ESTRO 2020 Abstract Book

S1058 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

against the largest distance between two markers found in the sagittal plane (A, figure 1). Also, the distribution of the measured pitches was determined. To determine the effect of the COG of the marker implant, the largest distance from the COG to the CTV in the sagittal plane was measured per patient (X, B, figure 1). Using goniometry, the distribution of the pitches, and distances A and distance B a margin recipe was then composed to compensate for rotations, namely PTV rot .

Figure 1

Results In 60% of all fractions the pitch measured was ≤ 4 ⁰ . For A > 2.5cm, 1.5cm ≤ A ≤ 2.5cm and for A < 1.5cm α was found to be 1 ⁰ , 2 ⁰ and 3 ⁰ respectively. The value of 4 ⁰ together with distance B and α as function of distance A was used to determine an individualized PTV rot margin recipe: PTV rot (A,B) = tan (α(A)+4 ⁰ ) * B Based on this recipe, PTV rot margins have been retrospectively calculated for 27 patients (figure 2).

Figure 2 Conclusion

A prostate marker implant geometry based PTV margin recipe is proposed. This margin compensates for prostate rotations of up to 4 ⁰ after daily setup based on the markers in combination with TI for intrafraction motion monitoring. Because a PTV margin also compensates for other uncertainties such as machine and dose delivery inaccuracies, and uncertainties in delineation, an additional margin should be added to PTV rot . This recipe will be clinically introduced for our prostate patients in the near future. IGRT instructions will include to maintain an online pitch < 4 ⁰ and use TI for intrafraction motion monitoring.