Session Item

To investigate the feasibility of 28 stereotactic single-dose radiation treatments for breast tumor in our institution. In elderly patients with localized breast cancer who refuse surgery, or for whom surgery is not indicated, SBRT could more effectively control local disease.

Between May 2017 and July 2021, 28 early-stage breast cancer patients were indicated for SABR treatment with MRI contraindication. We used a breast stereotactic prototype with ribs dampening (BSRD), developed and patented in our hospital.

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 4π 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.

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).

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.