ESTRO 2020 Abstract book

S1118 ESTRO 2020

PO-1908 Comparing soft tissue and bone to verify radiotherapy treatment position for lung cancer patients A. Sweeney 1 , D. Finn 1 1 Edinburgh Cancer Centre, Western General Hospitel, Edinburgh, United Kingdom

Conclusion We have evaluated the tumour and patient intrafraction movements along a SBRT treatment. Both are not negligible and depend on many factors. Using the tumour for correcting the patient setup is a safe way to ensure that it is inside the treatment area. Tumour movement resulted to be smaller than patient movement and could be correlated. The implementation of a CBCT after fraction increases the total treatment time, but we do consider it necessary for a qualified treatment. PO-1907 The effectiveness of SGRT for patient set-up in thoracic deep target radiotherapy S. Leva 1 , G. Libonati 1 , M. Tettamanti 1 , M. Casiraghi 2 , S. Presilla 2 , M.C. Valli 1 , A. Richetti 1 1 Ente Ospedaliero Cantonale, Radiotherapy, Bellinzona, Switzerland ; 2 Ente Ospedaliero Cantonale, Medical Physics Division- Istituto Imaging della Svizzera Italiana, Bellinzona, Switzerland Purpose or Objective Surface Guided Radiotherapy (SGRT) has been proven to improve set-up accuracy for breast radiotherapy. However, few data on the use of SGRT in the treatment of deep tumors are available up to now. The aim of this study is to analyze the benefit of SGRT with Catalyst HD® C-RAD for the positioning of patients treated for thoracic deep tumors. Material and Methods 80 patients treated with VMAT-RapidArc® for mediastinal and esophageal cancer were retrospectively evaluated. 42 patients were positioned using three tattoos and subsequently verified using Cone Beam Computed Tomography (CBCT) imaging for the first three fractions and then every five fractions (group A). 38 patients were positioned using Catalyst HD-C-RAD® in addition to tattoos and verified with CBCT according to aforementioned schedule. (group B).A total of 272 and 268 shifts along the three axes for group A and B, were respectively analyzed for the assessment of patient setup accuracy. The shift distribution along the three axes was compared to the two groups. Results An improved setup accuracy was observed for group B compared to group A. The standard deviation of the shift distribution decreases of 41%, 33% and 50% for the lateral, vertical and longitudinal direction, respectively, when SGRT positioning procedure is used. The histograms of the 3D displacement vector for group A and group B are shown in figure 1.

Purpose or Objective Introduction

As the complexity of radiotherapy treatments increase, more on treatment imaging is being used in routine clinical practice. Studies vary in which anatomical landmark to match to in lung cancer treatment, to ensure treatment accuracy. Prior to transitioning from bone matching using two-dimensional kilovoltage (kV) imaging to 3D soft tissue matching using Cone Beam CTs (CBCTs), we retrospectively assessed matching to bone, carina and the tumour to determine the optimum landmark for image- matching purposes. Material and Methods Five radiographers conducted automatic and manual matches to bone, carina and tumour in 88 CBCTs of 20 patients using CBCT. For each of the 2600 matches, couch shifts were recorded in the anterior/posterior, left/right and superior/inferior directions. Tumour coverage was graded using target volume margins. The level of agreement between automatic and manual matches and the percentage of set-up errors out of tolerance (5mm) were calculated. CBCT feasibility was assessed by examining inter-observer reliability, reporting difficult matches and comparing timings of CBCTs with kV images. Results There was a significant improvement in target coverage when matching to tumour, instead of bone or carina (P<0.001). However, Bland-Altman analysis demonstrated tumour matching had the lowest levels of automatic and manual agreement. Tumour matching detected the highest proportion of set-up errors (26.1%), then carina (19.05%) and bone (18.41%). All methods demonstrated good or excellent inter-observer reliability (intraclass correlation 0.871-0.957). The carina was used to guide difficult matches (20%) n the mediastinum and timings were comparable with 2DkV imaging (4 minutes). Conclusion This study supports the introduction of CBCT imaging and soft tissue matching to tumour as routine clinical practice for radical non-SABR lung radiotherapy. Manual inspection of the visible tumour is highly recommended after automatic matching to ensure accurate target coverage, with the use of the carina as a guide when problematic matching occurs. Further dosimetric assessment of direct tumour matching is advised PO-1909 Interobserver variability of CBCT for prostate radiation therapy: Fiducial Markers vs CTV/PTV F.A. Lima Aires 1 , D. Saraiva 1 , F. Costa 1 , P. Peixoto 1 , A. Monteiro 1 , S. Garcia 1 , M.G. Pinto 1 1 Centro Hospitalar Universitário de São João, Radioncology, Porto, Portugal Purpose or Objective The usage of image guidance is crucial for the radiation therapy treatment (RT) of prostate cancer with VMAT. For this purpose a daily CBCT is acquired prior to the treatment and then it is then compared to a reference CT scan for deviation assessment. At our department two methods may be used to assess these deviations: using implanted fiducial markers as a reference or, when in lack of such markers, using the target volumes (CTV/PTV) boundaries. The purpose of this study is to compare both methods in order to evaluate which presents less interobserver variability. Material and Methods Forty-four patients diagnosed with prostate cancer, treated between March and September 2019, are included

Conclusion SGRT allows an improvement of setup accuracy for the treatment of deep targets in the thoracic region.