ESTRO 2020 Abstract book

S966 ESTRO 2020

OAR did not exceed the constraints, indicating that the plans were robust to the variations, special attention should be given to large variations since a large deviation in the dose to the parotid gland was observed for one patient. In further studies, the cohort of patients will be extended to include patients with larger anatomical variations in order to validate the findings of this study. PO-1665 The mesorectum motion using MR-guided Radiotherapy: an exploratory study to quantify PTV margins A. Romano 1 , G. Chiloiro 1 , L. Boldrini 1 , D. Cusumano 1 , L. Placidi 1 , M.V. Antonelli 1 , V. Pollutri 1 , C. Votta 1 , M.A. Gambacorta 1 , V. Valentini 1 1 Fondazione Policlinico Universitario A.Gemelli IRCCS, Dipartimento di Diagnostica per immagini - Radioterapia Oncologica ed Ematologia, Roma, Italy Purpose or Objective Previous data about the quantification of mesorectal motion (MM) have been published using cone-beam computed tomography (CBCT) imaging during neoadjuvant chemoradiotherapy (nCRT) for locally advanced rectal cancer (LARC). The purpose of this analysis is to quantify the inter-fraction MM using the higher soft tissue contrast provided by daily magnetic resonance imaging (MRI) obtained on a 0.35 Tesla MR hybrid accelerator. Material and Methods The patients (pts) included in our study underwent nCRT with a total dose of 55 Gy/2.2 Gy/die on gross tumor volume plus the corresponding mesorectum and 45 Gy/1.8 Gy/die on total mesorectum and nodal volumes at risk. For each patient, a true fast imaging with steady state precession sequence (TRUFI) was acquired during the simulation and every day of the MRgRT treatment. The mesorectum was outlined by an expert radiation oncologist on all the volumetric MRI (17-175 seconds). The total mesorectum and the lower mesorectum section below the recto-uterine pouch for female, and the recto- vesical pouch for male have been identified for the analyses. Six PTVs were obtained adding a 0.5 cm, 0.7 cm, 1 cm, 1.3 cm, 1.5 cm and 2 cm margin to the total and lower mesorectum CTVs respectively, starting from the simulation MRI (Figure). The inter-fractional MM was studied considering the mean displacement of the mesorectum in all the 6 spatial directions (left, right, anterior, posterior, cranial, caudal). The margins including 95% of the mesorectal structures during the whole treatment in 95% of pts were considered adequate. Pearson's Correlation Coefficient was performed to correlate the MM and treatment time both for the total and lower mesorectum. Results Data of 12 consecutive pts (10 male, 2 female) affected by LARC who underwent nCRT by MRgRT were retrospectively analyzed, with a total of 300 fractions. Five, 2 and 5 pts had the disease above, below and in correspondence of the recto-uterine/recto-vesical pouch, respectively. The ideal PTV margins taking into account the motion both of the total and lower mesorectum are summarized in the table. No correlation between MM and mesorectal volume with the delivered fractions number was found. Only in one case, the anterior MM was correlated with the mesorectal volume reduction during treatment.

Total (cm)

Mesorectum

Lower

Mesorectum

(cm)

Left

1,5 1,5

1,3

Right

1

Anterior 2 Posterior 1,3 Cranial 0,7 Caudal 1

1,5 1,3

na

1

Conclusion This study confirms that MM occurs mainly in the anterior direction, and this finding appears to be applicable also for lower mesorectum. The results of this exploratory study recommend that the PTV margin should be defined taking into account tumor’s location and using asymmetric margins. PO-1666 Comparison of dose accumulation based on CBCT and CT for lung cancer patients T. Kanehira 1 , S. R. van Kranen 1 , W. V. Vogel 1 , J. Sonke 1 1 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Daily cone-beam CT (CBCT) is frequently applied for setup error correction. The application of CBCT can be extended to dose accumulation (DA). The accuracy of CBCT based DA for lung cancer radiotherapy, however, can be degraded due to both low image quality and limited field of view (FOV) of CBCT. The purpose of this study was to dosimetrically evaluate accumulated doses with CBCT compared to CT for lung cancer patients. Material and Methods We retrospectively randomly included 8 lung cancer patients who were planned to receive a prescribed dose of 66 Gy in 24 fractions with daily CBCT guidance (FOV: 25x25x25cm 3 ) and 5 weekly PET-CT scans for tumor response evaluation. DA with CBCT was performed by 1) deforming the planning CT (pCT) on the daily CBCT after setup correction using deformable image registration (DIR), 2) patching this simulated daily CT (sCT) using the pCT, 3) recalculating the treatment plan on the sCT, and 4) deformably accumulating the recalculated fraction- doses back to a reference of the pCT for all fractions. For DA with weekly CT (wCT), the treatment plan was recalculated on the wCT whose setup was simulated similar to the CBCT, and accumulated fraction-dose was calculated. Missing fractions were handled by scaling the accumulated dose to the full treatment (i.e. multiply with 24/5). Dosimetric parameters for the accumulated doses based on 24 CBCTs (D_CB) and wCT (D_CT) were evaluated for PTV and organs at risk (OARs). Additionally, D_CB was compared to accumulated dose based on 5 CBCTs at the same scan dates of the wCTs (D_sampledCB) to evaluate sampling effects. Results Modest differences in mean dose (MD), D1, and D99 caused by anatomical changes (planned dose vs. D_CT) were observed in our patients: medians from 0.0 (heart (HT)) to 0.7 (spinal cord (SC)), 0.0 (HT) to 0.8 (esophagus (ES)), and 0.2 (PTV) to 0.9 Gy (SC) (Figure 1). Small differences between and were observed in the target (Figure 2). Comparing between D_CB and D_CT, differences in MD, D1, and D99 were small except for Dmean for HT and SC: medians from 0.1 (ES) to 0.2 (HT), 0.3 (Lung-GTV) to 0.1 (PTV), and 0.4 (SC) to 0.0 Gy (ES). These medians for MD and D1 were smaller than those caused by anatomical changes except for MD for HT. Comparing between D_CB and D_sampledCB, dosimetric differences were also small (≤0.3 Gy). This indicates that the effect of a small number of the wCT compared to the daily CBCT was small.