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

doi:10.1016/j.ijrobp.2012.07.2367

International Journal of Radiation OncologyBiologyPhysics

Volume 84, Issue 5, 1 December 2012, Pages e655-e661

https://doi.org/10.1016/j.ijrobp.2012.07.2367 Get rights and content

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.

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)

R. de Crevoisier 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)
P.A. Kupelian et al.
Impact of image guidance on outcomes after external beam radiotherapy for localized prostate cancer
Int J Radiat Oncol Biol Phys
(2008)
H.M. Sandler 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)
B. Cho et al.
First demonstration of combined kV/MV image-guided real-time dynamic multileaf-collimator target tracking
Int J Radiat Oncol Biol Phys
(2009)
P. Kupelian 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)
T. Shchory 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)
M. van Herk
Errors and margins in radiotherapy
Semin Radiat Oncol
(2004)
P.R. Poulsen 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)
P.R. Poulsen 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)
J.M. Balter et al.
Accuracy of a wireless localization system for radiotherapy
Int J Radiat Oncol Biol Phys
(2005)

T.R. Willoughby et al.

Target localization and real-time tracking using the Calypso 4D localization system in patients with localized prostate cancer

Int J Radiat Oncol Biol Phys

(2006)

Cited by (95)

Monitoring Intrafraction Motion of the Prostate During Radiation Therapy: Suggested Practice Points From a Focused Review
2024, Practical Radiation Oncology
External beam radiation therapy to the prostate is typically delivered after verification of prostatic position with image guidance. Prostate motion can occur during the delivery of each radiation treatment between the time of localization imaging and completion of treatment. The objective of this work is to review the literature on intrafraction motion (IFM) of the prostate during radiation therapy and offer clinical recommendations on management.
A comprehensive literature review was conducted on prostate motion during prostate cancer radiation therapy. Information was organized around 3 key clinical questions, followed by an evidence-based recommendation.
IFM of the prostate during radiation therapy is typically ≤3 mm and is unlikely to compromise prostate dosimetry to a clinically meaningful degree for men treated in a relatively short treatment duration with planning target volume (PTV) margins of ≥3 to 5 mm. IFM of 5 mm or more has been observed in up to ∼10% of treatment fractions, with limited dosimetric effect related to the infrequency of occurrence and longer fractionation of therapy. IFM can be monitored in continuous or discontinuous fashion with a variety of imaging platforms. Correction of IFM may have the greatest value when tighter PTV margins are desired (such as with stereotactic body radiation therapy or intraprostatic nodule boosting), ultrahypofractionated courses, or when treatment time exceeds several minutes.
This focused review summarizes literature and provides practical recommendations regarding IFM in the treatment of prostate cancer with external beam radiation therapy.
Clinical Experience and Feasibility of Using 2D-kVimage Online Intervention in the Ultrafractionated Stereotactic Radiation Treatment of Prostate Cancer
2023, Practical Radiation Oncology
This study reports clinical experience and feasibility of using a 2-dimensional (2D)-kV image system with online intervention in the ultrafractionated stereotactic body radiation treatment (UF-SBRT) of prostate cancer.
Fifteen patients with prostate cancer who had a low- to intermediate-risk marker implanted received UF-SBRT with online 2D-kV image tracking and a manual beam interruption strategy with a 2-mm motion threshold. A total of 180 kV paired setup images and 1272 intrabeam 2D-kV images were analyzed to evaluate the setup deviation and intratreatment target deviation. Correlation of expected treatment interruptions with a set of parameters (eg, image and treatment time; direction of deviation) was performed (Spearman test). A subset of the data from 22 fractions was re-evaluated to check the differences in analysis results between using the planning position and using the pretreatment setup position as a reference. Margins based on the derived system and random errors were calculated to evaluate the feasibility of the workflow in ensuring prostate coverage during treatment.
