Main Session

Sep 29

PQA 07 - Head and Neck Cancer, Lung Cancer/Thoracic Malignancies, and Nursing and Supportive Care

3479 - The Impact of Rib Avoidance in Thoracic Stereotactic Ablative Radiation Therapy on Fracture Incidence

03:45pm - 05:00pm ET

Poster Hall - Exhibit Hall A

Screen: 31

POSTER

Presenter(s)

Holly Grace, MD - UT Southwestern Medical Center, Dallas, TX

H. E. Grace¹, S. J. Domal¹, J. Visak¹, Y. Zhang¹, S. N. Badiyan¹, P. Iyengar², R. D. Timmerman¹, and K. D. Westover¹; ¹Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, ²Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s):

Stereotactic ablative radiation (SABR) is an established treatment for early-stage lung cancer and metastatic thoracic lesions. Radiation-induced rib fracture is a known potential toxicity of thoracic SABR. In efforts to combat this risk, the ribs have been contoured and excluded from the planned treatment volume (PTV) for a cohort of patients. We hypothesized that the incidence of rib fractures would be lower among patients receiving SABR to the lung whose ribs were excluded from the PTV, opposed to cases in which the ribs are contoured but not excluded from the PTV.

Materials/Methods:

Patients who completed a five-fraction regimen of SABR to 50-60 Gy to the lung for primary and metastatic tumors in the lung between 2018 and 2022 were retrospectively evaluated. Clinical data was extracted from the electronic medical record system and dosimetric data and treatment plans were extracted from the treatment planning software. The typical five fraction rib dose constraint used was a maximum point dose of 57 Gy. The PTV must have included at least 0.03 cc of the ribs unless cropped. Radiation treatment plans were categorized as either including the ribs in the PTV or excluding the ribs from the PTV. Radiographic rib fractures were identified by manual chart review. Wilcoxon, Shapiro-Wilk, and T-tests were used for data analysis.

Results:

127 patients met inclusion criteria. The ribs were included in the PTV for 63 patients, and ribs were excluded from the PTV for 64 patients. The overall fracture rates were 19% (12/63) among the cohort with ribs included, and 22% (14/64) among the cohort with ribs excluded. The average maximum dose delivered to 5 cc of rib was 47.20 Gy among plans with the rib included (Shapiro-Wilk W = 0.979, p = 0.351) and 39.25 Gy among plans with the rib excluded (Shapiro-Wilk W = 0.974, p = 0.196). The maximum dose delivered to 5 cc of rib was statistically significantly different between plans with the ribs included and the ribs excluded from the PTV (Wilcoxon rank sum test W = 1403, p = 0.00314).

The average maximum dose delivered to 5 cc of rib was 47.27 Gy among patients who developed a rib fracture (Shapiro-Wilk W = 0.963, p = 0.447), and 42.14 Gy among patients who did not develop a rib fracture (Shapiro-Wilk W = 0.982, p = 0.188). The difference between the groups was not statistically significant (Wilcoxon rank-sum test W = 1105, p = 0.216).

Conclusion:

Excluding ribs from the PTV in thoracic SABR significantly reduces dose to the ribs but does not significantly reduce rib fracture incidence. Additional analysis of the dose threshold of the ribs is warranted to reduce the incidence of post-treatment fracture.