ABSTRACT
Manual fusion [MF] is a readily available image registration technique that does not require matching algorithms. The operator performs rigid-body transformations interactively. The precision of MF [as implemented on the Philips Pinnacle treatment planning system] was required for cranial CT-MR images used in radiotherapy planning for typical centrally located planning target volumes [PTVs]. A multi-stage MF procedure was developed which 11 observers followed to match the head contour, bones, soft tissues and contoured structures for 5 patient image-sets. Registration parameters were calculated by solving the transformation matrix following a consistent order of translations [T] and rotations [R]. The mean position of centre of each PTV averaged over all observers was used as the reference. The effect of misregistration on the PTV co-ordinates and the volume increase resulting from application of a margin for registration uncertainty were calculated. Mean intra- and inter-observer T/R SDs were 0.5mm/0.4, respectively. Mean intra- and inter-observer registration error and 1.1mm/1.0 [3D distance of each PTV centre from the mean position for all observers] was +/- 0.7 mm respectively, the latter reducing to 1.4 +/- 0.3 mm [1 SD] and 1.6 +/- 0.7 0.6 mm excluding the 3 least experienced operators. A subsequent 2 mm margin for misregistration on average increased the PTV volume by 27%. Moderately trained operators produced clinically acceptable results while experienced operators improved the precision. MF still has an important role in the registration of cranial CT and MR images for radiotherapy planning especially for under-resourced centers