(P070) Quantitative Analysis of Stereotactic Body Radiation Therapy (SBRT)-Induced Lung Injuries

Javad Rahimian, PhD, Luis Mariscal, MD, Michael R. Girvigian, MD, Arturo Alfaro, BSc, Michael J. Miller, MD; Southern California Permanente Medical Group

Purpose: Radiographic lung density changes are observed in most patients after stereotactic body radiotherapy (SBRT) for lung cancer. In this study, we assessed the relationship between SBRT dose and our treatment technique. Follow-up CT density changes were used as a surrogate for lung injury from SBRT.

Methods: Six patients with non–small-cell lung carcinoma (NSCLC) were retrospectively assessed. Patients’ 4D-CT scans were acquired, and the reconstructed phase images were imported into the Phillips Medical Systems Pinnacle treatment planning computer. The internal target volume (ITV) for each patient was contoured using 10 to 14 phase image datasets. The planning target volume (PTV) was generated by adding a 5-mm margin to the ITV in the lateral and anterior-posterior directions and by adding a 7.5-mm cranial-caudal margin. Radiation Therapy Oncology Group (RTOG) protocol 0618 was generally followed for PTV dose coverage. A total dose of 48–54 Gy (52.33 ± 2.66 Gy) in three to five (3.50 ± 0.84) fractions to the isocenter was prescribed. Further, seven to nine coplanar and nonopposing conformal beams (8.50 ± 0.84 beams) were placed. The convolution dose algorithm with heterogeneity correction was used for dose calculations. Treatments were delivered on alternate days under cone beam CT (CBCT) image guidance. A follow-up positron emission tomography (PET)/CT scan was acquired 6–14 months (8.50 ± 3.02 mo) after SBRT treatment completion. The follow-up scan was then fused with the original planning CT. The high radiographic density region was contoured to determine the net SBRT-induced volume of lung tissue injury. The minimum dose causing radiographic injury was then determined.

Results: The average and standard deviation for ITV, PTV, radiographic injury, and total lung volumes were calculated to be 12.74 ± 14.70 cm³, 49.67 ± 45.40 cm³, 16.49 ± 13.57 cm³, and 3,452.00 ± 235.00 cm³, respectively. The average radiographic injury volume-to-PTV ratio was 32.27 ± 21.67% (range: 0%–62%). The threshold dose responsible for radiographic injury was 10.70 ± 0.84 Gy, based on the scan acquired at an average of 8.50 months post-SBRT. There is a linear relationship between the biological effective dose (BED) with the radiographic injury volume.

Conclusions: In this study, increased CT density changes, a surrogate for damage to the normal lung tissue, was associated with higher BED and increasing PTV size, with a threshold dose of 10.70 Gy. Therefore, to reduce SBRT-induced lung injury, at least nine conformal equally weighted beams should be used to distribute dose evenly in normal lung tissue. Further, the noncoplanar and nonopposing conformal beams should be used if possible to minimize the radiographic injury. A study of serial follow-up scans of patients, at regular intervals, is warranted to determine the early and late SBRT-induced lung injuries.