Influence of rotations on dose distributions in spinal stereotactic body radiotherapy (SBRT).
OBJECTIVE
To evaluate the impact of rotational setup errors on dose distribution in spinal stereotactic body radiotherapy (SBRT).


METHODS
Thirty-nine cone beam computed tomography (CBCT) scans from 16 SBRT treatment courses were analyzed. Alignment (including rotation) to the treatment planning computed tomography was performed, followed by translational alignment that reproduced the actual positioning. The planned fluence was then applied to determine the delivered dose to the targets and organs at risk.


RESULTS
The mean planning target volume (PTV) was 71.01 mL (SD +/- 60.05; range, 22.62-250.65 mL). Prescribed dose (to the 62-82% isodose) was 14-30 Gy in one to six fractions. The average rotational displacements were 0.38 +/- 1.21, 1.12 +/- 1.82, and -0.51 +/- 2.0 degrees with maximal rotations of -4.29, 5.76, and -6.64 degrees along the x (pitch), y (yaw), and z (roll) axes, respectively. PTV coverage changed by an average of -0.07 Gy (SD +/- 0.20 Gy) between the rotated and the original plan, representing 0.92% of prescription dose (SD +/- 2.65%). For the spinal cord, planned with 2-mm expansion to create a planning organ at risk volume (PRV), the difference in minimum dose to the upper 10% of the PRV volume was 0.03 +/- 0.3 Gy (maximum, 0.9 Gy). Other organs at risk saw insignificant changes in dose.


CONCLUSIONS
PRV expansion generally assures safe treatment delivery in the face of typically encountered rotations. Given the variability of delivered dose within this expansion for certain cases, caution should be taken to properly interpret doses to the cord when considering clinical dose limits.
Gutfeld
Orit
O
Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109-5010, USA.
Kretzler
Annette E
AE
Kashani
Rojano
R
Tatro
Daniel
D
Balter
James M
JM
D054893:Cone-Beam Computed Tomography; D004947:Esophagus; D006801:Humans; D007103:Immobilization / Q000379:methods; D009929:Organ Size; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D012189:Retrospective Studies; D012399:Rotation; D013116:Spinal Cord / Q000033:anatomy & histology; D013125:Spinal Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy / Q000601:surgery*; D014132:Trachea; D047368:Tumor Burden