Sequential monoscopic image-guided motion compensation in tomotherapy stereotactic body radiotherapy (SBRT) for prostate cancer

PURPOSE

To manage intra-fractional motions, recent developments in tomotherapy enable a unique capability of adjusting MLC/jaw to track the moving target based on the intra-fractional motions detected by sequential monoscopic imaging. In this study, we evaluated the effectiveness of motion compensation with a realistic imaging rate for prostate stereotactic body radiotherapy (SBRT). The obtained results will guide optimizing treatment parameters and image-guided radiation therapy (IGRT) in tomotherapy using this approach.

METHODS

Ten retrospective prostate cases with actual prostate motion curves previously recorded through the Calypso system were used in this study. Based on the recorded peak-to-peak motion, these cases represented either large (> 5 mm) or median (≤ 5 mm) intra-fractional prostate motions. All the cases were re-planned on tomotherapy using 35 Gy/5 fractions SBRT regimen and three different jaw settings of 1 cm static, 2.5 cm static, and 2.5 cm dynamic jaw. Two motion compensation methods were evaluated: a complete compensation that adjusted the jaw and MLC every 0.1 s (the same rate as the Calypso motion trace), and a realistic compensation that adjusted the jaw and MLC at an average imaging interval of 6 s from sequential monoscopic images. An in-house 4D dose calculation software was then applied to calculate the dosimetric outcomes from the original motion-free plan, the motion-contaminated plan, and the two abovementioned motion-compensated plans. During the process, various imaging rates were also simulated in one case with unusually large motions to quantify the impact of the KV-imaging rate on the effectiveness of motion compensation.

RESULTS

The effectiveness of motion compensation was evaluated based on the PTV coverage and OAR sparing. Without any motion-compensation, the PTV coverage (PTV V100%) of patients with large prostate motions decreased remarkably to 55%-82% when planning with the 1 cm jaw but to a less level of 67-94% with the 2.5 cm jaw. In contrast, motion compensation improved the PTV coverage (>92%) when combined with the 2.5 cm jaw, but less effective, around 75%-94%, with the 1 cm jaw. For OAR sparing, the bladder D1cc, bladder D10cc, and rectum D1cc all increased in the motion-contaminated plans. Motion compensation improved OAR sparing to the equivalent level of the original motion-free plans. For patients with median prostate motion, motion-induced degradation in PTV coverage was only observed when planning with the 1 cm jaw. After motion compensation, the PTV coverage improved to better than 94% for all three jaw settings. Additionally, the effectiveness of motion compensation depends on the imaging rate. Motion compensation with a typical rate of two KV images per gantry rotation effectively reduces motion-induced dosimetric uncertainties. However, a higher imaging rate is recommended when planning with a 1 cm jaw for patients with large motions.

CONCLUSION

Our results demonstrated that the performance of sequential monoscopic imaging-guided motion compensation on tomotherapy depends on the amplitude of intra-fractional prostate motion, the plan parameter settings, especially jaw setting, gantry rotation, and the imaging rate for motion compensation. Creating a patient-specific imaging guidance protocol is essential to balance the effectiveness of motion compensation and achievable imaging rate for intra-fractional motion tracking.

Using 4D dose accumulation to calculate organ-at-risk dose deviations from motion-synchronized liver and lung tomotherapy treatments

Tracking systems such as Radixact Synchrony change the planned delivery of radiation during treatment to follow the target. This is typically achieved without considering the location changes of organs at risk (OARs). The goal of this work was to develop a novel 4D dose accumulation framework to quantify OAR dose deviations due to the motion and tracked treatment. The framework obtains deformation information and the target motion pattern from a four-dimensional computed tomography dataset. The helical tomotherapy treatment plan is split into 10 plans and motion correction is applied separately to the jaw pattern and multi-leaf collimator (MLC) sinogram for each phase based on the location of the target in each phase. Deformable image registration (DIR) is calculated from each phase to the references phase using a commercial algorithm, and doses are accumulated according to the DIR. The effect of motion synchronization on OAR dose was analyzed for five lung and five liver subjects by comparing planned versus synchrony-accumulated dose. The motion was compensated by an average of 1.6 cm of jaw sway and by an average of 5.7% of leaf openings modified, indicating that most of the motion compensation was from jaw sway and not MLC changes. OAR dose deviations as large as 19 Gy were observed, and for all 10 cases, dose deviations greater than 7 Gy were observed. Target dose remained relatively constant (D95% within 3 Gy), confirming that motion-synchronization achieved the goal of maintaining target dose. Dose deviations provided by the framework can be leveraged during the treatment planning process by identifying cases where OAR doses may change significantly from their planned values with respect to the critical constraints. The framework is specific to synchronized helical tomotherapy treatments, but the OAR dose deviations apply to any real-time tracking technique that does not consider location changes of OARs.

Tracking target/chest relationship changes during motion‐synchronized tomotherapy treatments

Background

Radixact Synchrony® is an intrafraction motion tracking system for helical tomotherapy treatments that uses kV radiographs of the target and LEDs on the patient's chest to synchronize the movement of the radiation beam with the respiratory motion of the target. Several works have demonstrated Synchrony's ability to track target motion when the chest and target motions are perfectly correlated.

