Biological image-guided radiotherapy in rectal cancer: challenges and pitfalls.
OBJECTIVE
To investigate the feasibility of integrating multiple imaging modalities for image-guided radiotherapy in rectal cancer.


METHODS
Magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) were performed before, during, and after preoperative chemoradiotherapy (CRT) in patients with resectable rectal cancer. The FDG-PET signals were segmented with an adaptive threshold-based and a gradient-based method. Magnetic resonance tumor volumes (TVs) were manually delineated. A nonrigid registration algorithm was applied to register the images, and mismatch analyses were carried out between MR and FDG-PET TVs and between TVs over time. Tumor volumes delineated on the images after CRT were compared with the pathologic TV.


RESULTS
Forty-five FDG-PET/CT and 45 MR images were analyzed from 15 patients. The mean MRI and FDG-PET TVs showed a tendency to shrink during and after CRT. In general, MRI showed larger TVs than FDG-PET. There was an approximately 50% mismatch between the FDG-PET TV and the MRI TV at baseline and during CRT. Sixty-one percent of the FDG-PET TV and 76% of the MRI TV obtained after 10 fractions of CRT remained inside the corresponding baseline TV. On MRI, residual tumor was still suspected in all 6 patients with a pathologic complete response, whereas FDG-PET showed a metabolic complete response in 3 of them. The FDG-PET TVs delineated with the gradient-based method matched closest with pathologic findings.


CONCLUSIONS
Integration of MRI and FDG-PET into radiotherapy seems feasible. Gradient-based segmentation is recommended for FDG-PET. Spatial variance between MRI and FDG-PET TVs should be taken into account for target definition.
Roels
Sarah
S
Department of Radiation Oncology, Leuven Cancer Institute, University Hospital Leuven, Herestraat 49, Leuven, Belgium.
Slagmolen
Pieter
P
Nuyts
Johan
J
Lee
John A
JA
Loeckx
Dirk
D
Maes
Frederik
F
Vandecaveye
Vincent
V
Stroobants
Sigrid
S
Ectors
Nadine
N
Penninckx
Freddy
F
Haustermans
Karin
K
D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D000964:Antimetabolites, Antineoplastic / Q000627:therapeutic use; D003131:Combined Modality Therapy / Q000379:methods; D005240:Feasibility Studies; D005260:Female; D019788:Fluorodeoxyglucose F18; D005472:Fluorouracil / Q000627:therapeutic use; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals; D011879:Radiotherapy Dosage; D012004:Rectal Neoplasms / Q000000981:diagnostic imaging / Q000188:drug therapy / Q000473:pathology / Q000532:radiotherapy*; D012074:Remission Induction; D014057:Tomography, X-Ray Computed / Q000379:methods*
D000964:Antimetabolites, Antineoplastic; D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18; D005472:Fluorouracil