Next-generation nonstop gated CBCT for respiratory gating lung radiotherapy: Scan time and imaging dose.

Background: Free-breathing gated CBCT (gCBCT) is commonly prescribed for lung cancer patients undergoing respiratory gating radiotherapy. Recently, the nonstop gated CBCT (ngCBCT) has been proposed to significantly reduce scanning time and imaging dose while preserving high image quality.

Purpose: To implement the novel ngCBCT imaging technique on a C-arm linear accelerator (LINAC) and quantitatively compare its scan time and imaging dose with those of the current clinical gCBCT.

Methods: ngCBCT was implemented via a customized XML file in the developer mode of a C-arm LINAC, while gCBCT was acquired in the clinical mode. Both techniques employed the same thorax imaging protocol (half fan, full trajectory). Scan times were calculated from the timestamps of acquired projection data. Imaging dose was characterized using the weighted Cone-Beam Dose Index (CBDI_w), measured with a standard CTDI body phantom and two pencil chambers placed centrally and peripherally. Respiratory motion was simulated using a CIRS motion platform with both Cos4 waveforms (3-6 s cycles) and seven clinical patient breathing traces. Gating duty cycles of 30%-60% were tested for Cos4 motion, while the same gating window was reproduced for each patient's breathing trace.

Results: Scan times for gCBCT ranged from 1.8 to 5 min, influenced by the gating duty cycle, breathing period, and waveform periodicity. In contrast, ngCBCT consistently achieved scan times of approximately 1 min. The imaging dose (CBDIw) for ngCBCT was reduced to 26.7%-60.1% of that for gCBCT, closely matching the respective gating duty cycles.

Conclusion: This study demonstrates that ngCBCT acquisition is feasible on C-arm LINAC and offers substantial improvements in scan time and dose reduction compared to current clinical gCBCT. This novel technique has the potential to enhance patient comfort and broaden access to respiratory gating radiotherapy.