverage. PO-1896 Implant geometry based PTV margins for prostate patients with daily setup on markers T. Rosario 1 , V. Spies 1 , M. Admiraal 1 1 VU University Medical Center, Radiation Oncology Department, Amsterdam, The Netherlands Purpose or Objective Interfraction changes of prostate orientation are well known to occur during the course of Radiotherapy, with the rotation along the left-right axis, pitch, being the most prominent. Daily online setup based on implanted markers can (partially) correct for interfraction translation and rotations. This marker implant is defined by two gold markers in the base of the prostate and one gold marker in the apex of the prostate. With a given PTV margin, the location of the center of gravity (COG) of the implant influences the tolerance for the remaining rotations after daily online setup. Although we have seen a great geometrical diversity in the prostate implants at our department, a fixed PTV margin for these patients was used. The purpose of the project is therefore to define an implant geometry based PTV margin recipe for prostate patients, where the effect of the spacing between the markers and its relation to the ability to measure rotations is also taken into account. Material and Methods Prostate patients were implanted with three gold markers and daily online setup was based on these markers. Triggered imaging (TI) was used to monitor intrafraction translations and rotations, and introduction of TI reduced our PTV margin from 0.6cm to 0.5cm. For 148 fractions (9 patients) the pitch on the markers was measured by two observers to determine if the spacing between markers has an effect on the variation in pitch measurement. The difference between the measurements (α) was plotted

outside of the PTV. For all other changes (e.g. changes in atelectasis, tumour regression) the action level is set at 1cm. The CBCT scan with clinically applied correction was resampled and sent to the treatment planning system (RayStation v7, Raysearch). The dose was recalculated on CBCT and compared to the planned dose. If a dose difference larger than 2% of the total treatment dose was found for a significant volume or if a hot/cold spot occurs a physician is consulted. The need for further actions was discussed taking into account the dose differences, the remaining amount of fractions and patient specific parameters such as e.g. previous irradiation. Results In the selected period 134 recalculations (11% of all patients, see table 1) were performed due to anatomical changes observed during treatment. In 84% of all recalculations it concerned a pelvis, chest wall / breast or lung case. The following changes were observed: in the pelvis region the body contour was reduced due to weight loss, in the chest wall / breast a change (in-/decrease) of the body contour due to seroma and in the lung changes in the amount of atelectasis. In total 27% of the cases needed a treatment plan adaptation. For pelvis the treatment was adapted in 39%, for chest wall / breast 30% and in lung 20% of cases. These adaptations were replanning based on a repeat CT scan or creating a new plan on the original CT scan with the additional information of the CBCT scan. For chest wall and breast patients we noticed that the dose delivered by a full IMRT or VMAT technique was less robust for anatomical changes compared to a tangential fields technique and therefore we had to decrease our action level for contour changes in that region from 1cm to 0.8cm. Conclusion In 11% of all patients an action level for anatomical changes was exceeded and in 27% of the cases treatment adaptation (i.e. ad-hoc ART) was necessary.

PO-1895 Introduction of surface imaging as part of repeat CT procedure for proton breast cancer patients M. KuijperS 1 , E. Batin 1 , T. Van Faassen-van Loenen 1 , A. Meijers 1 , A. Crijns 1 , J.A. Langendijk 1 1 University Medical Center Groningen, Department of Radiation Oncology/GPTC, Groningen, The Netherlands Purpose or Objective Control CT imaging (repeat CT) during the course of treatment is becoming a standard for most proton institutions, to evaluate anatomical changes and their impact on the treatment being delivered. For these repeat CTs, patient positioning is often based only on lasers and tattoos and therefore may differ from the treatment position: especially the arm position for breast patients (figure 1). Uncertainties in repositioning may result in unnecessary or suboptimal plan-adaptation. For breast treatments differences in arm positioning may hamper accurate visualization of treatment delivered to axillary/supraclavicular nodes (L1 to L4). Surface imaging for breast patients was introduced in the repeat CT workflow to improve the reproducibility of the treatment