ESTRO 2022 - Abstract Book

S1042

Abstract book

ESTRO 2022

At BSRD patients sit perpendicular to the CT couch on a rotating platform that fixes the symphysis and trochants. Legs are raised and the platform rotates until it locks at couch advance direction. Patients lean on an antirotation truncated V- shaped surface that leaves the spinous processes exposed and unsupported, placing their weight on the paraspinal musculature. Afterward, a thermoplastic mold with a compression belt is placed from the submammary crease to the end of the costal arch that avoid any ribs movements for an unexpected deep breathing or coughing. Finally, in the breast 5 BB's mark the 5 laser crossings corresponding to 5 sagittal references of the stereotactic arch and the axial laser placed on the tumor. These lines are covered by transparent waterproof adhesive dressings. Thus, we control the position of the patient and the breast separately. Targets definition in slowCT and HRCT allows automatic contouring based on Hounsfield numbers histogram. Even it is obtained a 4DCT like a dynamic acquisition for cardiology, that is, without couch displacement and with the simultaneous use of 16 detector rows with the aim of tumor movement analysis. Treatment planning is designed with Pinnacle using the negative margin technique for 4 π treatment with 15 beams, which allows a high conformation to target, maintaining the dose in organs at risk below the established limits. Malignant breast tumors that measured < 40 mm in greatest dimension were treated by single fraction SABR of 24Gy.The Pinnacle scorecard OAR constraints are lung MLD<3.6Gy, heart V 2.8Gy <10%, chestwall D 20cc <16.3Gy, skin(5mm) D 1cc <16Gy and breast MBD<9Gy. The patient position was evaluated with CBCT registered to the planning CT. A total of 30 cone-beams were analysed. Absolute averages, statistical means, standard deviations, and root mean square values of observed setup error were calculated. Couch shifts were registered to obtain Kernel coordinates. Results The target position deviations are below 1.3 mm (table 1) and the mean dose to GTV and PTV was 25.14Gy and 24.45Gy respectively with control of the lesions and without producing any toxicity (Table 2).

Conclusion We have implemented a procedure of high geometric and dosimetric precision, potentially leading to reduced PTV margins and excellent plan quality that allows a safe and effective breast SBRT. It showed inspiring results that may potentially enhance the Breast SBRT ratio. A larger prospective trial is ongoing.

PO-1235 Dosimetric comparison and optimal patient selection of VMAT for breast cancer with nodal irradiation

M. Ogita 1 , Y. Nozawa 1 , H. Yamashita 1

1 The University of Tokyo Hospital, Radiology, Tokyo, Japan

Purpose or Objective 3D-CRT is a widely used standard technique for breast radiotherapy, but it is technically difficult to cover the target with sufficient dose while sparing the organs at risk, especially for the locally advanced left-sided breast cancer due to the heart. We hypothesized that VMAT could improve target coverage and decrease the dose to the organs at risk. We aimed to compare the dosimetric parameters between 3D-CRT and VMAT for left-sided breast radiotherapy with regional nodal irradiation (RNI) and clarify the clinical factors to select ideal patient population for VMAT. Materials and Methods We made both 3D-CRT and VMAT plans using actual planning CT scans in 24 left-sided breast cancer patients who received adjuvant radiotherapy with RNI. The target volume was the chest wall or breast, and supraclavicular region with or without internal mammary nodes (IMN). Tangential photon beams technique with matching fields was used for 3D-CRT. Two partial arcs with 6MV were used for VMAT. A dose of 50 Gy in 25 fractions was prescribed to the reference point for 3D-CRT, and to cover 75% of the PTV for VMAT. VMAT plans were made for actual clinical treatment. 3D-CRT plans were made for the