Reduction of Cone-Beam CT scan time without compromising the accuracy of the image registration in IGRT

Figures & data

Table I. Settings for the CBCT scan.

Patient group	H&N	Lung
Start angle	–110°	–177°
Stop angle	90°	181°
Energy (kV)	100	120
Current (mA)	10	25
Time (ms)	10	40
kVCollimator	S20	M20
kVFilter	F0	Fl
Number of projections	∼365	–640

Table II. Definition of edges of the clipbox used for the image registration.

Edge of clipbox	H&N patients type 1	H&N patients type 2	Stereotactic lung patients
Anterior	Frontal part of back bone	Frontal sinus	Edge of PTV
Posterior	Dorsal part of spinous process	Frontal part of foramen magnum	Edge of PTV
Craniel	Foramen magnum	Base of skull	Edge ot PTV
Caudal	C4 or C6	Hard palate	Edge of PTV
Lateral	Foramen magnum	1-2 cm lateral of frontal sinus	Edge of PTV

H&N type 1 and 2 are related to tumours below or above the hard palate, respectively.

Figure 1. The visual differences when reducing the number of projections. Slices from the reconstruction using 90% and 75% are omitted because of the very little visual changes. There were too few projection images to reconstruct the CBCT in the 10% case for the H&N patients.

Figure 2. The components of ΔT for the lung and H&N patients. The average translation in each direction left-right (LR), anterior-posterior (AP) and cranio-caudal (CC) are marked with the asterisk symbol, and the error bars shows ± 1 standard deviation.

Figure 3. Probability histogram showing the probability of having a translational error of length larger than a given length. It is seen that no deviation larger than 0.7 mm is seen in the H&N group. Likewise, it is seen for the lung group that there is a small probability of deviations above 1 mm, however deviation above 1.4 mm is not observed in any of the reduced data sets.