A rationale for cone beam CT with extended longitudinal field-of-view in image guided adaptive radiotherapy

To investigate the efficacy of using cone beam CT with extended longitudinal field-of-view (CBCT_eLFOV) for image guided adaptive radiotherapy (IGART).

The protocol acquires two CBCT scans with a linear translation of treatment couch in the patient plane, allowing a 1 cm penumbral overlap (i.e. cone beam abutment) and fused as a single DICOM set (CBCT_eLFOV) using a custom-developed software script (coded in MatLab®) for extended localization. Systemic validation was performed to evaluate the geometric and Hounsfield Units accuracy at the overlapping regions of the CBCT_eLFOV using a Catphan®-504 phantom. Two case studies were used to illustrate the CBCT_eLFOV-based IGART workflow in terms of dosimetric and clinical perspectives. Segmentation accuracy/association between repeat CT (re-CT) and CBCT_eLFOV was evaluated. Moreover, the efficacy of the CBCT_eLFOV image data in deformable registration was also described.

Slice geometry, spatial resolution, line profiles and HU accuracy in the overlapping regions of the CBCT_eLFOV yielded identical results when compared with reference CBCT. In patient studies, the dice-similarity-coefficient evaluation showed a good association (>0.9) between re-CT and CBCT_eLFOV. Dosimetric analysis of the CBCT_eLFOV-based adaptive re-plans showed excellent agreement with re-CT based re-plans. Moreover, a similar and consistent pattern of results was also observed using deformed image data (initial planning CT deformed to CBCT_eLFOV) with extended longitudinal projection and the same frame-of-reference as that of the CBCT_eLFOV.

Utilization of CBCT_eLFOV proves to be clinically appropriate and enables accurate prediction of geometric and dosimetric consequences within the planned course of treatment. The ability to compute CBCT_eLFOV-based treatment plans equivalent to re-CT promises a potential improvement in IGART practice.