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Thoracic SBRT Induces Early Deterioration of Cortical Bone in Ribs

      Purpose/Objective(s)

      Thoracic stereotactic body radiation therapy (SBRT) leads to an increased incidence of spontaneous rib fractures often occurring early after treatment. The etiology of radiation induced rib fracture (RIRF) is unclear, but likely results from bone damage and deterioration. Spontaneous fractures are believed to initiate within cortical bone; cortical thickness (C.Th) is a major determinant of the bone strength. Unfortunately, thin laminar structures comprising the bone cortex of ribs are poorly visualized on clinical CT images due to limited spatial resolution. The objective of this study was to assess early radiation effects on the C.Th of ribs after SBRT using a novel C.Th mapping technique capable of resolving thickness differences >30 μm.

      Materials/Methods

      Rib cortical thickness maps were constructed from pretreatment and follow-up CT scans acquired 4 months after SBRT for 40 patients treated for primary and metastatic lung malignancies. Patients were treated with several dose fractionation schedules: 50 Gy × 10 fractions (fr) (n = 5), 50 Gy × 5 fr (n = 13), 45 Gy × 3 fr (n = 5), and 54 Gy × 3 fr (n = 7). Post-treatment scans were registered to corresponding pretreatment scans using rigid body transformation. Regions of interest (ROI) along the length of each rib were defined in 10 Gy increments (0 – 50 Gy) and clustered into the following groupings for analysis based on absorbed dose: 0–10 Gy; 20–30 Gy; 30–40 Gy; and >40 Gy. Regions receiving dose on the internal and external surface were analyzed independently, and irradiated regions with volumes <0.5 cm3 were excluded. The mean C.Th within each ROI was determined pre-SBRT and post-SBRT, and the percent difference was calculated. Data were compared using Kruskal-Wallace ANOVA to identify main effects of dose on C.Th, with Dunn’s post-hoc analysis for between group differences.

      Results

      Exposure to radiation at doses >20 Gy resulted in significant thinning of cortical bone at the both the internal (P = .029) and external (P = .008) rib surfaces. While no significant cortical thinning was observed at either the internal (+0.5%) or external (-2.9%) surfaces in ribs absorbing 0–10 Gy; significant thinning occurred at, i] the internal surface at locations absorbing 20–30 Gy (-12.7%; P = .013), 30–40 Gy (-12.3%; P = .017); and marginally lower at 40+ Gy (-9.5%; P = 0.08); and ii] the external surface at ROIs absorbing 20–30 Gy (-12.0%; P = .004), 30–40 Gy (-15.4%, P = .003), and 40+ Gy (-9.6%, P = .024). This substantial thinning of cortical bone at both external and internal surfaces could greatly increase the risk of spontaneous fracture.

      Conclusion

      Stereotactic body radiation therapy induced rapid thinning of the cortical bone throughout ribs that absorb >20 Gy. The thinning of cortical bone in irradiated regions of the rib likely contributes to the occurrence of RIRF in patients treated with SBRT.

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