ESTRO 2022 - Abstract Book

S1661

Abstract book

ESTRO 2022

X. Feng 1 , F. Huan 1 , H. Chen 1 , F. Lu 1 , Z. Li 1 , H. Li 1 , W. Li 1 , G. Wei 1 , B. Wan 1 , Y. Zhang 1 , H. Jing 1 , S. Wang 1

1 National Cancer Center/ National Clinical Research Center for Cancer /Cancer Hospital, Chinese Academy of Medical Sciences, Peking Union Medical College, Radiation Oncology, Beijing, China Purpose or Objective Supraclavicular fossa position reproducibility and accuracy has been an existing challenge for breast cancer radiotherapy patients. This issue is detrimental especially for mastectomy patients. With the utilisation of VMAT technique, the accuracy of the arm position is even more crucial to ensure the treatment precision. Our clinical team would like to find out if the use of SGRT technology could improve the supraclavicular fossa position accuracy and reproducibility. Materials and Methods 32 mastectomy patients with VMAT (chest-wall and supraclavicular field) technique were aligned using SGRT (AlignRT, Vision RT) workflow. All patients were immobilized with an opened thermoplastic on an all-in-one board. They were firstly aligned base on the skin marking and then fine-tuned using SGRT. The Region of Interest (ROI) was drawn on the ipsilateral chest-wall only. After that, the therapist activated the ‘treatment capture’ function in SGRT system to check on the shoulder and arm position relative to the CT position. Patients’ arms were adjusted accordingly. Lastly, the therapists will check on the ROI again to ensure the overall positioning was near to zero. 142 CBCT images were acquired. During the image registration, chest-wall position (CW) was matched based on sternum, chest-wall contour, and spine. Once the chest- wall was in great position, the supraclavicular fossa positioning discrepancies were calculated based on the acromioclavicular joint (ACJ) matching. All three dimensions x (lateral), y (longitudinal) and z (vertical) were recorded. Results The supraclavicular fossa accuracy in three dimensional were calculated using the formula: Δ X= ｜ X-cw － X-acj ︱ , Δ Y = ｜ Y-cw － Y-acj ︱ , Δ Z = ｜ Z-cw － Z-acj ︱ . The vector is measured using d= √ ( Δ X ² + Δ Y ² + Δ Z ² ). The result showed that the supraclavicular fossa positioning discrepancies were 0.12±0.10cm, 0.10±0.13cm and 0.11±0.12cm in x, y and z directions. The vector, d = 0.16±0.14cm. Conclusion ‘Treatment capture’ function can provide accurate 3D arm position information. It ensures an accurate and reproducible supraclavicular fossa position which benefits to the dose delivery precision for mastectomy patients and potentially protects the organs at risk such as spinal cord, thyroid gland and brachial plexus. 1 Aster Malabar Institute of Medical Sciences, Oncology, Calicut, India; 2 Karunya Institute of Technology and SCiences, Physics, Coimbatore, India; 3 Karunya Institute of Technology and Sciences, Physics, Coimabtore, India; 4 Karunya Institute of Technology and Sciences, Physics, Coimbatore, India; 5 Fortis Cancer Institute, Fortis Hospital, Radiation Oncology, Punjab, India Purpose or Objective High levels of conformity and steep dose gradient from the periphery of target to surrounding tissues are the characteristics of Stereotactic Radiosurgery (SRS) plans. Routinely the dose distribution and Dose Volume Histograms (DVH) are used for the evaluation of plans. There are various parameters used for evaluation of SRS plan to ensure its quality. The objective of the present study focused on dosimetric evaluation of different size of brain tumor volumes in Stereotactic Radiosurgery. Materials and Methods This is a retrospective study of 15 brain metastasis patients treated using TrueBeam™ STx linear accelerator with RapidARC® Radiosurgery. A total of 15 SRS patients of volume ranging from 2.9cc to 34.8cc planned with High definition multileaf collimator (HDMLC) with 2.5mm at isocenter. All the plans were approved by experienced radiation oncologist. The quality of plans were compared using different treatment coverage index such as Radiation Therapy Oncology Group (RTOG) Conformity Index (CI RTOG ), RTOG Quality of Coverage (Q RTOG ), Lomax Conformity Index (CI Lomax ), Paddick’s Conformity Index (CI Paddick ) and RTOG Homogeneity Index (HI RTOG ) for all the SRS plans. In addition, dose gradient parameters from Paddick’s Gradient Index (GI Paddick ), Equivalent Fall-off Distance (EFOD), Quality of coverage (QOC), Geometric overlap ratio (GOR) and R50% (ratio of 50% isodose line and tumour volume) were evaluated for the comparison of different size of tumour volumes. Results The results were analyzed and recorded for different CI respectively: CI RTOG ranges from 0.92 to 1.6 with an average of 1.15±0.16; The CI Paddick ranges from 0.63 to 0.95 with an average of 0.85±0.08; The CI Lomax ranges from 0.92 to 1.0 with an average of 0.99±0.02. Similarly, the HI RTOG was observed as range of 1.09 to 1.33 with an average of 1.18±0.08. The GI Paddick was found ranging from 2.9 to 4.0 with an average of 3.4±0.3. The results of QOC, GOR and R50% were ranging from 0.8 to 1.14; 0.85 to 1.0; 3.04 to 5.36 and with an average of 0.96±0.09, 0.98±0.04 and 3.99±0.72 respectively. The EFOD were seen range of 0.15 to 0.46 and with an average of 0.28±0.09. In addition, an increase of EFOD was observed linearly when tumor volume increased. PO-1875 Dosimetric plan evaluation of different size of tumour volumes in Stereotactic radio surgery A. Raj 1,2 , D. Khanna 3 , H. VT 1,4 , S. P 1 , A. Malik 1 , P. Mohandass 5

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