ESTRO 2020 Abstract book

S1003 ESTRO 2020

PO-1720 Evaluation of the compatibility and accuracy of lung tumor ITV between 4D-CT and 4D-CBCT MIP images Y. Tseng 1 , M. Zhang 2 , Y. Song 2 1 Columbia University, Medical Physics, New York, USA ; 2 Memorial Sloan Kettering Cancer Center, Medical Physics, Montvale, USA Purpose or Objective 4D imaging protocols are routinely used to generate maximum intensity projection (MIP) images for internal target volume (ITV) delineation in respiratory-gated radiotherapy treatment planning. During the patient setup, the current approach is to match the 4D-CT-defined ITV or PTV with that observed on a free-breathing 3D-CBCT on the linac. With the advent of 4D-CBCT, people are now exploring the possibility of matching 4D-CT-defined ITV with 4D-CBCT-defined ITV for a more accurate and reproducible patient setup. Although the differences in image acquisition time and technique, beam energy, and reconstruction algorithm between 4D-CT and 4D-CBCT could potentially affect ITV determination, ITVs defined by these two imaging modalities are currently assumed to be identical in clinical practice. In this project, we attempted to evaluate the ITV compatibility between 4D-CT and 4D- CBCT. Material and Methods Breathing motion was simulated by a commercial respiratory phantom with a lung structure and a target. Amplitudes of 5, 10 and 20 mm combined with frequencies of 15, 20 and 30 breath cycle/minute (BPM) were programmed to produce motion patterns commonly observed on lung cancer patients. For each amplitude and frequency combination, three scans were acquired to improve image statistics. Following each 4D scan, 0-90% MIP images were reconstructed for ITV delineation. To make the process reproducible and objective, the mean intensities of a 7x7mm ROI in the most uniform area within the normal lung structure and the center of target were defined as 0% and 100% intensity, respectively. Then, 50% intensity was derived to be the threshold and used to delineate the intensity-defined ITV (Fig 1). The ITV differences were calculated by comparing the intensity- defined ITVs with the theoretical ITVs that represent tumor’s true geometric volume for given amplitudes. Statistical t -test was performed to derive p -value for each comparison with α=0.05.

Conclusion Our study revealed that ITV was overestimated more by 4D-CBCT than by 4D-CT. In addition, 4D-CBCT-defined ITV was significantly larger than 4D-CT-defined ITV for most clinical scenarios. Based on the trend, we can expect that the ITV difference will be even more significant for patients with respiratory motion amplitudes greater than 20 mm. PO-1721 A voxel-based method to quantify longitudinal MRI changes after pediatric brain irradiation. L. Toussaint 1 , S. Eskildsen 2 , O. Casares-Magaz 1 , C. Stokkevåg 3 , Y. Lassen-Ramshad 4 , H. Hasle 5 , K. Tofting- Olesen 5 , D. Grosshans 6 , R. Mohan 6 , M. Høyer 4 , L. Muren 1 1 Aarhus University Hospital, Department of Medical Physics and Oncology, Aarhus N, Denmark ; 2 Aarhus University, Center of Functionally Integrative Neuroscience, Aarhus, Denmark ; 3 Haukeland University Hospital, Department of Medical Physics and Oncology, Bergen, Norway ; 4 Danish Centre for Particle Therapy, Radiation Oncology, Aarhus N, Denmark ; 5 Aarhus University Hospital, Department of Pediatrics, Aarhus N, Denmark ; 6 University of Texas MD Anderson Cancer Center, Department of Radiation Oncology, Houston, USA Purpose or Objective The acquisition of magnetic resonance images (MRI) for follow-up evaluation after radiotherapy (RT) of pediatric brain tumor patients allows for longitudinal analysis of imaging changes. However, assessing brain changes in the pediatric population is challenged by the ongoing normal growth of the patient, as well as several other concomitant factors including surgery, hydrocephalus or the tumor itself. Our study explored the feasibility of applying a voxel-based method adapted from the neuroscience field to identify longitudinal MRI changes following RT for pediatric brain tumor patients. More specifically, our aim was to investigate how the volume changes as measured with our method compared with the volume changes in a healthy pediatric population only influenced by growth- induced changes. Material and Methods Longitudinal series of MRI scans from seven pediatric brain tumor patients treated with either photon- or proton- based RT were analyzed. For a specific patient, the series consisted of a median of 8 [range 4–14] non-contrast T1 sequences acquired from the same scanner, using the same field strength (1.5 or 3T) and acquisition parameters. From each series, a patient-specific average template was created to which the MRIs from all time points were registered. For both the individual time points and the

Results Table 1 summarizes the ITV differences in percentage. 1) Motion amplitude had a significant impact on the ITV definition, while motion frequency had virtually no effect on it for both modalities. 2) Both 4D-CT-defined ITVs and 4D-CBCT-defined ITVs deviated significantly from their corresponding theoretical ITVs. 3) The magnitude of deviation was roughly linearly propositional to the motion amplitude. 4) 4D-CBCT always overestimated ITV more than 4D-CT. 5) 4D-CBCT-defined ITVs were always larger than 4D-CT-defined ITVs for any given amplitude and frequency combination. 6) 4D-CBCT-defined ITVs were significantly different from 4D-CT-defined ITVs for 6 out of 9 motion patterns.