Adaptive Radiation Therapy (ART) of Spinal Metastases: Motion Between a Rock and a Hard Place

Summary: Online adaptive radiation therapy (ART) is a treatment technique in which imaging and replanning is performed in the treatment room with the patient in the treatment position immediately prior to each fraction. A scheduled plan (developed using the treatment plan from simulation and on-board imaging) is then compared to an adaptive plan (created with updated contours based on this imaging) for final selection dependent on clinical goals. Whether this would be meaningful in the treatment of spine targets is unknown. We retrospectively review the ART delivered to patients with spinal metastases at our institution and hypothesize that adaptation would decrease dose to organs at risk (OARs). This was a single institution retrospective analysis of patients with vertebral body metastases treated with Online Adaptive Stereotactic Body Radiation Therapy (OA-SBRT) on an online adaptive CT-guided linear accelerator from 3/21/23 to 04/08/2024. The prescribed dose per fraction (d/fx) ranged from 6 to 18 Gy and patients received either 1, 3, or 5 fractions total. In our analysis, comparisons were made among each fraction with an assessment of both the scheduled and adaptive plans for target coverage and dose to OARs. A total of 38 fractions were delivered. A majority of the fractions were to the thoracic spine (n=22, 57%). The adaptive plan was chosen in 29/38 (76%) treatments including 17/22 (77%) of patients with T-spine targets. The most common recorded reason for adaptation was superior target coverage (62% of the adapted fractions) followed by less dose to the esophagus (41%). The mean percentage of the d/fx achieved was 99.1% (88.6 - 110.4%) for scheduled and 99.2% (93.4 – 103.1%) for adaptive plans. The mean reduction in maximum point dose to the esophagus, when comparing scheduled to adaptive plans for each adapted fraction chosen, was 10.6% (0.9 – 22.6%, 0.1 – 1.72 Gy). 16/22 (72.7%) of the fractions delivered to the T-spine had a reduced dose to the esophagus in the adaptive plan when compared to the scheduled (regardless of whether the adaptive plan or scheduled plan was selected). 6/38 (15.8%) fractions were adapted for less maximum point dose to the spinal cord and the mean dose reduction when comparing the adaptive to scheduled plans was 6.5% (1.0 – 19.3%, 0.14 – 1.4 Gy). Other OARs that were adapted for reduced dose included pharyngeal constrictors, trachea, lung, ribs, stomach, duodenum, and cauda equina. The following OARs were not adapted for given negligible differences in dose between the scheduled and adapted plans: heart, brachial plexus, skin, bronchial tree, small bowel, and large bowel. OA-SBRT decreases the dose to the esophagus in almost half of T-spine targets. The majority of patients achieve better target coverage with ART to an extent deemed meaningful by the treating physician. Robert H. Freeman (he/him/his), MD (Presenting Author) - Fox Chase Cancer Center; Joshua Meyer, MD (Co-Author) - Fox Chase Cancer Center; Mark Hallman, MD (Co-Author) - Fox Chase Cancer Center; Eric Horwitz, MD (Co-Author) - Fox Chase Cancer Center; Rebecca Shulman, MD (Co-Author) - Fox Chase Cancer Center; Sameera Kumar, MD (Co-Author) - Fox Chase Cancer Center; Jessica Wong, MD (Co-Author) - Fox Chase Cancer Center; Charlie Ma, PHD (Co-Author) - Fox Chase Cancer Center; Ahmed Eldib, PHD (Co-Author) - Fox Chase Cancer Center; Joseph Panetta, PhD (Co-Author) - Fox Chase Cancer Center; Alexander Lukez, MD (Co-Author) - Fox Chase Cancer Center; Thomas Galloway, MD (Co-Author) - Fox Chase Cancer Center