First clinical release of an online, adaptive, aperture-based image-guided radiotherapy strategy in intensity-modulated radiotherapy to correct for inter- and intrafractional rotations of the prostate.
OBJECTIVE
We developed and evaluated a correction strategy for prostate rotations using direct adaptation of segments in intensity-modulated radiotherapy (IMRT).


METHODS
Implanted fiducials (four gold markers) were used to determine interfractional translations, rotations, and dilations of the prostate. We used hybrid imaging: The markers were automatically detected in two pretreatment planar X-ray projections; their actual position in three-dimensional space was reconstructed from these images at first. The structure set comprising prostate, seminal vesicles, and adjacent rectum wall was transformed accordingly in 6 degrees of freedom. Shapes of IMRT segments were geometrically adapted in a class solution forward-planning approach, derived within seconds on-site and treated immediately. Intrafractional movements were followed in MV electronic portal images captured on the fly.


RESULTS
In 31 of 39 patients, for 833 of 1013 fractions (supine, flat couch, knee support, comfortably full bladder, empty rectum, no intraprostatic marker migrations >2 mm of more than one marker), the online aperture adaptation allowed safe reduction of margins clinical target volume-planning target volume (prostate) down to 5 mm when only interfractional corrections were applied: Dominant L-R rotations were found to be 5.3° (mean of means), standard deviation of means ±4.9°, maximum at 30.7°. Three-dimensional vector translations relative to skin markings were 9.3 ± 4.4 mm (maximum, 23.6 mm). Intrafractional movements in 7.7 ± 1.5 min (maximum, 15.1 min) between kV imaging and last beam's electronic portal images showed further L-R rotations of 2.5° ± 2.3° (maximum, 26.9°), and three-dimensional vector translations of 3.0 ±3.7 mm (maximum, 10.2 mm). Addressing intrafractional errors could further reduce margins to 3 mm.


CONCLUSIONS
We demonstrated the clinical feasibility of an online adaptive image-guided, intensity-modulated prostate protocol on a standard linear accelerator to correct 6 degrees of freedom of internal organ motion, allowing safe and straightforward implementation of margin reduction and dose escalation.
University Clinic for Radiotherapy and Radio-Oncology, Salzburg, Austria. h.deutschmann@salk.at
Heinz
H
Deutschmann
Gerhard
G
Kametriser
Philipp
P
Steininger
Philipp
P
Scherer
Helmut
H
Schöller
Christoph
C
Gaisberger
Michaela
M
Mooslechner
Bernhard
B
Mitterlechner
Harald
H
Weichenberger
Gert
G
Fastner
Karl
K
Wurstbauer
Stephan
S
Jeschke
Rosemarie
R
Forstner
Felix
F
Sedlmayer
D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D006046:Gold; D006801:Humans; D008297:Male; D009068:Movement; D011467:Prostate / Q000000981:diagnostic imaging*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011859:Radiography; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012007:Rectum / Q000000981:diagnostic imaging
D006046:Gold