3151 - Assessment of Intrafraction Motion During Spine SBRT Treatment Per Ongoing Phase II Randomized Controlled Trial Protocol

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 23

POSTER

Presenter(s)

Andrew Thompson, MS - OU Health – University of Oklahoma Medical Center, Oklahoma City, OK

A. Thompson^1,2, M. P. Heard³, R. Boopathy⁴, and S. McClelland III⁵; ¹Duke University, Durham, NC, ²Stephenson Cancer Center, Oklahoma City, OK, ³Department of Radiation Medicine, Oregon Health & Science University, Portland, OR, ⁴University of Oklahoma Health Science center, OKlahoma City, OK, ⁵University of Oklahoma Stephenson Cancer Center, oklahoma, OK

Purpose/Objective(s):

We hypothesize that intrafraction motion has the potential to inhibit target coverage and organ-at risk (OAR) avoidance for 1 and 2 fraction spine stereotactic body radiation therapy (SBRT) treatments. The ongoing Spine Patient Optimal Radiosurgery Treatment for Symptomatic Metastatic Neoplasms (SPORTSMEN) phase II randomized controlled trial (clinicaltrials.gov ID NCT05617716) evaluates spinal radiation therapy involving single-fraction and two-fraction SBRT as treatment arms. These fractionation schemes are especially susceptible to position uncertainty. Quantification of intrafraction motion may provide insight into the dosimetric risks of 1 and 2 fraction SBRT and the necessary accommodations to prevent dose-position error, such as planning target volume (PTV) expansions, organ avoidance expansions, immobilization devices, and triggered imaging.

Materials/Methods:

Five patients were treated on or per SPORTSMEN trial protocol with 1 or 2-fraction spine SBRT. 2mm PTV expansions and spinal cord planning risk volume (PRV) expansions were used. Eight fractions were treated in total. Each patient was imaged with a pre-treatment (TX) cone beam CT (CBCT), triggered imaging, and a post-TX CBCT. CBCTs were rigidly auto-registered based on DICOM origin. The relative translational and rotational shifts in each direction were extrapolated from the transformation matrix and corrected for couch motion between images. The translational shifts in each direction were added in quadrature to quantify the total translational shift for each fraction. Shifts for each degree of freedom were averaged across all delivered fractions.

Results:

The total translational shift values for the 8 delivered fractions were 1.18, 1.72, 2.00, 2.22, 2.25, 2.82, 3.85, & 6.20 mm, yielding a mean value of 2.78 ± 1.33 mm (95% confidence interval). 6 of the 8 delivered fractions contained translational shifts that exceeded the 2mm PTV and PRV expansions. Average, minimum, and maximum values for each shift are displayed in the table below.

Conclusion:

These results demonstrate that intrafraction motion in spine SBRT can inhibit target coverage and OAR avoidance by creating positioning errors significantly exceeding that which are accounted for with 2mm PTV and PRV expansions in the majority of treatment instances. Additional safety measures are therefore necessary to avoid under-irradiation of the target and over-irradiation of OARs for 1 and 2 fraction treatment regimens, where 50-100% of dose can be delivered with these positional errors. These may include enlargement of PTV and PRV expansions and improving immobilization. The maximum shift values strongly support the use of triggered imaging, for repositioning after large shifts before the fraction is completed.

	Total Translation	Left	Vertical	Longitudinal	Pitch	Roll	Yaw
Mean:	2.78 mm	1.84 mm	0.70 mm	1.54 mm	1.87°	1.26°	1.57°
Max:	6.20 mm	3.80 mm	1.20 mm	4.80 mm	3.30°	2.80°	4°
Min:	1.18 mm	0.37 mm	0.10 mm	0.20 mm	0.50°	0.09°	0.10°