Mean target motion in 3D propagated from 1.0 mm (setup at 0 minutes) to 2.0 mm (beam on at 7 minutes) to 2.4 mm (end at 13.5 minutes). Out of 75 fractions, 50 were found to require beam interruption. Interruption had a strong correlation with prostate motion along the longitudinal direction and had moderate correlation with prostate motion along the vertical direction and the prostate's treatment starting position along vertical and longitudinal directions. Using the pretreatment position as a reference for intrabeam monitoring, the magnitude of motion deviation from the reference position was reduced by 0.3 mm at a vertical direction and 0.4 mm at lateral and longitudinal directions. The calculated 3D margin to ensure target coverage was 3.7 mm, 4.6 mm, and 5.0 mm in lateral, vertical, and longitudinal directions, respectively.
Prostate motion propagated over time. It is feasible to use a 2D-kV online intrabeam monitoring system with a proper intervention scheme to perform UF-SBRT for prostate cancer.
Men's preferences for image-guidance in prostate radiation therapy: A discrete choice experiment
2022, Radiotherapy and Oncology
There are several options for real-time prostate monitoring during radiation therapy including fiducial markers (FMs) and transperineal ultrasound (TPUS). However, the patient experience for these procedures is very different. This study aimed to determine patient preferences around various aspects of prostate image-guidance, focusing on FMs and TPUS.
A discrete choice experiment (DCE) was conducted, describing the image-guidance approach by: pain, cost, accuracy, side effects, additional appointments, and additional time. Participants were males with prostate cancer (PCa) and from the general Australian population. A DCE survey required participants to make hypothetical choices in each of 8 choice sets. Multinomial logit modelling and Latent Class Analysis (LCA) were used to analyse the responses. Marginal willingness to pay (mWTP) was calculated.
476 respondents completed the survey (236 PCa patients and 240 general population). The most important attributes for both cohorts were pain, cost and accuracy (p < 0.01). PCa patients were willing to pay more to avoid the worst pain than the general population, and willing to pay more for increased accuracy. LCA revealed 3 groups: 2 were focused more on the process-related attributes of pain and cost, and the third was focused on the clinical efficacy attributes of accuracy and side effects.
Both cohorts preferred less cost and pain and improved accuracy, with men with PCa valuing accuracy more than the general population. In addition to the clinical and technical evidence, radiation oncology centres should consider the preferences of patients when considering choice of image-guidance techniques.
Intrafraction motion monitoring to determine PTV margins in early stage breast cancer patients receiving neoadjuvant partial breast SABR
2021, Radiotherapy and Oncology
Citation 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).
To quantify intra-fraction tumor motion using imageguidance and implanted fiducial markers to determine if a 5 mm planning-target-volume (PTV) margin is sufficient for early stage breast cancer patients receiving neoadjuvant stereotactic ablative radiotherapy (SABR).
A HydroMark© (Mammotome) fiducial was implanted at the time of biopsy adjacent to the tumor. Sixty-one patients with 62 tumours were treated prone using a 5 mm PTV margin. Motion was quantified using two methods (separate patient groups): 1) difference in 3D fiducial position pre- and post-treatment cone-beam CTs (CBCTs) in 18 patients receiving 21 Gy/1fraction (fx); 2) acquiring 2D triggered-kVimages to quantify 3D intra-fraction motion using a 2D-to-3D estimation method for 44 tumours receiving 21 Gy/1fx (n = 22) or 30 Gy/3fx (n = 22). For 2), motion was quantified by calculating the magnitude of intra-fraction positional deviation from the pretreatment CBCT. PTV margins were derived using van Herkian analysis.
The average ± standard deviation magnitude of motion across patients was 1.3 ± 1.15 mm Left/Right (L/R), 1.0 ± 0.9 mm Inferiorly/Superiorly (I/S), and 1.8 ± 1.5 mm Anteriorly/Posteriorly (A/P). 85/105 (81%) treatment fractions had dominant anterior motion. 6/62patients (9.7%) had mean intra-fraction motion during any fraction > 5 mm in any direction, with 4 in the anterior direction. Estimated PTV margins for single and three-fx patients in the L/R, I/S, and A/P directions were 6.0x4.1x5.9 mm and 4.5x2.9x4.3 mm, respectively.
