Kilovoltage Intrafraction Monitoring for Prostate Intensity Modulated Arc Therapy: First Clinical Results

Abstract

Summary Most cancer radiation therapy accelerators purchased today have gantry-mounted imagers, typically used to image the patient prior to treatment. We imaged prostate cancer patients during their treatment. Combining images with marker segmentation software and a 2- to 3-dimensional reconstruction method, we were able to measure prostate motion during the treatment to within submillimeter accuracy. Because intrafraction prostate monitoring method uses widely available clinical equipment, intratreatment prostate motion monitoring could become routine. Purpose Most linear accelerators purchased today are equipped with a gantry-mounted kilovoltage X-ray imager which is typically used for patient imaging prior to therapy. A novel application of the X-ray system is kilovoltage intrafraction monitoring (KIM), in which the 3-dimensional (3D) tumor position is determined during treatment. In this paper, we report on the first use of KIM in a prospective clinical study of prostate cancer patients undergoing intensity modulated arc therapy (IMAT). Methods and Materials Ten prostate cancer patients with implanted fiducial markers undergoing conventionally fractionated IMAT (RapidArc) were enrolled in an ethics-approved study of KIM. KIM involves acquiring kV images as the gantry rotates around the patient during treatment. Post-treatment, markers in these images were segmented to obtain 2D positions. From the 2D positions, a maximum likelihood estimation of a probability density function was used to obtain 3D prostate trajectories. The trajectories were analyzed to determine the motion type and the percentage of time the prostate was displaced ≥3, 5, 7, and 10 mm. Independent verification of KIM positional accuracy was performed using kV/MV triangulation. Results KIM was performed for 268 fractions. Various prostate trajectories were observed (ie, continuous target drift, transient excursion, stable target position, persistent excursion, high-frequency excursions, and erratic behavior). For all patients, 3D displacements of ≥3, 5, 7, and 10 mm were observed 5.6%, 2.2%, 0.7% and 0.4% of the time, respectively. The average systematic accuracy of KIM was measured at 0.46 mm. Conclusions KIM for prostate IMAT was successfully implemented clinically for the first time. Key advantages of this method are (1) submillimeter accuracy, (2) widespread applicability, and (3) a low barrier to clinical implementation. A disadvantage is that KIM delivers additional imaging dose to the patient.

The first clinical implementation of a real-time six degree of freedom target tracking system during radiation therapy based on Kilovoltage Intrafraction Monitoring (KIM)

“…The KIM translational accuracy and precision in this study are consistent with previous studies [12,14]. …”

“…Ideally, realtime image guidance would be performed using a standard linear accelerator (linac) without relying on additional hardware. Reported to have similar accuracy (sub-millimetre) to the Calypso system is the Kilovoltage Intrafraction Monitoring (KIM) system which estimates tumour position in real-time based on kV images from a single on-board imager (OBI) mounted on a standard linac [12,13]. The first clinical trial implementing real-time KIM for prostate cancer radiation therapy with beam gating and couch correction to re-align the tumour position with the treatment isocenter was reported by Keall et al [14,15].…”

Real‐time intrafraction prostate motion during linac based stereotactic radiotherapy with rectal displacement

“…Less motion was observed in the patient cohort considered here, treated with a Rectafix in place, when compared with the data reported by Ng et al17 who found, in a study of 10 patients without an RDD, that the prostate was displaced less than 1 mm from its initial position only 62.4% of the time, and was more than 3 mm from its initial position 4.7% of the time. Keall et al23 reported on the use of KIM in an interventional study for 197 prostate fractions delivered to 6 patients using VMAT, where, if no repositioning had occurred, the prostate would have been greater than 3 mm from its isocentre position for 20% of the beam on treatment time and greater than 5 mm from the isocentre position for 4% of the treatment time.…”

“…The geometrical accuracy of KIM has been established, and the software has successfully been used to measure prostate displacement during treatment in noninterventional17 and interventional studies 18…”

Technical note: TROG 15.01 SPARK trial multi‐institutional imaging dose measurement

“…KIM determines the position of the prostate in three dimensions from 2D kV projections using a probability density function 9, 10. KIM has successfully been used to measure prostate displacement during treatment in retrospective11 and interventional studies 12…”