2563 - Identifying Clinical Determinants of Intrafraction Motion during Spine Stereotactic Body Radiotherapy

Purpose/Objective(s): In spine stereotactic body radiotherapy (SBRT) patient motion tracking is critical to provide adequate target coverage and sparing of the spinal cord and cauda equina. We aim to identify patient, tumor and treatment characteristics associated with intrafraction motion. We hypothesize that older patients with symptomatic spinal disease and who have multiple sites treated are more likely to experience intrafraction motion.

Materials/Methods: In this single institution retrospective study, patients with metastatic cancer who received spine SBRT in 2023 with image guidance using the ExacTrac Dynamic system were included. SBRT fractionation schemes included 18-30 Gy in 1-5 fractions. Patients with up to 2 intrafraction motions, defined as movement beyond 1 mm/1°, were identified. Chi square and Fisher’s exact tests were used to identify associations between characteristics and intrafraction motion. Local control (LC), progression free survival (PFS) and overall survival (OS) were evaluated using the Kaplan-Meier method and log rank test.

Results: There were 54 patients with 70 sites treated and a median follow up of 12.8 months. The majority were male (N=35, 64.8%), with a median age of 66 years old (range 34-84). The most common tumor histology was non-small cell lung cancer (N=16, 29.6%). Thirty-four patients (63%) had symptomatic spinal metastasis. Seven patients received SBRT after decompressive surgery. Most patients (77.8%) received single site treatment with the thoracic spine as the most common site irradiated (N=30; 63%). The median dose to tumor was 27 Gy in median of 3 fractions. The 1-year LC and OS were 80.3% and 59.3% respectively. Median PFS was 4 months (95%CI: 0.87 to 7.06). Of the 34 patients with symptomatic metastatic lesions, 30 (85.7%) achieved pain relief. There were 10 patients with an adverse radiation effect, all of which were grade 1-2. The incidence of new or worsening vertebral compression fracture was 16.7%. Sixteen (29.6%) patients had intrafraction motion. A greater proportion of patients with painful spinal metastatic lesions were found to have intrafraction motion though not statistically significant (38.23% vs 15%, p=0.07). There was a trend of increased presence of spinal cord or cauda compression in the intrafraction motion group (66% vs 25%, p=0.06). There was no significant association between age, spinal level, single versus multi-site treatment and type of immobilization with intrafraction motion. There was no significant difference in LC, PFS, OS and adverse radiation effects for patients with and without intrafraction motion.

Conclusion: Patients with painful spinal metastasis or spinal cord/cauda compression may be more likely to have intrafraction motion during spine SBRT. The presence of intrafraction motion was not associated with worse clinical outcomes. These findings emphasize the need for adequate pain control for precise and efficient spine SBRT delivery.