SU‐E‐J‐128: Setup Variation in Deep Inspiration Breath‐Hold for Radiotherapy of Breast Cancer Tumor Bed Assessed with RPM‐Guided CBCT

Purpose: Deep inspiration breath hold (DIBH) can reduce irradiated cardiac volume during tangential left breast radiotherapy. Commonly the setup position for DIBH is verified using gated on‐board orthogonal kV imaging (OBI) of the rib cage. This study assessed positional variation of the tumor bed in the boost treatment with gated orthogonal kV image‐guided setup and CBCT. Methods: 8 patients with median age of 66 (57–85), received a boost dose to the tumor bed in the left breast. After setup positions were aligned using gated orthogonal kV imaging, RPM system‐guided, voluntary DIBH, CBCTs were acquired in 2 DIBH of 30 sec or 3 DIBH of 20 sec. Positional difference of the tumor bed between the planning CT and CBCT were assessed. Results: Left breast volume range was 611–2184 cm3 among 8 patients. Positional change in tumor bed was 0–1.5 cm lateral, 0–2.5 cm vertical, and 0–0.7 cm longitudinal, resulting in 0.4−3.0 cm 3D positional change. The magnitude of 3D positional change was associated with breast volume (r=0.83). The magnitude increased as breast volume increased, with a change of ∼ 1.5 cm for a breast volume of ∼ 1500 cm3. Conclusion: Feedback‐guided, DIBH CBCT can be used to verify tumor bed position for boost treatment. DIBH setup using gated orthogonal kV imaging may be adequate for patients with small breasts. More data are needed for this ongoing study to establish the minimum breast volume for which by DIBH CBCT guided setup becomes necessary.