Accumulated dose in liver stereotactic body radiotherapy: positioning, breathing, and deformation effects.
OBJECTIVE
To investigate the accumulated dose deviations to tumors and normal tissues in liver stereotactic body radiotherapy (SBRT) and investigate their geometric causes.


METHODS
Thirty previously treated liver cancer patients were retrospectively evaluated. Stereotactic body radiotherapy was planned on the static exhale CT for 27-60 Gy in 6 fractions, and patients were treated in free-breathing with daily cone-beam CT guidance. Biomechanical model-based deformable image registration accumulated dose over both the planning four-dimensional (4D) CT (predicted breathing dose) and also over each fraction's respiratory-correlated cone-beam CT (accumulated treatment dose). The contribution of different geometric errors to changes between the accumulated and predicted breathing dose were quantified.


RESULTS
Twenty-one patients (70%) had accumulated dose deviations relative to the planned static prescription dose >5%, ranging from -15% to 5% in tumors and -42% to 8% in normal tissues. Sixteen patients (53%) still had deviations relative to the 4D CT-predicted dose, which were similar in magnitude. Thirty-two tissues in these 16 patients had deviations >5% relative to the 4D CT-predicted dose, and residual setup errors (n = 17) were most often the largest cause of the deviations, followed by deformations (n = 8) and breathing variations (n = 7).


CONCLUSIONS
The majority of patients had accumulated dose deviations >5% relative to the static plan. Significant deviations relative to the predicted breathing dose still occurred in more than half the patients, commonly owing to residual setup errors. Accumulated SBRT dose may be warranted to pursue further dose escalation, adaptive SBRT, and aid in correlation with clinical outcomes.
Radiation Medicine Program, Princess Margaret Hospital, University Health Network, Toronto, ON, Canada. michael.velec@rmp.uhn.on.ca
Michael
M
Velec
Joanne L
JL
Moseley
Tim
T
Craig
Laura A
LA
Dawson
Kristy K
KK
Brock
D000368:Aged; D000369:Aged, 80 and over; D054893:Cone-Beam Computed Tomography / Q000379:methods; D005260:Female; D056973:Four-Dimensional Computed Tomography / Q000379:methods; D006801:Humans; D007103:Immobilization / Q000379:methods; D008099:Liver / Q000000981:diagnostic imaging / Q000528:radiation effects*; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000556:secondary / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D009068:Movement; D058958:Organs at Risk / Q000528:radiation effects*; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods; D012119:Respiration; D012189:Retrospective Studies; D035501:Uncertainty