ESTRO 2020 Abstract book

S1117 ESTRO 2020

Other then saving time, effectuating CBCT not on a daily base, reduces the total dose of radiation as opposed to a daily CBCT radiation dose. PO-1906 Use of cone beam CT (CBCT) to evaluate the intrafraction movements during lung SBRT A. Giraldo Marin 1 , A. Seoane 1 , M.M. Marti Laosa 1 , S. Mico 1 , M. Ramos 1 , J. Giralt 1 1 Hospital Universitari Vall d`Hebron, Radiation Oncology, Barcelona, Spain Purpose or Objective Our standard IGRT protocol for SBRT of pulmonary lesions consists of an initial CBCT (BT-CBCT) to determine the couch shift that adjusts the tumour position to the planning CT. We have also acquired a CBCT post treatment (PT-CBCT) after every fraction with the objective of evaluating the intrafraction tumor and patient movement. Material and Methods We analyzed 92 Lung-SBRT patients who have PT-CBCT at least in half of the fractions, treated either with IMRT or VMAT. All patients were immobilized with a vacuum customized cushion and with arms raised above the head. We adopted abdominal compression as the motion compensation strategy. 4D-PET images were used for internal target volume (ITV) generation, and a 5 mm isotropic margin was added to create the planning target volume (PTV). The prescribed dose and number of fractions were selected according to our institution protocol. Prior to each fraction, the patient was positioned and aligned with the lasers on the reference tattoos. The BT- CBCT was obtained and registered to the tumor in the planning CT, in order to obtain and apply the setup corrections (rotation was not allowed). After the treatment, the PT-CBCT was acquired and registered to the planning CT. The tumour intrafraction movement in the three principal axes (left-right, LR; cranio-caudal, CC and antero- posterior, AP) was calculated as the difference between PT-CBCT and BT-CBCT registrations. Group Mean (GM), systematic (∑) and random (σ) errors were obtained from these data. To study the intrafraction patient movement, we registered again both CBCTs to the planning CT using the closest vertebrae to the tumour as the matching structure instead of tumor and made the same analysis. Results A total of 384 BT-CBCT and PT-CBCT were analyzed. The tumour intrafraction analysis resulted in a group mean error of LR = -0.2 mm (range -7 to 3 mm); CC = -0.9 mm (range -8 to 4 mm) and AP = -0.5 mm (range -5 to 7 mm). The systematic and random setup errors (σ and ∑) are displayed in Table 1. Intrafraction tumor motion exceeded 3 and 5 mm margins in 5% and 2% of the fractions, respectively, remaining the lateral axis the one with the smallest errors. In the retrospective evaluation carried out with vertebrae matching, the mean systematic error was in LR = 0.0 mm (range -4 to 5 mm); CC = 0.3 mm (range -11 to 14 mm) and AP = 0.5 mm (range -9 to 12 mm).

topic because of the physical fragility and psychological stress that these patients may experience during their treatment. The purpose is to test the possibility of applying no daily scheduled imaging without compromising set up accuracy. Material and Methods This study was conducted on 7 patients that were undergoing radiotherapy of the brain with a number of fractions above 16 with daily CBCT, for a total of 143 CBCT analysed. For each single fraction, the following values of the translational movements (mm) along the 3 axes X (lateral), Y (longitudinal) and Z (vertical) obtained in the matching phase were considered. On the 7 patients imaging, 5 different protocols were applied. In the first 3 IGRT protocols ("3 fractions", "5 fractions" and "7 fractions") the average shifts were calculated respectively in the 3, 5 and 7 first fractions. These average shifts for every axis were then applied to the remaining fractions and the differences between the calculated and the delivered positions were recorded. The remaining 2 protocols considered "3 fractions plus weekly imaging" and "5 fractions plus weekly imaging". The weekly imaging became the reference point for the following weekly sessions. Results Table 1 presents the absolute average of the differences derived from the shift during the daily session and those from the protocol chosen. The average shift value for each patient is within 3 mm tolerance. Comparing all the data the protocol “5 fractions + weekly imaging” seems to be the optimal choice.

Table 1: Box Plots Conclusion The protocol "5 fractions + weekly imaging" does not show any outlier. It is necessary to increase the number of patients in future studies to enhance study significance. The application of the optimal protocol is limited by tumor localization. The proximity to organs at risk requires daily imaging.