Our results suggest that a 5 mm PTV margin is sufficient for the I/S and A/P directions if a lateral kV image is acquired immediately before treatment. For the L/R direction, either further immobilization or a larger margin is required.
Prospective dual-surrogate validation study of periodic imaging during treatment for accurately monitoring intrafraction motion of prostate cancer patients
2021, Radiotherapy and Oncology
The goal of this prospective study is to validate the use of periodic imaging during treatment with a fiducial marker detection algorithm using radiofrequency transponders for prostate cancer patients undergoing treatment for radiation therapy.
Ten male patients were enrolled in this study and treated for prostate cancer with implanted electromagnetic monitoring beacons. We evaluated the accuracy and limitations of Intrafraction Motion Review (IMR) by comparing the known locations of the beacons using the electromagnetic monitoring system to the position data reported from IMR images.
A total of 4054 images were taken during treatment. The difference in vector magnitude of the two methods is centered around zero (mean: 0.03 cm, SD: 0.16 cm) and Lin’s Concordance Correlation Coefficient (CCC) is 0.99 (95% CI: 0.98, 1) overall. The Euclidean distance between the two methods was close to zero (median: 0.09 cm, IQR: 0.06, 0.14 cm). The difference in distance between any two markers was centered around zero (mean: 0.01 cm, SD: 0.12 cm) and Lin’s CCC is 0.97 (95% CI: 0.96, 0.98) overall.
The accuracy of the algorithm for detected markers within the 2D images is comparable to electromagnetic monitoring for fiducial identification when detected. IMR could provide an alternate solution for patients with contraindications of use of an electromagnetic monitoring system and a cost effective alternative to the acquisition of an additional system for patient monitoring, but does not provide data for pre-treatment set-up verification and real-time 3D positioning during treatment.
Dosimetric impact of intrafraction prostate rotation and accuracy of gating, multi-leaf collimator tracking and couch tracking to manage rotation: An end-to-end validation using volumetric film measurements
2021, Radiotherapy and Oncology
Both gating and tracking can mitigate the deteriorating dosimetric impact of intrafraction translation during prostate stereotactic body radiotherapy (SBRT). However, their ability to manage intrafraction rotation has not yet been thoroughly investigated. The dosimetric accuracy of gating, MLC tracking and couch tracking to manage intrafraction prostate rotation was investigated.
Treatment plans for end-to-end tests of prostate SBRT with focal boosting were generated for a dynamic anthropomorphic pelvis phantom. The phantom applied internal lateral rotation (up to 25°) and coupled vertical and longitudinal translation of a radiochromic film stack that was used for dose measurements. Dose was delivered for each plan while the phantom applied motion according to three typical prostate motion traces without compensation (i), with gating (ii), with MLC tracking (iii) or with couch tracking (iv). Measured doses for the four motion compensation strategies were compared with the planned dose in terms of γ-index analysis, target coverage and organs at risk (OAR) sparing.
Intrafraction rotation reduced the 3%(global)/2mm γ-index passing rate (γPR) for the prostate target volume by median (range) –33.2% (−68.6%, −4.1%) when no motion compensation was applied. The use of motion compensation improved the γPR by 13.2% (−0.4%, 32.9%) for gating, by 6.0% (−0.8%, 27.7%) for MLC tracking and by 11.1% (1.2%, 22.9%) for couch tracking. The three compensation techniques improved the target coverage in most cases. Gating showed better OAR sparing than MLC tracking or couch tracking.
Compensation of intrafraction prostate rotation with gating, MLC tracking and couch tracking was investigated experimentally for the first time. All three techniques improved the dosimetric accuracy, but residual motion-related dose errors remained due to the lack of rotation correction.

View all citing articles on Scopus

: 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.

View full text

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

Purpose

Methods and Materials

Results

Conclusions

Introduction

Section snippets

Overview of KIM

Prostate trajectory types

Patient motion statistics

Discussion

Conclusions

Acknowledgment

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Urology

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Semin Radiat Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

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