ESTRO 2020 Abstract Book

S81 ESTRO 2020

use this approach when using VMAT planning for bilateral breast cancer patients. PH-0167 Dosimetric stability of left breast cancer irradiation in DIBH triggered with RPM S. Bermejo 1 , G. Gómez de Segura Melcón 2 , X. Nolla Nieto 2 , N. Ventosa Lli 1 , P. Carrasco de Fez 2 1 Hospital de la Santa Creu i Sant Pau, Servei d'Oncologia Radioteràpica, Barcelona, Spain ; 2 Hospital de la Santa Creu i Sant Pau, Servei de Radiofísica i Radioprotecció, Barcelona, Spain Purpose or Objective Deep Inspiration Breath Hold (DIBH) has emerged as a cost- effective and easy-to-implement strategy to minimize the absorbed dose at the heart for left breast cancer treatments with external beam radiotherapy (EBRT). Among the several strategies for DIBH guidance, RPM relies on an external marker block placed at the chest of the patient to describe the chest movement in real-time. This study is aimed to quantify how stable and reproducible is the internal anatomy in terms of dosimetry indices for breast cancer treatments in DIBH triggered with the RPM system. Material and Methods For 33 left breast cancer patients treated with EBRT (3DCRT or IMRT) we acquired between three and four DIBH-CBCT exams during the EBRT course. Body, heart and lung volumes were segmented and treatment plans were copied onto CBCT exams that had been co-registered on- line. For lung volumes, new ipsi and contralateral volumes were segmented on the planning CT with the same length as in the CBCT. Then, the dose distribution was calculated, and the following DVH parameters were obtained: V 30 , V 25 and D mean for the heart, and V 20 for the lung. We analyzed differences between the values derived from the planning CT study and those calculated on CBCTs. We performed a t-test to see whether the distribution of values obtained in CBCTs were different to those obtained on the planning CT. We also analyzed intra-patient stability as the 95% confidence interval (2σ) for each parameter for each Figure 1 shows that differences between values calculated on the planning CT compared against those calculated on the various CBCTs were around 0 for each parameter. Standard deviations of the distributions were around 1%. The distribution was less peaked for lung V 20 than for the heart parameters. patient. Results

significantly different from those calculated on CBCT exams. Intrapatient stability was around 1% volume or 0.6Gy for the heart parameters, and around 2% for lung V 20 . The larger difference for the lung parameter could stem from the process of cropping the planning CT lung volume to calculate in the same length of lung as in CBCT exams. Table I. Statistics of the differences between each index calculated on CBCT and that calculated on the planning CT exam

Conclusion The RPM system guarantees the stability of the internal patient geometry on DIBH left breast cancer radiotherapy treatments. The relevant dose-volume parameters of lung and heart during the treatment therefore agreed with those calculated on the planning CT. PH-0168 Can the use of PET/MR improve target delineation accuracy in RT planning for H&N cancer patients? Y.S. Wong 1 , M. Collins 2 , G. Chiu 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Happy Valley, Hong Kong SAR China ; 2 Sheffield Hallam University, Department of Allied Health Professions, Sheffield, United Kingdom Purpose or Objective In H&N cancers, MRI are often registered to planning CTs for better target delineation. Multi-parametric MRI such as DWI can be an alternative to PET images. High b value DWI can generate high intensity signal in tumour regions, differentiating tumour from surrounding healthy tissues. This has allowed multi-parametric MRI to improve target delineation for RT planning. Applying semi-automated segmentation for PET and DWI scans may improve target delineation accuracy, which may lead to precise radiotherapy delivery. The aim of this study was to find out if PET/MR can improve target delineation accuracy for RT planning. In this study, we determined the correlation between SUV from PET images and the apparent diffusion coefficient (ADC) from MR images for H&N cancer patients. Subsequently, the similarity of tumour volumes delineated by oncologists in planning CTs and the tumour volumes determined by the use of functional PET/MR images were compared. Material and Methods 12 patients with H&N cancers were recruited. They had PET/MR scan prior to radiotherapy planning. DWI (b0 and b700) images were used to calculate the ADC values then plotted against SUV from PET images to determine the correlation. Rigid and deformable registrations of PET/MR images to planning CTs were performed. Similarity indexes, dice similarity coefficient (DSC) and jaccard similarity coefficient (JSC) were calculated to compare the overlapping of DWI-based target volume (DWITV) and PET- based target volume (PETTV) against targets delineated by treating oncologists. Rigid and deformable registration similarity results were compared. Statistical analysis was performed in SPSS. Kendall’s tau b was used to compute the correlation of SUVmax and ADC non-parametrically. Wilcoxon signed-rank test was used. It was considered as statistically significant if p-value < 0.05.

Figure 1. Difference between each index calculated in CBCT exams and that calculated in the planning CT. Table I shows the statistics of the differences between each index calculated on CBCT and that calculated on the planning CT exam. Average differences in volumes were around 0.2%, well below 1%, and 0 Gy in dose. p-values were much larger than 0.05 for all indices. This means that values calculated on planning CTs were not statistically