Abstract book - ESTRO meets Asia

S17 ESTRO meets Asia 2018

This provides correction strategy however results are not uniform and depend on the clinical implementation of OBI by RTT. Improvements have been shown through accurate and efficient online and offline strategies for patients. Analysis of these errors allows the definition of meaningful margins and changes treatment out come with external- beam radiation therapy without increasing the work burden excessively. Key words: Target Localization using OBI (MV-CBCT & KV- CBCT) best clinical outcome OC-045 Is surface imaging a reliable target indicator for pelvic radiation therapy? R. Chua 1 1 Icon Cancer Centre, The Valley, Wantirna South, Australia Purpose or Objective Precise target localization is a requirement of modern radiation therapy treatments. This is achievable through a variety of methods, most of which are reliant on the administration of additional radiation dose to patients. Surface Guided Radiation Therapy (SGRT) has emerged as a radiation independent and non-invasive way of verifying a patient’s position. Its ability to improve setup accuracy and reduce the translational corrections required during imaging procedures has been well established for externally visible structures, such as breasts. Its efficacy for pelvic volumes, where the target is deep-seated, however, has not been consistently proven. To investigate the reliability of three-dimensional (3D) surface imaging, this study compares the discrepancy in target matching errors between a video-based surface imaging system, AlignRT (Vision RT, London, UK) with cone beam CTs (CBCTs) and orthogonal kilovoltage (kV) paired images acquired for patients undergoing Radiation Over 200 imaging sessions from a sample of patients with urological and gastrointestinal malignancies were analyzed for this study. Utilizing AlignRT’s three in-room camera system, daily video-based surface renders were recorded for patients once aligned to their initial planned position. Simultaneously a CBCT or orthogonal kV pair was acquired and registered to the relevant CT simulation dataset to quantify target error. The average of the daily patient track compared to the reference topographic surface created from simulation was used to calculate the displacement error for AlignRT. The difference between these two values was then used to evaluate the positional disagreement between the two systems. These translations were classified as below 3mm, between 3- 5mm and above 5mm. Results On average, AlignRT demonstrated a 4.6mm, 4.9mm and 2.6mm target misalignment as visualized through CBCT and kV images in the anterior-posterior, superior-inferior and right-left directions, respectively. For the majority (62.9%) of treatments a target discrepancy of >5mm in any direction was evident. When evaluating each axis separately, the displacement error was more commonly <3mm than between 3-5mm or >5mm. The greatest correlation was seen in the lateral direction, with 68.3% of treatment sessions demonstrating a <3mm misalignment in this plane. These results are inconsistent with Zhao et al.’s study. 1 Conclusion The above results suggest SGRT may assist in improving the initial positioning of patients receiving radiation therapy to the pelvis, particularly in the left-right direction. However it is not a surrogate for imaging. Future work should consider corroborating this trend in a broader range of patients and all positional axes. References: Therapy to the pelvis. Material and Methods

Results According to the ten repetitive measurement, the reproducibility (1 standard deviation: 1 SD) for θ A pitch , θ A roll , and θ A yaw was 0.03 degrees, 0.02 °, and 0.02 °, respectively. The reproducibility for θ B pitch , θ B roll , and θ B yaw was 0.03 °, 0.03 °, and 0.03 °, respectively. The differences (average ± 1 SD) of θ A pitch , θ A roll , and θ A yaw between the calculated and the measured values in the combinations of twenty patters for the screw were 0.00 ± 0.06 °, － 0.01 ± 0.06 °, and － 0.04 ± 0.04 °, respectively. The differences of θ B pitch , θ B roll , and θ B yaw were － 0.03 ± 0.04 °, 0.03 ± 0.05 °, and 0.02 ± 0.05 °, respectively. Conclusion We developed the twist-correction system for the head and neck positions, and then the operation accuracy was confirmed. The feasibility of this concept is expected in future. OC-044 Clinical Analysis of Geometric Accuracy using On-Board Imager in Radiotherapy S.S. Tomar 1 , M. Gairola 2 , G. Yadav 2 , G. wadhawan 2 1 Rajiv gandhi cancer institute and research centre, Radiation oncology, Delhi, India 2 RGCI&RC, Radiation oncology, Delhi, India Purpose or Objective Accurate target localization is necessary for successful outcome in radiation therapy treatments. On-Board Imager (OBI) an advanced tool with digital technology to improve and maintain clinical efficiency is essential prerequisite to practice IMRT/IGRT and forms an important factor in the quality of actual radiation delivery. Generating data in a quantitative manner to improve treatment accuracy for any treatment site via MV-CBCT/KV-CBCT in a single-gantry rotation, allows 3D visualization of the tumor prior to the delivery of treatment and correlation with reference plan CT data. This permits corrections of shifts to an acceptable limit Material and Methods Prior to treatment, 2D and/or CBCT (on ARTISTE Siemens and Varian True Beam) was acquired and setup errors with reference to X, Y, Z were corrected online in 30 patients of Breast, Head & Neck (H&N) and Prostate. A second CBCT was acquired after the correction process and coordinates for daily set-up and images were obtained Results A total number of 211 CBCT/ or 2D images were performed in 30 patients. The sites included – Breast (n=10), H&N (n=10) and prostate (n=10). Images were evaluated for 95, 58 and 58 fractions respectively. The shifts observed in X, Y and Z axes are summarized below: In addition, rotational errors were observed in 7% (15/211 images). These include breast (2%), H&N (1%) and prostate (4%), which were also corrected by IGRT Conclusion Despite immobilization devices, shifts beyond the acceptable limits of 2mm were observed during online CBCT or 2D imaging with IGRT in breast (79.9%), H&N (49.2%) and prostate (96.6%). IGRT permits detection and online corrections of these shifts which would have been otherwise gone unnoticed leading to dosimetric errors during radiation therapy.