What CTV-to-PTV margins should be applied for prostate irradiation? Four-dimensional quantitative assessment using model-based deformable image registration techniques.
OBJECTIVE
To quantify adequate anisotropic clinical target volume (CTV)-to-planning target volume (PTV) margins for three different setup strategies used during prostate irradiation: (1) no setup corrections, (2) on-line corrections determined from bony anatomy, and (3) on-line corrections determined from gold markers.


METHODS
Three radiation oncologists independently delineated the CTV on computed tomography images of 30 prostate cancer patients. Eight repeat scans were acquired to allow simulation of the delivered dose distributions in changing geometry. Different registration approaches were taken to mimic the different setup strategies. A surface model-based deformable image registration system was used to warp the delivered dose distributions back to the dose in the planning computed tomography scan. On the basis of the geometric extent of the underdosed areas, a set of anisotropic margins was derived to ensure a minimal dose to the CTV of 95% for 90% of the patients.


RESULTS
Without setup correction, margins of approximately 11 mm for the corpus of the prostate and 15 mm for the seminal vesicles were required. These margins could be reduced to 8 and 13 mm when aligning the patient to the bony anatomy and to 3 and 8 mm aligning the patient to implanted gold markers. A larger margin at the apex was required to account for the significant observer variability and steep dose gradients at this location (11 mm using skin marker registration, 9 mm using bony anatomy registration, and 6 mm using gold marker registration).


CONCLUSIONS
Novel voxel tracking techniques have enabled us to calculate accumulated dose distributions and design accurate three-dimensional CTV-to-PTV margins for prostate irradiation.
Meijer
Gert J
GJ
Department of Radiotherapy, Catharina Hospital, Eindhoven, The Netherlands.
de Klerk
Jeroen
J
Bzdusek
Karl
K
van den Berg
Hetty A
HA
Janssen
Rogier
R
Kaus
Michael R
MR
Rodrigus
Patrick
P
van der Toorn
Peter-Paul
PP
D002985:Clinical Protocols; D006801:Humans; D008297:Male; D011467:Prostate / Q000033:anatomy & histology / Q000473:pathology; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D011877:Radionuclide Imaging; D011880:Radiotherapy Planning, Computer-Assisted; D015203:Reproducibility of Results; D012669:Seminal Vesicles / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D012867:Skin / Q000473:pathology / Q000528:radiation effects; D014057:Tomography, X-Ray Computed