Study suggests role for 4D-CBCT in liver SBRT planning

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Kumamoto University Hospital researchers find that inter- and intrafractional liver tumor motion changes are rare in patients who had liver SBRT with abdominal compression (AC)

  • For stereotactic body radiation therapy (SBRT) of liver tumors, tumor motion induced by respiration must be taken into account in planning and treatment.
  • Liver tumor motion at the planning simulation represents liver tumor motion during SBRT. Inter- and intrafractional liver tumor motion changes were small in patients with AC.

Therapeutically potent SBRT for liver tumors can be an attractive alternative to surgery or RF ablation. To visualize liver tumor motion pre-SBRT, physicians use CT or cone-beam CT (CBCT) to track fiducial markers in or near the lesion. In the planning stage, 4D-CT can define tumor trajectories and the internal target volume (ITV), assuming that the tumor motion of 4D-CT at planning represents tumor motion throughout the course of SBRT.

The problem is that 4D-CT is not installed at all centers. Researchers at Kumamoto University Hospital (Shimohigashi and Toya, et al) explored whether the combined use of 4D-CBCT (e.g., Elekta’s Symmetry™ software) and fiducial markers could provide precise liver SBRT at both the planning and treatment stages1“We evaluated whether liver tumor motion at the planning simulation represents liver tumor motion during SBRT, and estimated changes of inter- and intrafractional tumor motion in patients undergoing liver SBRT with abdominal compression [AC] using 4D-CBCT and fiducial markers.”

The retrospective study included 10 patients (5 HCC, 2 CCC, 3 liver mets), all of whom had 1-2 gold fiducial markers implanted in the liver. In the planning simulation CT, patients were told to breathe shallowly to form a thermoplastic body shell and AC was applied in all patients to achieve reproducible tumor motion for planning and treatment. Free-breathing contrast-enhanced CT and slow CT were performed and the images were exported to the treatment planning system.

Estimation of tumor motion on 4D-CBCT

On the same day as CT planning simulation, a 4D-CBCT (Symmetry) scan was performed to evaluate liver tumor motion for planning. Projection data were sorted into 10 respiratory-phase bins. Tumor motion was measured using Elekta’s XVI software (v4.5): “The translational distances at the center positions of the fiducial markers from all 10 phases on the 4D-CBCT images were measured as the extent of the tumor motion in the left-right [LR], anterior-posterior [AP], and superior-inferior [SI] directions. In each patient, fiducial markers were measured individually, with an average of 1.8 markers and a total of 18 markers.”

Treatment planning

The GTV was defined with CT and MRI images and CTV was defined with 3D margins of 0-3 mm to the GTV. The ITV was developed by applying margins to the CTV, which were based on the calculated results of the tumor motion. The PTV was created by adding margins ranging from 1-3 mm, 1-3 mm and 3-6 mm to the ITV for LR, AP and SI directions, respectively.

“All structures of these tumor and target volumes, and the organs at risk were delineated on the slow CT images. Each fiducial marker was also delineated, as a covering volume with margins determined according to the calculated results of the tumor motion generated on the slow CT images, referred to as the ‘internal marker target volume’. All slow CT data and structures were exported into the Elekta XVI as the reference for image guidance.”

Treatment delivery

To localize the SBRT targets, Kumamoto clinicians performed 4D-CBCT in a similar way as the planning simulation and used Elekta XVI to co-register the 4D-CBCT and slow CT images. Then, the treatment couch was repositioned with translational correction and liver SBRT was delivered. Just after therapy, a 4D-CBCT scan was acquired to evaluate the intrafractional setup error of the target position. Treatment time (pre-SBRT 4D-CBCT to post-SBRT 4D-CBCT) ranged from 11.1 to 25.7 minutes, with an average of 17.7 minutes.

Results – 4D-CBCT should be considered for both planning and treatment

The investigators found a strong correlation in the LR, AP and SI directions between the liver tumor motion of the planning simulation and the mean liver tumor motion of the pre-SBRT. In addition: “Interfractional liver tumor motion changes of >3 mm occurred in 10% of treatment fractions, in the SI direction alone…intrafractional liver motion changes of >3 mm occurred in 2% of treatment fractions in the SI direction alone.

The researchers indicated – based on the strong correlation between liver tumor motion on the planning simulation and pre-SBRT – that these data suggest that liver tumor motion at the planning simulation could represent liver tumor motion during SBRT. The implication is that 4D-CBCT could be introduced not only for treatment but also for planning liver SBRT, as a useful modality for evaluating tumor motion in centers that do not have 4D-CT.

“Liver tumor motion changes were small in most patients who underwent liver SBRT with AC, and inter- and intrafractional motion changes of >3 mm were rare in any direction…Precise liver SBRT should be realized by the combined use of 4D-CBCT and fiducial markers at both the planning and treatment stages. Moreover, our results suggested that adding 3D margins of 3 mm or less to liver tumor motion at the planning simulation might be adequate to cover the tumor motion at the treatment if AC is available.”


  1. Shimohigashi Y, Toya R, Saito T, et al. Tumor motion changes in stereotactic body radiotherapy for liver tumors: an evaluation based on four-dimensional cone-beam computed tomography and fiducial markers. Radiation Oncology (2017) 12:61. DOI 10.1186/s13014-017-0799-7.
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