Purpose

The purpose of this work was to determine Synchrony's ability to accurately adapt to scenarios with a changing target/chest correlation.

Methods

A custom ion chamber mimicking plug with embedded fiducials was placed inside a Delta4 Phantom+ and used as the tracking object. A separate motion stage was programmed to mimic chest motion. The target and chest surrogate phantom were programmed to move sinusoidally and two types of target/chest relationship changes were introduced: rigid shifts and linear drifts of the target position but not surrogate position. Tracking analysis was performed by comparing programmed phantom motion to log files of the Synchrony‐modeled motion. No dosimetry was performed in this work.

Results

At the fastest imaging rate of 2 s/img, Synchrony accurately adapted for gradual drifts in the target location (up to 5 mm/min) with minor increases in tracking errors and adapted for an abrupt 5 mm shift after about 30 s (with an auto‐pause threshold at 60 s). When the imaging period was longer (> 4 s/img), larger tracking errors (> 5 mm) were observed, and the treatment would be paused. The measured delta (MD) parameter (2D target localization error on the most recent image) was found to be a more responsive indicator of tracking errors than the potential difference (PD) parameter (3D estimator of tracking error based on all images in the model). Lastly, the effect of a recent update to the tracking algorithm was found to improve the ability of Synchrony to track target/chest relationship changes.

Conclusions

This work demonstrated that Synchrony can adapt to gradual changes (drifts) in the target/chest relationship, but it takes a finite amount of time to adapt to abrupt shifts. Ability to adapt to these changes increases with increasing imaging frequency. Larger tracking errors were observed in this work than others have reported in the literature due to the introduction of target/chest correlation changes in this work. Future work needs to be performed investigating what type and magnitude of target/chest miscorrelations occur in patients. Lastly, users should ensure they are using the most recent software (3.0.1 or newer) to improve the ability of Synchrony to track these movements.

Incorporation of tumor motion directionality in margin recipe: The directional MidP strategy

PURPOSE

Planning target volume (PTV) definition based on Mid-Position (Mid-P) strategy typically integrates breathing motion from tumor positions variances along the conventional axes of the DICOM coordinate system. Tumor motion directionality is thus neglected even though it is one of its stable characteristics in time. We therefore propose the directional MidP approach (MidP dir), which allows motion directionality to be incorporated into PTV margins. A second objective consists in assessing the ability of the proposed method to better take care of respiratory motion uncertainty.

METHODS

11 lung tumors from 10 patients with supra-centimetric motion were included. PTV were generated according to the MidP and MidP dir strategies starting from planning 4D CT.

RESULTS

PTV_{MidP dir} volume didn't differ from the PTV_MidP volume: 31351 mm³ IC95% [17242-45459] vs. 31003 mm³ IC95% [ 17347-44659], p = 0.477 respectively. PTV_{MidP dir} morphology was different and appeared more oblong along the main motion axis. The relative difference between 3D and 4D doses was on average 1.09%, p = 0.011 and 0.74%, p = 0.032 improved with directional MidP for D_99% and D_95%. D_2% was not significantly different between both approaches. The improvement in dosimetric coverage fluctuated substantially from one lesion to another and was all the more important as motion showed a large amplitude, some obliquity with respect to conventional axes and small hysteresis.

CONCLUSIONS

Directional MidP method allows tumor motion to be taken into account more tightly as a geometrical uncertainty without increasing the irradiation volume.

Evaluation of radixact motion synchrony for 3D respiratory motion: Modeling accuracy and dosimetric fidelity

The Radixact® linear accelerator contains the motion Synchrony system, which tracks and compensates for intrafraction patient motion. For respiratory motion, the system models the motion of the target and synchronizes the delivery of radiation with this motion using the jaws and multi-leaf collimators (MLCs). It was the purpose of this work to determine the ability of the Synchrony system to track and compensate for different phantom motions using a delivery quality assurance (DQA) workflow. Thirteen helical plans were created on static datasets from liver, lung, and pancreas subjects. Dose distributions were measured using a Delta4® Phantom+ mounted on a Hexamotion® stage for the following three case scenarios for each plan: (a) no phantom motion and no Synchrony (M0S0), (b) phantom motion and no Synchrony (M1S0), and (c) phantom motion with Synchrony (M1S1). The LEDs were placed on the Phantom+ for the 13 patient cases and were placed on a separate one-dimensional surrogate stage for additional studies to investigate the effect of separate target and surrogate motion. The root-mean-square (RMS) error between the Synchrony-modeled positions and the programmed phantom positions was <1.5 mm for all Synchrony deliveries with the LEDs on the Phantom+. The tracking errors increased slightly when the LEDs were placed on the surrogate stage but were similar to tracking errors observed for other motion tracking systems such as CyberKnife Synchrony. One-dimensional profiles indicate the effects of motion interplay and dose blurring present in several of the M1S0 plans that are not present in the M1S1 plans. All 13 of the M1S1 measured doses had gamma pass rates (3%/2 mm/10%T) compared to the planned dose > 90%. Only two of the M1S0 measured doses had gamma pass rates > 90%. Motion Synchrony offers a potential alternative to the current, ITV-based motion management strategy for helical tomotherapy deliveries.

