SU‐E‐T‐408: Dosimetric Effect of Intrafraction Motion in Spine SRS: A Retrospective Study

Purpose: To investigate the dosimetric consequence of intrafraction motion in stereotactic radiosurgery (SRS) to the spine. Methods: Post‐treatment CBCT registration results from 103 spine SRS cases were used as surrogates to analyze the scale of intrafraction motion. For only those cases with motion vectors greater than the 2 mm planning margin (subgroup, n=20), dose was re‐calculated in the TPS assuming a worst case scenario where the entire treatment was delivered in the post‐treatment position. Dosimetric data was evaluated only for subgroup patients. DVH parameters, including PTV V100%, V90%, D99%, D80%, and maximum cord dose in equivalent dose of 2 Gy (EQD2_cord, α/β=2 Gy) were compared with those in the original plans. The correlations between DVH differences, treatment time, and motion distance were calculated. RESULTS: The intrafraction motions for all cases and the subgroup were 1.3±1.2 mm and 3.1±1.4 mm, respectively, with a max of 6.5 mm. In the subgroup, the re‐calculated PTV V100%, V90%, D99% and D80% values were significantly less than in the original plan: 78.6%, 90.6%, 60.2%, and 99.6% vs. 83.2%, 92.8%, 66.3%, and 102.2%, respectively (paired t‐tests, p<0.01). The corresponding differences were significantly correlated to motion distances, with V100% the best correlated parameter (R=0.834 and R2=0.695, p<0.01). The subgroup maximum EQD2_cord dose in the re‐calculated vs. original plans was 11.7±5.3 Gy vs. 10.4±4.4 Gy (p=0.14), with one case violating in‐house criteria. For all patients, the mean treatment time was 31.8±12.9 minutes, and no significant correlation was found between total treatment time and motion distance (R=−0.12, p=0.23). CONCLUSION: Intrafraction motion in spine SRS patients can cause significant deviations from planned tumor dose proportional to the motion magnitude. Since local failure is significantly correlated to PTV coverage, and intrafraction motion may exceed the 2mm PTV margin, intrafraction imaging and motion management could improve spine SRS local failure rates.