Validated image fusion of dedicated PET and CT for external beam radiation and therapy in the head and neck area.

AIM: Integration of positron emission tomography (PET) information into computer tomography (CT)- based intensity modulated external beam radiation therapy (IMRT) allows adaptation of the target volume to functional parameters, but only when the image registration procedure is reliable. The aim of this study was to select the optimal method for software fusion of dedicated PET and CT, and to validate the procedure for IMRT head-neck area.

METHOD: Fifteen patients with head and neck squamous cell carcinoma underwent separate CT and F-18-fluorodeoxyglucose positron emission tomography(FDG-PET), both in a custom-moulded rigid mask fitted with 4 multimodality fiducial markers. Five image registration methods were applied . PET emission and CT were registered manually (ME) and using the landmarks (LM). PET transmission and CT were registered manually (MT) using a mutual information-based method (MI) and an iterative closest point method (ICP). The error in image registration of each method was determined by evaluating the markers.

RESULTS: LM showed an average registration error of 1.4 mm at the location of the markers, and 0.3mm in the planning area. However, this method proved to be laborious. Apart from LM, the best method was ICP, with registration errors of 3 and 2mm, respectively. The respective errors were 4.7 and 3.5 mm with ME, 3.6 and 2.7 mm with MT, and 5.3 and 4.1mm with MI.

CONCLUSION: Image fusion of dedicated PET and CT of the head-neck area can be performed reliably using the operator-independent ICP method with no need for laborious markers. The achieved accuracy permits implementation of dedicated PET images in external beam radiation therapy.