TU-C-351-05: Daily Alignment Results for In-Room CT-Guided Stereotactic Body Radiation Therapy for Lung Cancer

Purpose: To assess daily bone alignment results and changes in soft tissuetumor position during hypofractionated, in‐room computed tomography (CT)‐guided stereotactic body radiation therapy(SBRT) of lungcancer.Method and Materials: Daily alignment results during SBRT were analyzed for 117 tumors in 112 patients. Patients received 40–50 Gy of SBRT in 4–5 fractions to the target using an integrated CT‐LINAC system. The free‐breathing CT scans acquired during treatment set‐up were retrospectively re‐aligned to match with each of the bony references and the gross tumor volume (GTV) defined on the reference CT by rigid registration, and the daily deviations were calculated. Results: The mean (±SD) three‐dimensional (3D) shift from the initial skin marks to the final bone‐aligned positions was 9.4 ± 5.7 mm. The mean daily GTV deviation from the bone position was 0.1 ± 3.8 mm in the anterior‐posterior (AP) direction, −0.01 ± 4.2 mm in the superior‐inferior (SI) direction, and 0.2 ± 2.5 mm in the lateral direction. A statistically significant trend (linear fit with R2>0.8) in the change in GTV position relative to the bone was observed in 15 (13%), 11 (9%), and 21 (18%) cases along the AP, SI, and lateral directions, respectively. There were no significant associations between the trends in GTV movement and clinical factors. A margin of 10 mm around the ITV covered the inter‐fractional organ motion errors in 96.4% of tumors in the AP direction, 100% of tumors in the SI direction, and 100% of tumors in the lateral direction. Conclusion: 3D bone alignment using daily in‐room CT‐guided SBRT has good accuracy. However, a substantial number of tumors showed trends in position changes over 4 or 5 days. An isotropic margin distance of 10 mm around the ITV was necessary for adequate coverage of inter‐fractional organ motion errors of all cases.