An evaluation of the mid-ventilation method for the planning of stereotactic lung plans

BACKGROUND AND PURPOSE

Stereotactic ablative body radiotherapy for lung plans requires 4DCT. Most radiotherapy centres use this to determine an internal target volume (ITV), despite studies suggesting that planning on a mid-ventilation (Mid-V) phase can reduce target volumes. The purpose of this study is two-fold: to determine whether the Mid-V approach provides adequate coverage and to discuss methods to enable the Mid-V approach to be applied more widely.

METHOD

4D scans of 79 patients were outlined on every phase. The mid-V phase was identified. Margins were determined from the range of motion, and plans generated with a 55 Gy prescription. A grid-based method was used to get the probability of tumour coverage in the presence of systematic and random uncertainties, with and without blurring for breathing motion.

RESULTS

For the Mid-V plans with the margins calculated from the van-Herk formula, after blurring doses for breathing, the coverage (dose covering 95% of the CTV 95% of the time) was greater than for plans with isotropic 5 mm margins uncorrected for breathing (58.2 Gy v 57.3 Gy). Similar results were obtained for a linear margin chosen as 0.15 of the breathing range. Deformable contour propagation in a commercial outlining system (ProSoma) identified the same mid-V phase in the majority of cases.

CONCLUSION

Our results confirm that a mid-V approach can be used to reduce the PTV size, with no loss of tumour coverage. We propose the use of a simplified margin formula equal to the margin ignoring breathing plus 0.15 of the range of motion.

Evaluation of TomoTherapy dose calculations with intrafractional motion and motion compensation

PURPOSE

Anatomical motion, both cyclical and aperiodic, can impact the dose delivered during external beam radiation. In this work, we evaluate the use of a research version of the clinical TomoTherapy^® dose calculator to calculate dose with intrafraction rigid motion. We also evaluate the feasibility of a method of motion compensation for helical tomotherapy using the jaws and MLC.

METHODS

Treatment plans were created using the TomoTherapy treatment planning system. Dose was recalculated for several simple rigid motion traces including a 4 mm step motion applied either longitudinally or transversely, and a sinusoidal motion. The calculated dose volumes were compared to dose measurements that were performed by translating the phantom with the same motion traces used in the calculations. Measurements were made using film and ion chambers. Finally, the delivery plans were modified to compensate for the motion by sweeping the jaws for longitudinal motion and shifting the MLC leaves for transverse motion, and the calculations and measurements were repeated.

RESULTS

A transverse step motion shifted the dose that was delivered after the step occurred, but otherwise did not impact the dose distribution. Film measurements agreed with dose calculations to within 2%/2 mm for 99% of dose points within the 50% isodose line. A shift in the MLC leaf delivery pattern successfully compensated for the step motion to within the 3 mm accuracy allowed by the finite leaf widths. A longitudinal step motion impacted the dose in the interior of the target volume to a degree that was dependent on the planning field width and step size. Film measurements agreed with dose calculations to within 2%/2 mm for 98% of dose points within the 50% isodose line. Shifts in the jaw position successfully compensated for the longitudinal step motion. Sinusoidal (breathing-like) motion was also studied, with similar results.

CONCLUSIONS

A research version of the clinical TomoTherapy dose calculator has been shown to accurately calculate the dose from treatment plans delivered in the presence of arbitrary rigid motion. Modifications to the delivery plan using jaw and MLC leaf shifts that follow the motion can successfully compensate for the target motion.

Evaluation of the radiobiological gamma index with motion interplay in tangential IMRT breast treatment

The purpose of this study was to evaluate the impact of the motion interplay effect in early-stage left-sided breast cancer intensity-modulated radiation therapy (IMRT), incorporating the radiobiological gamma index (RGI). The IMRT dosimetry for various breathing amplitudes and cycles was investigated in 10 patients. The predicted dose was calculated using the convolution of segmented measured doses. The physical gamma index (PGI) of the planning target volume (PTV) and the organs at risk (OAR) was calculated by comparing the original with the predicted dose distributions. The RGI was calculated from the PGI using the tumor control probability (TCP) and the normal tissue complication probability (NTCP). The predicted mean dose and the generalized equivalent uniform dose (gEUD) to the target with various breathing amplitudes were lower than the original dose (P < 0.01). The predicted mean dose and gEUD to the OARs with motion were higher than for the original dose to the OARs (P < 0.01). However, the predicted data did not differ significantly between the various breathing cycles for either the PTV or the OARs. The mean RGI gamma passing rate for the PTV was higher than that for the PGI (P < 0.01), and for OARs, the RGI values were higher than those for the PGI (P < 0.01). The gamma passing rates of the RGI for the target and the OARs other than the contralateral lung differed significantly from those of the PGI under organ motion. Provided an NTCP value <0.05 is considered acceptable, it may be possible, by taking breathing motion into consideration, to escalate the dose to achieve the PTV coverage without compromising the TCP.