ESTRO 2023 - Abstract Book

S559

Sunday 14 May 2023

ESTRO 2023

expiration phases, however, each beam on period covers an end-exhalation state that may last longer than necessary. We propose and evaluate a novel dose modulation scheme to reduce the associated unnecessary dose to the patient. Materials and Methods Dose down- and up-regulation were defined rule-based, based on the patient's breathing signal and the representative breathing cycle learned before and during scanning: For dose down-regulation in prolonged exhalation, time (wait until expiration is reached), amplitude (below a pre-defined threshold) and phase (arc interval in phase space) criteria were to be fulfilled; up-regulation after exhalation (usually easier to detect) was defined by an amplitude criterion. The corresponding potential of dose reduction was evaluated in-silico (ie, by simulation of the scanning process) using 4DCT breathing signals of 167 patients (signals acquired by the Varian RPM system). Dose reduction was determined as the fraction of the down-regulated dose delivery time to the overall beam-on time. Assuming standard 10-phase 4DCT, periods at risk (‘fails’) were defined as breathing cycles for which the down-regulation period covered the entire phase-specific amplitude range for a specific breathing phase (ie, no appropriate reconstruction of the phase image possible for the corresponding beam-on period). Results The proposed dose modulation strategy resulted in an average dose reduction of 11+/-5% per patient (maximum: 28%). As intended, down-regulation mainly affected the end-exhalation phase (91%) and adjacent expiration (6%) and inhalation (1%) phases. For the proposed rules and inherent algorithm parameters, ‘Fails’ were observed in 3% of the analyzed beam on periods (66 of 1935 beam-on periods). The amount of dose reduction was not significantly correlated with the average breathing cycle length of the patient (Spearman’s rho 0.12, p=0.12), but the fraction of ‘fails’ was anti-correlated with the average cycle length (rho = 0.36; p<0.001). Among patients with the 10% longest average breathing cycles, the fraction of ‘fails’ was reduced to <1%. Conclusion The results support the potential for significant dose reduction in respiratory signal-guided 4DCT by online dose modulation. The reverse correlation of average breathing cycle length and the fraction of ‘fails’ could allow for the identification of patients that safely (ie, without loss of image quality) benefit from the proposed dose modulation scheme when extending the study to a larger cohort and adapting the applied dose modulation criteria parameters. Purpose or Objective We have recently introduced ultra-fractionated SBRT for prostate cancer (5 x 7.25 Gy) on a 0.35T ViewRay MRIdian MR Linac system. As part of the MR-guided radiotherapy (MRgRT) workflow, a simulation scan is performed on the MR-Linac using a 3D balanced steady-state free-precession (aka TrueFISP) imaging sequence. This scan differs substantially from a standard 2D T2w Turbo spin-echo (2D T2w-TSE) 1.5T MRI simulation scan that is typically used in our conventional, non MRgRT, workflow. The aim of this study was therefore to investigate whether this transition has an impact on the delineations performed by the radiation oncologist. To our knowledge this is the first study investigating the influence on delineation between 1.5T MRI-sim and 0.35T MRgRT imaging. Materials and Methods Both 1.5T T2w-TSE (res. = 0.8 x 0.8 x 3 mm) and 0.35T TrueFISP (res. = 1.5 mm3 isotropic) scans were acquired in 10 patients with localized prostate cancer. For this interim analysis, 4 experienced radiation oncologists (RTOs) delineated the prostate CTV and base of the seminal vesicles in five patients on both scans. The 0.35T scan was delineated first during which the RTO was allowed to have access to the unmatched diagnostic MRI scan (not MRI-sim) as per clinical workflow. The 1.5T scans were delineated in a second round, once all the 0.35T scans had been delineated. With the lack of a histopathological ground truth, the intra- and inter-modality consistency between multiple observers and the potential discrepancy in delineated volumes were assessed. Intra-modality differences were calculated by comparing the specialists’ contours within one patient. Inter-modality differences were calculated by comparing the differences between a specialists’ 0.35T and 1.5T contours within one patient. Differences in contours were quantified by three metrics: CTV volume differences, Dice similarity/mismatch coefficients and Hausdorff distances. Results Fig. 1 shows example slices on both systems and the delineations in one patient. On both modalities the main differences in contours were found at the apex and the distal part of the seminal vesicles. Contours on 0.35T showed higher consistency than on 1.5T, as depicted by the Dice similarity coefficients and Hausdorff distances (Fig. 2). Inter-modality differences were fairly consistent over all specialists (data not shown). No significant volume difference (2.3% median with 15% inter quartile range) was found between delineated CTVs on 1.5T and 0.35T MR images. Conclusion Intra-modality contour differences where smaller with the 0.35T images. Therefore, there is no indication that a 1.5T simulation scan is necessary for the standard workflow of the 0.35T MRIdian. PD-0667 The effect of low MR field strength on prostate volume delineation (an MR-Linac study) B. kalkhoven 1 , H. Peulen 1 , T. Budiharto 1 , P. van der Toorn 1 , M. Hilberts 1 , S. Tetar 1 , R. Tijssen 1 1 Catharina ziekenhuis Eindhoven, Radiation oncology, Eindhoven, The Netherlands