Symptomatic radiation-induced rib fractures after stereotactic body radiotherapy for early-stage non-small cell lung cancer.

Background and purpose: The present study investigated the relationships between the risk of radiation-induced rib fractures (RIRF) and clinical and dosimetric factors in stereotactic body radiotherapy (SBRT) for early-stage non-small cell lung cancer (NSCLC). We also examined dosimetric parameters associated with symptomatic or asymptomatic RIRF and the dosimetric threshold for symptomatic RIRF. Materials and methods: We reviewed 244 cases of early-stage NSCLC treated with SBRT. Gray's test and the Fine-Gray model were performed to examine the relationships between clinical and dosimetric factors and grade ≥ 2 (i.e., symptomatic) RIRF. The effects of each dose parameter on grade ≥ 1 and ≥ 2 RIRF were assessed with the Fine-Gray model. The t-test was used to compare each dose parameter between the grade 1 and grade ≥ 2 groups. Optimal thresholds were tested using receiver operating characteristic (ROC) curves. Results: With a median follow-up period of 48 months, the 4-year cumulative incidence of grade ≥ 1 and grade ≥ 2 RIRF were 26.4 % and 8.0 %, respectively. Regarding clinical factors, only age was associated with the development of grade ≥ 2 RIRF (p = 0.024). Among dosimetric parameters, only V40Gy significantly differed between the grade 1 and grade ≥ 2 groups (p = 0.015). The ROC curve analysis of grade ≥ 2 RIRF showed that the optimal diagnostic thresholds for D3cc, D4cc, D5cc, and V40Gy were 45.86 Gy (area under the curve [AUC], 0.706), 39.02 Gy (AUC, 0.705), 41.62 Gy (AUC, 0.702), and 3.83 cc (AUC, 0.730), respectively. These results showed that V40Gy ≤ 3.83 cc was the best indicator of grade ≥ 2 RIRF. The 4-year incidence of grade ≥ 2 RIRF in the V40Gy ≤ 3.83 cc vs. > 3.83 cc groups was 1.8 % vs. 14.2 % (p = 0.001). Conclusion: The present results recommend V40Gy ≤ 3.83 cc as the threshold for grade ≥ 2 RIRF in SBRT.