PTV margin definition in hypofractionated IGRT of localized prostate cancer using cone beam CT and orthogonal image pairs with fiducial markers.
OBJECTIVE
To evaluate PTV margins for hypofractionated IGRT of prostate comparing kV/kV imaging or CBCT.


METHODS
Between 2009 and 2012, 20 patients with low- (LR), intermediate- (IR) and high-risk (HR) prostate cancer were treated with VMAT in supine position with fiducial markers (FM), endorectal balloon (ERB) and full bladder. CBCT's and kV/kV imaging were performed before and additional CBCT's after treatment assessing intra-fraction motion. CTVP for 5 patients with LR and CTVPSV for 5 patients with IR/HR prostate cancer were contoured independently by 3 radiation oncologists using MRI. The van Hark formula (PTV margin =2.5Σ +0.7σ) was applied to calculate PTV margins of prostate/seminal vesicles (P/PSV) using CBCT or FM.


RESULTS
172 and 52 CBCTs before and after RT and 507 kV/kV images before RT were analysed. Differences between FM in CBCT or in planar kV image pairs were below 1 mm. Accounting for both random and systematic uncertainties anisotropic PTV margins were 5-8 mm for P (LR) and 6-11 mm for PSV (IR/HR). Random uncertainties like intra-fraction and inter-fraction (setup) uncertainties were of similar magnitude (0.9-1.4 mm). Largest uncertainty was introduced by CTV delineation (LR: 1-2 mm, IR/HR: 1.6-3.5 mm). Patient positioning using bone matching or ERB-matching resulted in larger PTV margins.


CONCLUSIONS
For IGRT CBCT or kV/kV-image pairs with FM are interchangeable in respect of accuracy. Especially for hypofractionated RT, PTV margins can be kept in the range of 5 mm or below if stringent daily IGRT, ideally including prostate tracking, is applied. MR-based CTV delineation optimization is recommended.
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. christoph.oehler@ksgr.ch.|Department of Radiation Oncology, Hospital Graubuenden, Chur, Switzerland. christoph.oehler@ksgr.ch.
Christoph
C
Oehler
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. stephanie.lang@usz.ch.
Stephanie
S
Lang
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. peter.dimmerling@usz.ch.
Peter
P
Dimmerling
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. cbolesch@student.ethz.ch.
Christian
C
Bolesch
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. stephan.kloeck@usz.ch.
Stephan
S
Kloeck
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. alessandra.tini@usz.ch.
Alessandra
A
Tini
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. christoph.glanzmann@usz.ch.
Christoph
C
Glanzmann
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. yousef.najafi@usz.ch.
Yousef
Y
Najafi
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. gabriela.studer@usz.ch.
Gabriela
G
Studer
Department of Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. daniel.zwahlen@gmx.ch.|Department of Radiation Oncology, Hospital Graubuenden, Chur, Switzerland. daniel.zwahlen@gmx.ch.
Daniel R
DR
Zwahlen
D054893:Cone-Beam Computed Tomography / Q000379:methods*; D019583:Dose Fractionation, Radiation; D057918:Fiducial Markers; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D056888:Patient Positioning; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology* / Q000532:radiotherapy*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated