Impact of immobilization on intrafraction motion for spine stereotactic body radiotherapy using cone beam computed tomography

Li, Winnie, Sahgal, Arjun, Foote, Matthew, Millar, Barbara-Ann, Jaffray, David A. and Letourneau, Daniel (2012) Impact of immobilization on intrafraction motion for spine stereotactic body radiotherapy using cone beam computed tomography. International Journal of Radiation Oncology Biology Physics, 84 2: 520-526. doi:10.1016/j.ijrobp.2011.12.039

Author Li, Winnie
Sahgal, Arjun
Foote, Matthew
Millar, Barbara-Ann
Jaffray, David A.
Letourneau, Daniel
Title Impact of immobilization on intrafraction motion for spine stereotactic body radiotherapy using cone beam computed tomography
Journal name International Journal of Radiation Oncology Biology Physics   Check publisher's open access policy
ISSN 0360-3016
Publication date 2012-10-01
Sub-type Article (original research)
DOI 10.1016/j.ijrobp.2011.12.039
Open Access Status Not Open Access
Volume 84
Issue 2
Start page 520
End page 526
Total pages 7
Place of publication Philadelphia, PA United States
Publisher Elsevier
Language eng
Formatted abstract

Spine stereotactic body radiotherapy (SBRT) involves tight planning margins and steep dose gradients to the surrounding organs at risk (OAR). This study aimed to assess intrafraction motion using cone beam computed tomography (CBCT) for spine SBRT patients treated using three immobilization devices.

Methods and Materials

Setup accuracy using CBCT was retrospectively analyzed for 102 treated spinal metastases in 84 patients. Thoracic and lumbar spine patients were immobilized with either an evacuated cushion (EC, n = 24) or a semirigid vacuum body fixation (BF, n = 60). For cases treated at cervical/upper thoracic (thoracic [T]1–T3) vertebrae, a thermoplastic S-frame (SF) mask (n = 18) was used. Patient setup was corrected by using bony anatomy image registration and couch translations only (no rotation corrections) with shifts confirmed on verification CBCTs. Repeat imaging was performed mid- and post-treatment. Patient translational and rotational positioning data were recorded to calculate means, standard deviations (SD), and corresponding margins ± 2 SD for residual setup errors and intrafraction motion.


A total of 355 localizations, 333 verifications, and 248 mid- and 280 post-treatment CBCTs were analyzed. Residual translations and rotations after couch corrections (verification scans) were similar for all immobilization systems, with SDs of 0.6 to 0.9 mm in any direction and 0.9° to 1.6°, respectively. Margins to encompass residual setup errors after couch corrections were within 2 mm. Including intrafraction motion, as measured on post-treatment CBCTs, SDs for total setup error in the left-right, cranial-caudal, and anterior-posterior directions were 1.3, 1.2, and 1.0 mm for EC; 0.9, 0.7, and 0.9 mm for BF; and 1.3, 0.9, and 1.1 mm for SF, respectively. The calculated margins required to encompass total setup error increased to 3 mm for EC and SF and remained within 2 mm for BF.


Following image guidance, residual setup errors for spine SBRT were similar across three immobilization systems. The BF device resulted in the least amount of intrafraction motion, and based on this device, we justify a 2-mm margin for the planning OAR and target volume.
Keyword Cone beam CT
Intrafraction motion
Spine SBRT
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Medicine Publications
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