International Journal of Radiation Oncology*Biology*Physics
Physics ContributionKilovoltage Intrafraction Monitoring for Prostate Intensity Modulated Arc Therapy: First Clinical Results
Introduction
Tumors move during radiation therapy treatment, resulting in reduced geometric and dosimetric accuracy. In standard clinical practice, this motion is not monitored during treatment. In prostate radiation therapy, the probability of biochemical and local control decreases and rectal toxicity increases when the rectum is distended during planning computed tomography (CT) simulations (1). In 2008, Kupelian et al (2) demonstrated that daily image guidance eliminates the error due to rectal distention. In 2010, Sandler et al (3) found that real-time motion monitoring using electromagnetic (EM) guidance and gating with a reduced planning target volume (PTV) margin resulted in reduced patient morbidity. From these previous data, it can be argued that real-time tumor localization and adaptation can improve clinical outcomes. Real-time adaptation is enabled by real-time localization. Hence, the introduction of a novel real-time tumor localization modality that is widely available may be beneficial for prostate cancer outcomes.
Several real-time tumor localization imaging modalities have been investigated; for example, ultrasonography (4), megavoltage (MV) imaging (5), combined MV and kV (6), Calypso EM guidance (7), and Navotek radioactive fiducial implant (8). However, some of these techniques are either still under development, not readily available, or are expensive.
Kilovoltage intrafraction monitoring (KIM) is a novel intrafraction real-time tumor localization modality. It involves a single gantry-mounted kV X-ray imager (which is widely available on most linear accelerators [LINACs]) acquiring 2-dimensional (2D) projections of implanted fiducial markers. Three-dimensional (3D) positions are then reconstructed by maximum likelihood estimation (MLE) of a 3D probability density function. In this study, we report the first use of KIM in a prospective clinical study with prostate cancer patients undergoing intensity modulated arc therapy (IMAT).
Section snippets
Overview of KIM
Figure 1 shows the workflow of the clinical study. As the gantry rotates around the patient during treatment (Fig. 1, 1), the kV imager acquires 2D projections of the prostate (Fig. 1, 2). The fiducial markers are segmented using software developed in-house (Fig. 1, 3). Three-dimensional positions are determined via MLE of a 3D probability density function (pdf) (Fig. 1, 4) (9). The 3D trajectory of the prostate is then plotted as a function of time (Fig. 1, 5).
This clinical study was
Prostate trajectory types
Examples of prostate trajectories observed using KIM are shown in Figure 2. These trajectory types are similar to those observed using the Calypso EM guidance modality for prostate tumor position localization (7). Of particular note is Figure 2D, representing persistent excursion, in which >12-mm displacement was observed for most of the treatment, indicating a large uncorrected geometric miss.
Patient motion statistics
Table 2 lists the motion statistics for all patients. It is evident that LR motion is nearly
Discussion
A new method of prostate intrafraction motion monitoring using kilovoltage imaging was successfully clinically implemented in a cohort of 10 patients undergoing conventionally fractionated IMAT. The measured motion information could be used with dose reconstruction tools to give an estimate of the dose delivered to the prostate. Real-time implementation of the KIM method (14) could be used with gating or tracking motion management. Although prostate cancer was the focus of the current study,
Conclusions
KIM for prostate IMAT was successfully implemented clinically for the first time, and its geometric accuracy was demonstrated. KIM is a clinically viable and objective method for monitoring prostate motion during treatment. 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. Several strategies to reduce the patient imaging dose are
Acknowledgment
Julie Baz, University of Sydney, improved the clarity and readability of the manuscript.
References (23)
- et al.
Increased risk of biochemical and local failure in patients with distended rectum on the planning CT for prostate cancer radiotherapy
Int J Radiat Oncol Biol Phys
(2005) - et al.
Impact of image guidance on outcomes after external beam radiotherapy for localized prostate cancer
Int J Radiat Oncol Biol Phys
(2008) - et al.
Reduction in patient-reported acute morbidity in prostate cancer patients treated with 81-Gy Intensity-modulated radiotherapy using reduced planning target volume margins and electromagnetic tracking: assessing the impact of margin reduction study
Urology
(2010) - et al.
First demonstration of combined kV/MV image-guided real-time dynamic multileaf-collimator target tracking
Int J Radiat Oncol Biol Phys
(2009) - et al.
Multi-institutional clinical experience with the Calypso System in localization and continuous, real-time monitoring of the prostate gland during external radiotherapy
Int J Radiat Oncol Biol Phys
(2007) - et al.
Tracking accuracy of a real-time fiducial tracking system for patient positioning and monitoring in radiation therapy
Int J Radiat Oncol Biol Phys
(2010) Errors and margins in radiotherapy
Semin Radiat Oncol
(2004)- et al.
Implementation of a new method for dynamic multileaf collimator tracking of prostate motion in arc radiotherapy using a single kV imager
Int J Radiat Oncol Biol Phys
(2010) - et al.
Dynamic multileaf collimator tracking of respiratory target motion based on a single kilovoltage imager during arc radiotherapy
Int J Radiat Oncol Biol Phys
(2010) - et al.
Accuracy of a wireless localization system for radiotherapy
Int J Radiat Oncol Biol Phys
(2005)
Target localization and real-time tracking using the Calypso 4D localization system in patients with localized prostate cancer
Int J Radiat Oncol Biol Phys
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2021, Radiotherapy and OncologyCitation Excerpt :However, for the L/R direction, the error is largely due to the fact that there is no kV image angle that allows for view of the L/R direction of the fiducial marker, and so the therapist has no indication that a correction is required. One way to alleviate this problem would be to use the 2D-3D position estimation that uses the pre-treatment CBCT projection data acquired to estimate the PDF and dynamically update and report the 3D position of the fiducial marker based on the triggered images as for prostate [21]. An alternative approach would be better immobilization as currently, no immobilization of the breast itself is used (discussed further below).
Funding support was received from the Australian National Health and Medical Research Council Australia Fellowship and US National Institutes of Health/National Cancer Institute grant CI R01CA93626. Drs Keall and Poulsen are inventors of the kilovoltage intrafraction motion monitoring method investigated clinically in this study. Stanford University has filed a US patent application (no. 20100172469) and has licensed the method to Varian Medical Systems. No commercial support was received for this study.
Conflict of interest: none.