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Purpose: Kilovoltage intrafraction monitoring (KIM) allows for real‐time image guidance for tracking tumor motion in six‐degrees‐of‐freedom (6DoF) on a standard linear accelerator. This study assessed the geometric accuracy and precision of KIM used to guide patient treatments in the TROG 15.01 multi‐institutional Stereotactic Prostate Ablative Radiotherapy with KIM trial and investigated factors affecting accuracy and precision. Methods: Fractions from 44 patients with prostate cancer treated using KIM‐guided SBRT were analyzed across four institutions, on two different linear accelerator models and two different beam models (6 MV and 10 MV FFF). The geometric accuracy and precision of KIM was assessed from over 33 000 images (translation) and over 9000 images (rotation) by comparing the real‐time measured motion to retrospective kV/MV triangulation. Factors potentially affecting accuracy, including contrast‐to‐noise ratio (CNR) of kV images and incorrect marker segmentation, were also investigated. Results: The geometric accuracy and precision did not depend on treatment institution, beam model or motion magnitude, but was correlated with gantry angle. The centroid geometric accuracy and precision of the KIM system for SABR prostate treatments was 0.0 ± 0.5, 0.0 ± 0.4 and 0.1 ± 0.3 mm for translation, and −0.1 ± 0.6°, −0.1 ± 1.4° and −0.1 ± 1.0° for rotation in the AP, LR and SI directions respectively. Centroid geometric error exceeded 2 mm for 0.05% of this dataset. No significant relationship was found between large geometric error and CNR or marker segmentation correlation. Conclusions: This study demonstrated the ability of KIM to locate the prostate with accuracy below other uncertainties in radiotherapy treatments, and the feasibility for KIM to be implemented across multiple institutions. [ABSTRACT FROM AUTHOR]

Purpose The focus of this work was to improve the available kV image quality for continuous intrafraction monitoring of the prostate during volumetric modulated arc therapy. This is investigated using a novel blade collimation system enabling tube current modulated (TCM) volume-of-interest (VOI) imaging of prostate fiducial markers during radiotherapy, and Monte Carlo simulation of MV scatter. Materials and methods A four-blade dynamic kV collimator was used to track a VOI containing gold fiducial markers embedded in a dynamic pelvis phantom during gantry rotation. For each fiducial, a VOI margin around each marker was set to be 2σ of the population covariance matrix characterizing prostate motion. This was used to conform to a single or several fiducials and compared to a static field. DRRs were used to calculate the kV attenuation for each VOI as a function of angle and used to optimize x-ray tube current during acquisition. Image quality was assessed with regard to contrast-to-noise ratio (CNR), fiducial detectability, and imaging dose. Monte Carlo simulations in EGSnrc were used to calculate the imaging dose to the phantom and MV scatter fluence to the imaging panel. Results Fiducials can be accurately located using a VOI containing a single or several fiducials using a relatively high constant kV output. However, when using a 6 × 6 cm2 field the dose can be upwards of 1.5 Gy in bone for constant kV output and 3.1 Gy when applying TCM at 1 Hz imaging over the course of 40 fractions. This can be mitigated through tailoring the imaging field to a single or several fiducials, in which the integral dose is reduced by a factor of 15.6 and 3.7, respectively. For a constant MV treatment field size, the scattered fluence reaching the kV panel varies by less than a factor of two for a completely rotation of the gantry. However, the MV scatter spectrum overlaps with the detector response for a deleterious effect, with a peak MV scatter energy of approximately 100 keV. TCM can be used to overcome the variability in image quality throughout the rotation and therefore improve fiducial CNR and detectability during periods of high kV attenuation. Conclusions The combination of VOI and TCM introduces an advantageous approach in intrafraction monitoring of the prostate during radiotherapy by both reducing and localizing the imaging dose, while improving image quality and fiducial detectability during periods of high kV attenuation. In addition, the influence of MV scatter has been shown to be most important in low attenuation regions, with a variation by a factor of two. [ABSTRACT FROM AUTHOR]

Purpose: Kilovoltage intrafraction monitoring (KIM) is a real-time image guidance method that uses widely available radiotherapy technology, i.e., a gantry-mounted x-ray imager. The authors report on the geometric and dosimetric results of the first patient treatment using KIM which occurred on September 16, 2014. Methods: KIM uses current and prior 2D x-ray images to estimate the 3D target position during cancer radiotherapy treatment delivery. KIM software was written to process kilovoltage (kV) images streamed from a standard C-arm linear accelerator with a gantry-mounted kV x-ray imaging system. A 120° pretreatment kV imaging arc was acquired to build the patient-specific 2D to 3D motion correlation. The kV imager was activated during the megavoltage (MV) treatment, a dual arc VMAT prostate treatment, to estimate the 3D prostate position in real-time. All necessary ethics, legal, and regulatory requirements were met for this clinical study. The quality assurance processes were completed and peer reviewed. Results: During treatment, a prostate position offset of nearly 3 mm in the posterior direction was observed with KIM. This position offset did not trigger a gating event. After the treatment, the prostate motion was independently measured using kV/MV triangulation, resulting in a mean difference of less than 0.6 mm and standard deviation of less than 0.6 mm in each direction. The accuracy of the marker segmentation was visually assessed during and after treatment and found to be performing well. During treatment, there were no interruptions due to performance of the KIM software. Conclusions: For the first time, KIM has been used for real-time image guidance during cancer radiotherapy. The measured accuracy and precision were both submillimeter for the first treatment fraction. This clinical translational research milestone paves the way for the broad implementation of real-time image guidance to facilitate the detection and correction of geometric and dosimetric errors, and resultant improved clinical outcomes, in cancer radiotherapy. [ABSTRACT FROM AUTHOR]

Purpose: Kilovoltage intrafraction monitoring (KIM) is a real-time 3D tumor monitoring system for cancer radiotherapy. KIM uses the commonly available gantry-mounted x-ray imager as input, making this method potentially more widely available than dedicated real-time 3D tumor monitoring systems. K IM is being piloted in a clinical trial for prostate cancer patients treated with VMAT (NCT01742403). The purpose of this work was to develop clinical process and quality assurance (QA) practices for the clinical implementation of KIM. Methods: Informed by and adapting existing guideline documents from other real-time monitoring systems, KIM-specific QA practices were developed. The following five KIM-specific QA tests were included: (1) static localization accuracy, (2) dynamic localization accuracy, (3) treatment interruption accuracy, (4) latency measurement, and (5) clinical conditions accuracy. Tests (1)-(4) were performed using K IM to measure static and representative patient-derived prostate motion trajectories using a 3D programmable motion stage supporting an anthropomorphic phantom with implanted gold markers to represent the clinical treatment scenario. The threshold for system tolerable latency is <1 s. The tolerances for all other tests are that both the mean and standard deviation of the difference between the programmed trajectory and the measured data are <1 mm. The (5) clinical conditions accuracy test compared the K IM measured positions with those measured by kV/megavoltage (MV) triangulation from five treatment fractions acquired in a previous pilot study. Results: For the (1) static localization, (2) dynamic localization, and (3) treatment interruption accuracy tests, the mean and standard deviation of the difference are <1.0 mm. (4) The measured latency is 350 ms. (5) For the tests with previously acquired patient data, the mean and standard deviation of the difference between KIM and kV/MV triangulation are <1.0 mm. Conclusions: Clinical process and QA practices for the safe clinical implementation of KIM, a novel real-time monitoring system using commonly available equipment, have been developed and implemented for prostate cancer VMAT. [ABSTRACT FROM AUTHOR]