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

PURPOSE

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

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Clinical implementation of magnetic resonance imaging guided adaptive radiotherapy for localized prostate cancer

Background and purpose

Magnetic resonance-guided radiation therapy (MRgRT) has recently become available in clinical practice and is expected to expand significantly in coming years. MRgRT offers marker-less continuous imaging during treatment delivery, use of small clinical target volume (CTV) to planning target volume (PTV) margins, and finally the option to perform daily plan re-optimization.

Materials and methods

A total of 140 patients (700 fractions) have been treated with MRgRT and online plan adaptation for localized prostate cancer since early 2016. Clinical workflow for MRgRT of prostate cancer consisted of patient selection, simulation on both MR- and computed tomography (CT) scan, inverse intensity-modulated radiotherapy (IMRT) treatment planning and daily plan re-optimization prior to treatment delivery with partial organs at risk (OAR) recontouring within the first 2 cm outside the PTV. For each adapted plan online patient-specific quality assurance (QA) was performed by means of a secondary Monte Carlo 3D dose calculation and gamma analysis comparison. Patient experiences with MRgRT were assessed using a patient-reported outcome questionnaire (PRO-Q) after the last fraction.

Results

In 97% of fractions, MRgRT was delivered using the online adapted plan. Intrafractional prostate drifts necessitated 2D-corrections during treatment in approximately 20% of fractions. The average duration of an uneventful fraction of MRgRT was 45 min. PRO-Q’s (N = 89) showed that MRgRT was generally well tolerated, with disturbing noise sensations being most commonly reported.

Conclusions

MRgRT with daily online plan adaptation constitutes an innovative approach for delivering SBRT for prostate cancer and appears to be feasible, although necessitating extended timeslots and logistical challenges.

Extended localization and adaptive dose calculation using HU corrected cone beam CT: Phantom study

BACKGROUND AND AIM

The practicability of computing dose calculation on cone beam CT (CBCT) has been widely investigated. In most clinical scenarios, the craniocaudal scanning length of CBCT is found to be inadequate for localization. This study aims to explore extended tomographic localization and adaptive dose calculation strategies using Hounsfield unit (HU) corrected CBCT image sets.

MATERIALS AND METHODS

Planning CT (pCT) images of the Rando phantom (T12-to-midthigh) were acquired with pelvic-protocol using Biograph CT-scanner. Similarly, half-fan CBCT were acquired with fixed parameters using Clinac2100C/D linear accelerator integrated with an on-board imager with 2-longitudinal positions of the table. For extended localization and dose calculation, two stitching strategies viz., one with "penumbral-overlap" (S1) and the other with "no-overlap" (S2) and a local HU-correction technique were performed using custom-developed MATLAB scripts. Fluence modulated treatment plans computed on pCT were mapped with stitched CBCT and the dosimetric analyses such as dose-profile comparison, 3D-gamma (γ) evaluation and dose-volume histogram (DVH) comparison were performed.

RESULTS

Localizing scanning length of CBCT was extended by up to 15 cm and 16 cm in S1 and S2 strategies, respectively. Treatment plan mapping resulted in minor variations in the volumes of delineated structures and the beam centre co-ordinates. While the former showed maximum variations of -1.4% and -1.6%, the latter showed maximum of 1.4 mm and 2.7 mm differences in anteroposterior direction in S1 and S2 protocols, respectively. Dosimetric evaluations viz., dose profile and DVH comparisons were found to be in agreement with one another. In addition, γ-evaluation results showed superior pass-rates (≥98.5%) for both 3%/3 mm dose-difference (DD) and distance-to-agreement (DTA) and 2%/2 mm DD/DTA criteria with desirable dosimetric accuracy.

CONCLUSION

Cone beam tomographic stitching and local HU-correction strategies developed to facilitate extended localization and dose calculation enables routine adaptive re-planning while circumventing the need for repeated pCT.