Intrafraction 4D-cone beam CT acquired during volumetric arc radiotherapy delivery: kV parameter optimization and 4D motion accuracy for lung stereotactic body radiotherapy (SBRT) patients

Abstract

Purpose: Elekta XVI 5.0 allows for four-dimensional cone beam computed tomography (4D CBCT) image acquisition during treatment delivery to monitor intrafraction motion. These images can have poorer image quality due to undersampling of kV projections and treatment beam MV scatter effects. We determine if a universal intrafraction preset can be used for stereotactic body radiotherapy (SBRT) lung patients and validate the accuracy of target motion characterized by XVI intrafraction 4D CBCT.

Methods: The most critical parameter within the intrafraction preset is the nominal AcquisitionInterval, which controls kV imaging acquisition frequency. An optimal value was determined by maximizing the kV frame number acquired up to 1000 frames, typical of pretreatment 4D CBCT. CIRS motion phantom intrafraction phase images for 16 SBRT beams were obtained. Mean target position, time-weighted standard deviation, and amplitude for these images as well as target motion for three SBRT lung patients were compared to respective pretreatment 4D CBCTs. Evaluation of intrafraction 4D CBCT reconstruction revealed inclusion of MV only images acquired to remove MV scatter effects. A workaround to remove these images was developed.

Results: AcquisitionInterval of 0.1°/frame was optimal. The number of kV frames acquired was 567-1116 and showed strong linear correlation with beam monitor unit (MUs). Phantom target motion accuracy was excellent with average differences in target position, standard deviation and amplitude range of ≤0.5 mm. Target tracking for SBRT patients also showed good agreement. Evaluation of phase sorting wave forms showed that inclusion of MV only images significantly impacts intrafraction image reconstruction for patients and use of workaround is necessary.

Conclusions: A universal intrafraction imaging preset can be used safely for SBRT lung patients. The number of kV projections with MV delivery parameters varies; however images with fewer kV projections still provided accurate target position information. Impact of the reconstruction workaround was significant and is mandated for all 4D CBCT intrafraction imaging performed at our institution.

Keywords: 4D CBCT; SBRT; intrafraction; motion validation.

Figure 1

CIRS dynamic thorax phantom. Left: thorax phantom with lung equivalent cylindrical rod, motion actuator box and surrogate platform is shown. Right: phantom with 1 cm bolus modification is shown and highlighted by red arrow.

Figure 2

Difference in phase sorting of intrafraction four‐dimensional cone beam computed tomography imaging using the CIRS phantom with and without a high‐density bolus attached for an 842 MU beam. Image projection angle = Gantry angle + 90.

Figure 3

Snippet of XVI internal data file that specifies what imaging frames were acquired and if they will be used for image reconstruction and phase sorting.

Figure 4

Correlation of total number of kV frames acquired with intrafraction imaging as a function of volume modulated arc therapy treatment beam MUs.

Figure 5

Target tracking for each of the 10 phases for the S10M10 phantom setup for three different treatment beams as well as the reference four‐dimensional cone beam computed tomography (kV only).

Figure 6

Relationship between target motion uncertainty in mean position, standard deviation and amplitude as a function of MV treatment beam gantry span, and MV treatment beam MUs for each of the four phantom setups (S10M10, S10M20, S20M10, S20M20). The Y‐axis title = chart title.

Figure 7

Intrafraction four‐dimensional cone beam computed tomography reconstructed S10M20 phantom images for three breathing phases (Top‐row: phase 0, Mid‐row: phase 9, Bottom‐row: phase 3). Phantom target is a sphere and should appear as a circle on reconstructed phase images. Deformation of target shape as a result of unevenly spaced and reduced amount of kV projections can be seen. The Window and Level settings used in the Pinnacle Planning system were: 259 and 238.

Figure 8

Target tracking for each of the 10 phases for each of the three stereotactic body radiotherapy patients for the pretreatment four‐dimensional cone beam computed tomography (4D CBCT) and intrafraction 4D CBCT. 0 = exhale phase, 5 = inhale phase. Phase position zero line corresponds to the treatment machine isocenter position.

Figure 9

Distribution of two‐dimensional projection images in each sorting phase.

Figure 10

(a) Original XVI phase sorting (blue curve) and manually detected diaphragm position (red curve) wave forms for a single stereotactic body radiotherapy lung patient. The black box highlights frames with phase sorting errors. (b) Modified phase sorting (blue curve) and manually detected diaphragm position (red curve) wave forms after modification of internal XVI file _frames.xml.

Figure 11

Comparison of intrafraction four‐dimensional cone beam computed tomography images for a stereotactic body radiotherapy lung patient with and without XVI file modification workaround. Same Window and Level was applied for both image displays. Without workaround there is significant blurring of the diaphragm position highlighted by the red arrows making tracking of the diaphragm position difficult.