ESTRO 2020 Abstract book

S1112 ESTRO 2020

against the largest distance between two markers found in the sagittal plane (A, figure 1). Also, the distribution of the measured pitches was determined. To determine the effect of the COG of the marker implant, the largest distance from the COG to the CTV in the sagittal plane was measured per patient (X, B, figure 1). Using goniometry, the distribution of the pitches, and distances A and distance B a margin recipe was then composed to compensate for rotations, namely PTV rot .

PO-1897 CBCT for evaluation setup errors among different fixation in proton therapy to intracranial lesions Y.Y. Huang 1 , S. Tsai 1 , Y.H. Lu 1 , C. Lin 1 , C. Kang 1 , Y. Huang 1 1 Proton Therapy Center- Department of Radiation Oncology, Kaohsiung Chang Gung Memorial Hospital, Kaohsiung, Taiwan Purpose or Objective For particle therapy, an effective fixation devices ensures excellent reproducibility. It is essential to perform a highly accurate positioning and verification of the patient before and during treatment. This study uses cone beam computerized tomographic (CBCT) image system to analyze the setup errors between two fixation modes of proton therapy in patients with intracranial tumors. Material and Methods In this study, we retrospective analyzed patients with intracranial tumors who received proton therapy between April 2019 and October 2019 (IRB:201801844B0), and divided them into two groups. All of them were using a base of skull (Bos) frame with thermoplastic head and neck fixed mold. One was supplemented by a bite block, the other without bite block. The corrected displacement values with 6-dimensional (6DOF) couch were evaluated and recorded for interfraction setup errors analysis. Results According to the results of daily CBCT images, total of 88 groups had pre-treatment positioning and verification. The displacement mean values with standard deviation of no bite block group from the 6DOF couch in the X, Y and Z axes were 0.04 ± 0.02 mm, 0.01±0.03mm and 0.06±0.01mm, respectively. The bite block group were 0.02±0.01 mm, 0.02±0.04mm, 0.07±0.07mm on the X, Y and Z axes, respectively. The average rotation errors with standard deviation in the yaw, pitch, and roll were 0.09 ± 0.22 , 0.01± 0.21 and 0.264± 0.29 in the no mouth bite group. The average rotation error of the bite block group were 0.07±0.14, 0.12±0.04 and 0.12±0.10 . The results of the translational and rotational corrections between two groups were checked with Paired Sample t test. The p - values on the X, Y and Z axes were: 0.097, 0.407, 0.466. The p -values of the rotational variation for yaw, pitch, and roll were 0.161, 0.224 and 0.065. There were no significant differences between two groups. Conclusion This study was using daily CBCT as a verification tool for every patient. The results show the trend that patients who have bite block may reduce errors than without bite block. Roll ( p -value=0.06) may be significant after more data available in the future. PO-1898 Do 2DKV images add value in online position verification of breast ca patients treated with protons? T. Van Faassen 1 , E. Batin 1 , M. Kuijpers 1 , A. Meijers 1 , A. Crijns 1 , J.A. Langendijk 1 1 University Medical Center Groningen, Department of Radiation Oncology / GPTC, Groningen, The Netherlands Purpose or Objective At our institution the initial positioning workflow for breast cancer patients treated with proton therapy consisted of surface imaging, followed by two dimensional kilovoltage (2DkV) images and cone beam computed tomography (CBCT). Tolerances of 3mm/1.5° were used for CBCT and it was assumed that surface imaging and 2DkV images would prevent a need for a second CBCT acquisition, resulting in reduction of the X-ray dose and shorter positioning time. In order to estimate the added value of 2DkV images, a second workflow was introduced using only surface images followed by CBCT. Material and Methods Nineteen breast cancer patients treated with protons were initially positioned using surface imaging (AlignRT TN ). 2DkV

Figure 1

Results In 60% of all fractions the pitch measured was ≤ 4⁰. For A > 2.5cm, 1.5cm ≤ A ≤ 2.5cm and for A < 1.5cm α was found to be 1⁰, 2⁰ and 3⁰ respectively. The value of 4⁰ together with distance B and α as function of distance A was used to determine an individualized PTV rot margin recipe: PTV rot (A,B) = tan (α(A)+4⁰) * B Based on this recipe, PTV rot margins have been retrospectively calculated for 27 patients (figure 2).

Figure 2 Conclusion

A prostate marker implant geometry based PTV margin recipe is proposed. This margin compensates for prostate rotations of up to 4⁰ after daily setup based on the markers in combination with TI for intrafraction motion monitoring. Because a PTV margin also compensates for other uncertainties such as machine and dose delivery inaccuracies, and uncertainties in delineation, an additional margin should be added to PTV rot . This recipe will be clinically introduced for our prostate patients in the near future. IGRT instructions will include to maintain an online pitch < 4⁰ and use TI for intrafraction motion monitoring.