Abstract
PurposeElekta 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.MethodsThe 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.ResultsAcquisitionInterval 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.ConclusionsA 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.
Highlights
Image guidance is considered a prerequisite for lung stereotactic body radiotherapy (SBRT) treatment
We investigate whether a standard universal intrafraction imaging preset could be used for all SBRT lung patients by evaluating preset parameters for a wide range of arc lengths and monitor unit (MU) deliveries
Differences in calculated target mean position, position standard deviation (SD), and amplitude for each of the intrafraction 4D cone beam computerized tomography (CBCT) images as compared to the reference 4D CBCT images are presented for the four phantom setups
Summary
Image guidance is considered a prerequisite for lung stereotactic body radiotherapy (SBRT) treatment. Among the various image guidance modalities, the linear accelerator gantry‐mounted kilovoltage (kV) cone beam computerized tomography (CBCT) has become one of the most frequently used in‐room imaging technologies With this system, the CBCT scanner acquires projections over a time period of 70 s to reconstruct a three‐dimensional CBCT volumetric image set (3D CBCT). The relatively long acquisition time of these 3D CBCT images makes them subject to significant respiratory motion artifacts when imaging the thoracic region.[1] To address this issue, respiratory‐correlated 4D CBCT has been developed and gradually incorporated into the lung SBRT workflow.[2,3]. In 4D CBCT, the projection images are retrospectively binned according to respiratory phase Those x‐ray projections that correspond to the same phase of the patient respiratory trace are reconstructed together, generating multiple phase‐based CBCT datasets which characterize the tumor motion at various phases of the breathing cycle.[1,2]. Those x‐ray projections that correspond to the same phase of the patient respiratory trace are reconstructed together, generating multiple phase‐based CBCT datasets which characterize the tumor motion at various phases of the breathing cycle.[1,2] These 4D CBCT image sets can be directly compared with 4DCT datasets acquired at the time of planning to guide patient setup based on tumor motion
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