Superiority of Deformable Image Co-registration in the Integration of Diagnostic Positron Emission Tomography-Computed Tomography to the Radiotherapy Treatment Planning Pathway for Oesophageal Carcinoma

Aims: To investigate the use of image co-registration in incorporating diagnostic positron emission tomography-computed tomography (PET-CT) directly into the radiotherapy treatment planning pathway, and to describe the pattern of local recurrence relative to the PET-avid volume. Materials and methods: Fourteen patients were retrospectively identified, six of whom had local recurrence. The accuracy of deformable image registration (DIR) and rigid registration of the diagnostic PET-CT and recurrence CT, to the planning CT, were quantitatively assessed by comparing co-registration of oesophagus, trachea and aorta contours. DIR was used to examine the correlation between PET-avid volumes, dosimetry and site of recurrence. Results: Positional metrics including the dice similarity coefficient (DSC) and conformity index (CI), showed DIR to be superior to rigid registration in the co-registration of diagnostic and recurrence imaging to the planning CT. For diagnostic PET-CT, DIR was superior to rigid registration in the transfer of oesophagus (DSC=0.75 versus 0.65, P <0.009 and CI=0.59 versus 0.48, P <0.003), trachea (DSC=0.88 versus 0.65, P <0.004 and CI=0.78 versus 0.51, P <0.0001) and aorta structures (DSC=0.93 versus 0.86, P <0.006 and CI=0.86 versus 0.76, P <0.006). For recurrence imaging, DIR was superior to rigid registration in the transfer of trachea (DSC=0.91 versus 0.66, P <0.03 and CI=0.83 versus 0.51, P <0.02) and oesophagus structures (DSC=0.74 versus 0.51, P <0.004 and CI=0.61 versus 0.37, P <0.006) with a non-significant trend for the aorta (DSC=0.91 versus 0.75, P <0.08 and CI=0.83 versus 0.63, P <0.06) structure. A mean inclusivity index of 0.93 (range 0.79-1) showed that the relapse volume was within the planning target volume (PTVPET-CT); all relapses occurred within the high dose region. Conclusion: DIR is superior to rigid registration in the co-registration of PET-CT and recurrence CT to the planning CT, and can be considered in the direct integration of PET-CT to the treatment planning process. Local recurrences occur within the PTVPET-CT, suggesting that this is a suitable target for dose-escalation strategies.