Effect of image-guidance frequency on geometric accuracy and setup margins in radiotherapy for locally advanced lung cancer.
OBJECTIVE
To assess the relative effectiveness of five image-guidance (IG) frequencies on reducing patient positioning inaccuracies and setup margins for locally advanced lung cancer patients.


METHODS
Daily cone-beam computed tomography data for 100 patients (4,237 scans) were analyzed. Subsequently, four less-than-daily IG protocols were simulated using these data (no IG, first 5-day IG, weekly IG, and alternate-day IG). The frequency and magnitude of residual setup error were determined. The less-than-daily IG protocols were compared against the daily IG, the assumed reference standard. Finally, the population-based setup margins were calculated.


RESULTS
With the less-than-daily IG protocols, 20-43% of fractions incurred residual setup errors ≥ 5 mm; daily IG reduced this to 6%. With the exception of the first 5-day IG, reductions in systematic error (∑) occurred as the imaging frequency increased and only daily IG provided notable random error (σ) reductions (∑ = 1.5-2.2 mm, σ = 2.5-3.7 mm; ∑ = 1.8-2.6 mm, σ = 2.5-3.7 mm; and ∑ = 0.7-1.0 mm, σ = 1.7-2.0 mm for no IG, first 5-day IG, and daily IG, respectively. An overall significant difference in the mean setup error was present between the first 5-day IG and daily IG (p < .0001). The derived setup margins were 5-9 mm for less-than-daily IG and were 3-4 mm with daily IG.


CONCLUSIONS
Daily cone-beam computed tomography substantially reduced the setup error and could permit setup margin reduction and lead to a reduction in normal tissue toxicity for patients undergoing conventionally fractionated lung radiotherapy. Using first 5-day cone-beam computed tomography was suboptimal for lung patients, given the inability to reduce the random error and the potential for the systematic error to increase throughout the treatment course.
Higgins
Jane
J
Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada. Jane.Higgins@rmp.uhn.on.ca
Bezjak
Andrea
A
Hope
Andrew
A
Panzarella
Tony
T
Li
Winnie
W
Cho
John B C
JB
Craig
Tim
T
Brade
Anthony
A
Sun
Alexander
A
Bissonnette
Jean-Pierre
JP
D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy; D002985:Clinical Protocols / Q000592:standards; D054893:Cone-Beam Computed Tomography / Q000379:methods* / Q000592:standards; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D058958:Organs at Risk / Q000528:radiation effects; D056888:Patient Positioning / Q000379:methods*; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D012015:Reference Standards; D055752:Small Cell Lung Carcinoma / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy