How much margin reduction is possible through gating or breath hold?

Biomedical advances and future prospects of high-precision three-dimensional radiotherapy and four-dimensional radiotherapy

Biomedical advances of external-beam radiotherapy (EBRT) with improvements in physical accuracy are reviewed. High-precision (±1 mm) three-dimensional radiotherapy (3DRT) can utilize respective therapeutic open doors in the tumor control probability curve and in the normal tissue complication probability curve instead of the one single therapeutic window in two-dimensional EBRT. High-precision 3DRT achieved higher tumor control and probable survival rates for patients with small peripheral lung and liver cancers. Four-dimensional radiotherapy (4DRT), which can reduce uncertainties in 3DRT due to organ motion by real-time (every 0.1-1 s) tumor-tracking and immediate (0.1-1 s) irradiation, have achieved reduced adverse effects for prostate and pancreatic tumors near the digestive tract and with similar or better tumor control. Particle beam therapy improved tumor control and probable survival for patients with large liver tumors. The clinical outcomes of locally advanced or multiple tumors located near serial-type organs can theoretically be improved further by integrating the 4DRT concept with particle beams.

Intrafraction motion during radiotherapy of breast tumor, breast tumor bed, and individual axillary lymph nodes on cine magnetic resonance imaging

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Intrafraction motion of the breast and individual axillary lymph nodes was studied.

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Displacements were investigated using cine magnetic resonance imaging.

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Motion was separated into breathing and drift components.

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Medians of the maximum displacements were small, <3 mm for breast and lymph nodes.

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Intrafraction motion of the tumor (bed) was less in prone than in supine position.

Background and purpose

In (ultra-)hypofractionation, the contribution of intrafraction motion to treatment accuracy becomes increasingly important. Our purpose was to evaluate intrafraction motion and resulting geometric uncertainties for breast tumor (bed) and individual axillary lymph nodes, and to compare prone and supine position for the breast tumor (bed).

Materials and methods

During 1–3 min of free breathing, we acquired transverse/sagittal interleaved 1.5 T cine magnetic resonance imaging (MRI) of the breast tumor (bed) in prone and supine position and coronal/sagittal cine MRI of individual axillary lymph nodes in supine position. A total of 31 prone and 23 supine breast cine MRI (in 23 women) and 52 lymph node cine MRI (in 24 women) were included. Maximum displacement, breathing amplitude, and drift were analyzed using deformable image registration. Geometric uncertainties were calculated for all displacements and for breathing motion only.

Results

Median maximum displacements (range over the three orthogonal orientations) were 1.1–1.5 mm for the breast tumor (bed) in prone and 1.8–3.0 mm in supine position, and 2.2–2.4 mm for lymph nodes. Maximum displacements were significantly smaller in prone than in supine position, mainly due to smaller breathing amplitude: 0.6–0.9 mm in prone vs. 0.9–1.4 mm in supine. Systematic and random uncertainties were 0.1–0.4 mm in prone position and 0.2–0.8 mm in supine position for the tumor (bed), and 0.4–0.6 mm for the lymph nodes.

Conclusion

Intrafraction motion of breast tumor (bed) and individual lymph nodes was small. Motion of the tumor (bed) was smaller in prone than in supine position.

Audiovisual biofeedback guided breath-hold improves lung tumor position reproducibility and volume consistency

Purpose

Respiratory variation can increase the variability of tumor position and volume, accounting for larger treatment margins and longer treatment times. Audiovisual biofeedback as a breath-hold technique could be used to improve the reproducibility of lung tumor positions at inhalation and exhalation for the radiation therapy of mobile lung tumors. This study aimed to assess the impact of audiovisual biofeedback breath-hold (AVBH) on interfraction lung tumor position reproducibility and volume consistency for respiratory-gated lung cancer radiation therapy.

Methods

Lung tumor position and volume were investigated in 9 patients with lung cancer who underwent a breath-hold training session with AVBH before 2 magnetic resonance imaging (MRI) sessions. During the first MRI session (before treatment), inhalation and exhalation breath-hold 3-dimensional MRI scans with conventional breath-hold (CBH) using audio instructions alone and AVBH were acquired. The second MRI session (midtreatment) was repeated within 6 weeks after the first session. Gross tumor volumes (GTVs) were contoured on each dataset. CBH and AVBH were compared in terms of tumor position reproducibility as assessed by GTV centroid position and position range (defined as the distance of GTV centroid position between inhalation and exhalation) and tumor volume consistency as assessed by GTV between inhalation and exhalation.

Results

Compared with CBH, AVBH improved the reproducibility of interfraction GTV centroid position by 46% (P = .009) from 8.8 mm to 4.8 mm and GTV position range by 69% (P = .052) from 7.4 mm to 2.3 mm. Compared with CBH, AVBH also improved the consistency of intrafraction GTVs by 70% (P = .023) from 7.8 cm³ to 2.5 cm³.

Conclusions

This study demonstrated that audiovisual biofeedback can be used to improve the reproducibility and consistency of breath-hold lung tumor position and volume, respectively. These results may provide a pathway to achieve more accurate lung cancer radiation treatment in addition to improving various medical imaging and treatments by using breath-hold procedures.

Margin selection to compensate for loss of target dose coverage due to target motion during external-beam radiation therapy of the lung

The aim of this study is to provide guidelines for the selection of external‐beam radiation therapy target margins to compensate for target motion in the lung during treatment planning. A convolution model was employed to predict the effect of target motion on the delivered dose distribution. The accuracy of the model was confirmed with radiochromic film measurements in both static and dynamic phantom modes. 502 unique patient breathing traces were recorded and used to simulate the effect of target motion on a dose distribution. A 1D probability density function (PDF) representing the position of the target throughout the breathing cycle was generated from each breathing trace obtained during 4D CT. Changes in the target D95 (the minimum dose received by 95% of the treatment target) due to target motion were analyzed and shown to correlate with the standard deviation of the PDF. Furthermore, the amount of target D95 recovered per millimeter of increased field width was also shown to correlate with the standard deviation of the PDF. The sensitivity of changes in dose coverage with respect to target size was also determined. Margin selection recommendations that can be used to compensate for loss of target D95 were generated based on the simulation results. These results are discussed in the context of clinical plans. We conclude that, for PDF standard deviations less than 0.4 cm with target sizes greater than 5 cm, little or no additional margins are required. Targets which are smaller than 5 cm with PDF standard deviations larger than 0.4 cm are most susceptible to loss of coverage. The largest additional required margin in this study was determined to be 8 mm.

PACS numbers: 87.53.Bn, 87.53.Kn, 87.55.D‐, 87.55.Gh

Suitability of markerless EPID tracking for tumor position verification in gated radiotherapy

PURPOSE

To maximize the benefits of respiratory gated radiotherapy (RGRT) of lung tumors real-time verification of the tumor position is required. This work investigates the feasibility of markerless tracking of lung tumors during beam-on time in electronic portal imaging device (EPID) images of the MV therapeutic beam.

METHODS

EPID movies were acquired at ∼2 fps for seven lung cancer patients with tumor peak-to-peak motion ranges between 7.8 and 17.9 mm (mean: 13.7 mm) undergoing stereotactic body radiotherapy. The external breathing motion of the abdomen was synchronously measured. Both datasets were retrospectively analyzed in PortalTrack, an in-house developed tracking software. The authors define a three-step procedure to run the simulations: (1) gating window definition, (2) gated-beam delivery simulation, and (3) tumor tracking. First, an amplitude threshold level was set on the external signal, defining the onset of beam-on/-off signals. This information was then mapped onto a sequence of EPID images to generate stamps of beam-on/-hold periods throughout the EPID movies in PortalTrack, by obscuring the frames corresponding to beam-off times. Last, tumor motion in the superior-inferior direction was determined on portal images by the tracking algorithm during beam-on time. The residual motion inside the gating window as well as target coverage (TC) and the marginal target displacement (MTD) were used as measures to quantify tumor position variability.

RESULTS

Tumor position monitoring and estimation from beam's-eye-view images during RGRT was possible in 67% of the analyzed beams. For a reference gating window of 5 mm, deviations ranging from 2% to 86% (35% on average) were recorded between the reference and measured residual motion. TC (range: 62%-93%; mean: 77%) losses were correlated with false positives incidence rates resulting mostly from intra-/inter-beam baseline drifts, as well as sudden cycle-to-cycle fluctuations in exhale positions. Both phenomena can lead to considerable deviations (with MTD values up to a maximum of 7.8 mm) from the intended tumor position, and in turn may result in a marginal miss. The difference between tumor traces determined within the gating window against ground truth trajectory maps was 1.1 ± 0.7 mm on average (range: 0.4-2.3 mm).

CONCLUSIONS

In this retrospective analysis of motion data, it is demonstrated that the system is capable of determining tumor positions in the plane perpendicular to the beam direction without the aid of fiducial markers, and may hence be suitable as an online verification tool in RGRT. It may be possible to use the tracking information to enable on-the-fly corrections to intra-/inter-beam variations by adapting the gating window by means of a robotic couch.

Retrospective evaluation of CTV to PTV margins using CyberKnife in patients with thoracic tumors

The objectives of this study were to estimate global uncertainty for patients with thoracic tumors treated in our center using the CyberKnife VSI after placement of fiducial markers and to compare our findings with the standard CTV to PTV margins used to date. Datasets for 16 patients (54 fractions) treated with the CyberKnife and the Synchrony Respiratory Tracking System were analyzed retrospectively based on CT planning, tracking information, and movement data generated and saved in the logs files by the system. For each patient, we analyzed all the main uncertainty sources and assigned a value. We also calculated an expanded global uncertainty to ensure a robust estimation of global uncertainty and to enable us to determine the position of 95% of the CTV points with a 95% confidence level during treatment. Based on our estimation of global uncertainty and compared with our general margin criterion (5 mm in all three directions: superior/inferior [SI], anterior/posterior [AP], and lateral [LAT]), 100% were adequately covered in the LAT direction, as were 94% and 94% in the SI and AP directions. We retrospectively analyzed the main sources of uncertainty in the CyberKnife process patient by patient. This individualized approach enabled us to estimate margins for patients with thoracic tumors treated in our unit and compare the results with our standard 5 mm margin.

PACS number: 87.55‐x

Four-dimensional CT-based evaluation of volumetric modulated arc therapy for abdominal lymph node metastasis from hepatocellular carcinoma

This study aimed to identify the potential benefits and limitations of a new volumetric modulated arc therapy (VMAT) planning system in Monaco, compared with conventional intensity-modulated radiotherapy (IMRT) and three-dimensional conformal radiotherapy (3DCRT). Four-dimensional CT scans of 13 patients with abdominal lymph node metastasis from hepatocellular carcinoma were selected. Internal target volume was defined as the combined volume of clinical target volumes (CTVs) in the multiple 4DCT phases. Dose prescription was set to 45 Gy for the planning target volume (PTV) in daily 3.0-Gy fractions. The PTV dose coverage, organs at risk (OAR) doses, delivery parameters and treatment accuracy were assessed. Compared with 3DCRT, both VMAT and IMRT provided a systematic improvement in PTV coverage and homogeneity. Planning objectives were not fulfilled for the right kidney, in which the 3DCRT plans exceeded the dose constraints in two patients. Equivalent target coverage and sparing of OARs were achieved with VMAT compared with IMRT. The number of MU/fraction was 462 ± 68 (3DCRT), 564 ± 105 (IMRT) and 601 ± 134 (VMAT), respectively. Effective treatment times were as follows: 1.8 ± 0.2 min (3DCRT), 6.1 ± 1.5 min (IMRT) and 4.8 ± 1.0 min (VMAT). This study suggests that the VMAT plans generated in Monaco improved delivery efficiency for equivalent dosimetric quality to IMRT, and were superior to 3DCRT in target coverage and sparing of most OARs. However, the superiority of VMAT over IMRT in delivery efficiency is limited.

[Advances of precise radiotherapy for lung cancer]

目前肺部肿瘤的放射治疗已进入精确放疗时代。实施精确放疗的具体方法主要包括：调强放疗（intensity modulated radiotherapy, IMRT）、图像引导放射治疗（image-guided radiotherapy, IGRT）和体部立体定向放射治疗（stereotactic body radiotherapy, SBRT）。在实施精确放疗过程中，对于以下问题：患者体位固定、肺部肿瘤运动的控制、影像技术的使用、PTV边界、剂量的处方和报道、射野的安排、剂量体积的控制和治疗的实施等，应给予充分的考虑和注意，以确保精确放疗能够精确执行。

A 4D IMRT planning method using deformable image registration to improve normal tissue sparing with contemporary delivery techniques

We propose a planning method to design true 4-dimensional (4D) intensity-modulated radiotherapy (IMRT) plans, called the t4Dplan method, in which the planning target volume (PTV) of the individual phases of the 4D computed tomography (CT) and the conventional PTV receive non-uniform doses but the cumulative dose to the PTV of each phase, computed using deformable image registration (DIR), are uniform. The non-uniform dose prescription for the conventional PTV was obtained by solving linear equations that required motion-convolved 4D dose to be uniform to the PTV for the end-exhalation phase (PTV50) and by constraining maximum inhomogeneity to 20%. A plug-in code to the treatment planning system was developed to perform the IMRT optimization based on this non-uniform PTV dose prescription. The 4D dose was obtained by summing the mapped doses from individual phases of the 4D CT using DIR. This 4D dose distribution was compared with that of the internal target volume (ITV) method. The robustness of the 4D plans over the course of radiotherapy was evaluated by computing the 4D dose distributions on repeat 4D CT datasets. Three patients with lung tumors were selected to demonstrate the advantages of the t4Dplan method compared with the commonly used ITV method. The 4D dose distribution using the t4Dplan method resulted in greater normal tissue sparing (such as lung, stomach, liver and heart) than did plans designed using the ITV method. The dose volume histograms of cumulative 4D doses to the PTV50, clinical target volume, lung, spinal cord, liver, and heart on the 4D repeat CTs for the two patients were similar to those for the 4D dose at the time of original planning.

Evaluation of a vessel-tracking-based technique for dynamic targeting in human liver

The purpose of this study was to evaluate a novel vessel-tracking-based technique for tracking of human liver. The novelty of the proposed technique is that it measures the translation and deformation of a local tissue region based on the displacements of a set of vessels of interest instead of the entire organ. The position of the target point was estimated from the relative positions of the center-of-masses of the vessels, assuming that the topological relationship between the target point and center-of-masses is unchanged during breathing. To reduce inaccuracy due to the delay between vessel image acquisition and sonication, the near-future target position was predicted based on the vessel displacements in the images extracted from an image library acquired before the tracking stage. Experiments on healthy volunteers demonstrated that regardless of the respiratory condition, appropriate combinations of three center-of-masses from the vessels situated around the target-tissue position yielded an estimation error of less than 2 mm, which was significantly smaller than that obtained when using a single center-of-mass trio. The effect of the tracking delay was successfully compensated, with a prediction error of less than 3 mm, by using over four images selected from the image library.

Lung tumor reproducibility with active breath control (ABC) in image-guided radiotherapy based on cone-beam computed tomography with two registration methods

PURPOSE

To study the inter- and intrafraction tumor reproducibility with active breath control (ABC) utilizing cone-beam computed tomography (CBCT), and compare validity of registration with two different regions of interest (ROI).

METHODS AND MATERIALS

Thirty-one lung tumors in 19 patients received conventional or stereotactic body radiotherapy with ABC. During each treatment, patients had three CBCT scanned before and after online position correction and after treatment. These CBCT images were aligned to the planning CT using the gray scale registration of tumor and bony registration of the thorax, and tumor position uncertainties were then determined.

RESULTS

The interfraction systematic and random translation errors in the left-right (LR), superior-inferior (SI) and anterior-posterior (AP) directions were 3.6, 4.8, and 2.9mm; 2.5, 4.5, and 3.5mm, respectively, with gray scale alignment; 1.9, 4.3, 2.0mm and 2.5, 4.4, 2.9mm, respectively, with bony alignment. The interfraction systematic and random rotation errors with gray scale and bony alignment groups ranged from 1.4° to 3.0° and 0.8° to 2.3°, respectively. The intrafraction systematic and random errors with gray scale registration in LR, SI, AP directions were 0.9, 2.0, 1.8mm and 1.5, 1.7, 2.9mm, respectively, for translation; 1.5°, 0.9°, 1.0° and 1.2°, 2.2°, 1.8°, respectively, for rotation. The translational errors in SI direction with bony alignment were significantly larger than that of gray scale (p<0.05).

CONCLUSIONS

With CBCT guided online correction the interfraction positioning errors can be markedly reduced. The intrafraction errors were not diminished by the use of ABC. Rotation errors were not very remarkable both inter- and intrafraction. Gray scale alignment of tumor may provide a better registration in SI direction.

A novel respiratory motion compensation strategy combining gated beam delivery and mean target position concept --a compromise between small safety margins and long duty cycles

PURPOSE

To evaluate a novel respiratory motion compensation strategy combining gated beam delivery with the mean target position (MTP) concept for pulmonary stereotactic body radiotherapy (SBRT).

MATERIALS AND METHODS

Four motion compensation strategies were compared for 10 targets with motion amplitudes between 6mm and 31mm: the internal target volume concept (plan(ITV)); the MTP concept where safety margins were adapted based on 4D dose accumulation (plan(MTP)); gated beam delivery without margins for motion compensation (plan(gated)); a novel approach combining gating and the MTP concept (plan(gated&MTP)).

RESULTS

For 5/10 targets with an average motion amplitude of 9mm, the differences in the mean lung dose (MLD) between plan(gated) and plan(MTP) were <10%. For the other 5/10 targets with an average motion amplitude of 19mm, gating with duty cycles between 87.5% and 75% reduced the residual target motion to 12mm on average and 2mm safety margins were sufficient for dosimetric compensation of this residual motion in plan(gated&MTP). Despite significantly shorter duty cycles, plan(gated) reduced the MLD by <10% compared to plan(gated&MTP). The MLD was increased by 18% in plan(MTP) compared to that of plan(gated&MTP).

CONCLUSIONS

For pulmonary targets with motion amplitudes >10-15mm, the combination of gating and the MTP concept allowed small safety margins with simultaneous long duty cycles.

Optimizing principal component models for representing interfraction variation in lung cancer radiotherapy

PURPOSE

To optimize modeling of interfractional anatomical variation during active breath-hold radiotherapy in lung cancer using principal component analysis (PCA).

METHODS

In 12 patients analyzed, weekly CT sessions consisting of three repeat intrafraction scans were acquired with active breathing control at the end of normal inspiration. The gross tumor volume (GTV) and lungs were delineated and reviewed on the first week image by physicians and propagated to all other images using deformable image registration. PCA was used to model the target and lung variability during treatment. Four PCA models were generated for each specific patient: (1) Individual models for the GTV and each lung from one image per week (week to week, W2W); (2) a W2W composite model of all structures; (3) individual models using all images (weekly plus repeat intrafraction images, allscans); and (4) composite model with all images. Models were reconstructed retrospectively (using all available images acquired) and prospectively (using only data acquired up to a time point during treatment). Dominant modes representing at least 95% of the total variability were used to reconstruct the observed anatomy. Residual reconstruction error between the model-reconstructed and observed anatomy was calculated to compare the accuracy of the models.

RESULTS

An average of 3.4 and 4.9 modes was required for the allscans models, for the GTV and composite models, respectively. The W2W model required one less mode in 40% of the patients. For the retrospective composite W2W model, the average reconstruction error was 0.7 +/- 0.2 mm, which increased to 1.1 +/- 0.5 mm when the allscans model was used. Individual and composite models did not have significantly different errors (p = 0.15, paired t-test). The average reconstruction error for the prospective models of the GTV stabilized after four measurements at 1.2 +/- 0.5 mm and for the composite model after five measurements at 0.8 +/- 0.4 mm.

CONCLUSIONS

Retrospective PCA models were capable of reconstructing original GTV and lung shapes and positions within several millimeters with three to four dominant modes, on average. Prospective models achieved similar accuracy after four to five measurements.

Optimal margin and edge-enhanced intensity maps in the presence of motion and uncertainty

In radiation therapy, intensity maps involving margins have long been used to counteract the effects of dose blurring arising from motion. More recently, intensity maps with increased intensity near the edge of the tumour (edge enhancements) have been studied to evaluate their ability to offset similar effects that affect tumour coverage. In this paper, we present a mathematical methodology to derive margin and edge-enhanced intensity maps that aim to provide tumour coverage while delivering minimum total dose. We show that if the tumour is at most about twice as large as the standard deviation of the blurring distribution, the optimal intensity map is a pure scaling increase of the static intensity map without any margins or edge enhancements. Otherwise, if the tumour size is roughly twice (or more) the standard deviation of motion, then margins and edge enhancements are preferred, and we present formulae to calculate the exact dimensions of these intensity maps. Furthermore, we extend our analysis to include scenarios where the parameters of the motion distribution are not known with certainty, but rather can take any value in some range. In these cases, we derive a similar threshold to determine the structure of an optimal margin intensity map.

Time delays in gated radiotherapy

In gated radiotherapy, the accuracy of treatment delivery is determined by the accuracy with which both the imaging and treatment beams are gated. Time delays are of four types: (1) beam on imaging time delay is the time between the target entering the gated region and the first gated image acquisition; (2) beam off imaging time delay is the time between the target exiting a gated region and the last image acquisition; (3) beam on treatment time delay is the time between the target entering the gated region and the treatment beam on; and (4) beam off treatment time delay is the time between the target exiting the gated region and treatment beam off. Asynchronous time delays for the imaging and treatment systems may increase the required internal target volume (ITV) margin. We measured time delay on three fluoroscopy systems, and three linear accelerator treatment beams, varying gating type (amplitude vs. phase), beam energy, dose rate, and period. The average beam on imaging time delays were −0.04±0.05sec (amplitude, 1 SD), −0.11±0.04sec (phase); while the average beam off imaging time delays were −0.18±0.08sec (amplitude) and −0.15±0.04sec (phase). The average beam on treatment time delays were +0.09±0.02sec (amplitude, 1 SD), +0.10±0.03sec (phase); while the average beam off time delays for treatment beams were +0.08±0.02sec (amplitude) and +0.07±0.02sec (phase). The negative value indicates the images were acquired early, and the positive values show the treatment beam was triggered late. We present a technique for calculating the margin necessary to account for time delays. We found that the difference between these imaging and treatment time delays required a significant increase in the ITV margin in the direction of tumor motion at the gated level.

PACS number: 87.53.Dq

Three-dimensional spatial modelling of the correlation between abdominal motion and lung tumour motion with breathing

The aim of this research was to investigate whether a spatial correlation could be found between an external 3-D respiratory signal and the tumour trajectory. The respiratory signal was obtained by tracking the abdominal movement and the tumour trajectory was obtained by automatically determining the tumour position in a series of portal images. Three different models, based on Systems Identification, are presented to model the correlation using a 1-D respiratory signal, a 3-D respiratory signal and a 3-D respiratory signal together with previously determined tumour positions. Adequate correlation was found for all models in the direction of the tumour movement with standard deviations of 0.89 mm, 0.72 mm and 0.75 mm, respectively, and model fit of Rt2 = 0.19, 0.63 and 0.82, respectively. Increasing the frame rate for the acquisition of portal images from 3 to 15 frames per second improved the standard deviation and model fit. In summary, it is possible to spatially correlate a 3-D respiratory signal with the tumour trajectory using this approach. The models presented provide a framework that can be extended to include more information if required. A 3-D respiratory signal is preferable to a 1-D signal in modelling the tumour motion that is not along the main axis of tumour movement.

Mid-ventilation concept for mobile pulmonary tumors: internal tumor trajectory versus selective reconstruction of four-dimensional computed tomography frames based on external breathing motion

PURPOSE

To evaluate the accuracy of direct reconstruction of mid-ventilation and peak-phase four-dimensional (4D) computed tomography (CT) frames based on the external breathing signal.

METHODS AND MATERIALS

For 11 patients with 15 pulmonary targets, a respiration-correlated CT study (4D CT) was acquired for treatment planning. After retrospective time-based sorting of raw projection data and reconstruction of eight CT frames equally distributed over the breathing cycle, mean tumor position (P(mean)), mid-ventilation frame, and breathing motion were evaluated based on the internal tumor trajectory. Analysis of the external breathing signal (pressure sensor around abdomen) with amplitude-based sorting of projections was performed for direct reconstruction of the mid-ventilation frame and frames at peak phases of the breathing cycle.

RESULTS

On the basis of the eight 4D CT frames equally spaced in time, tumor motion was largest in the craniocaudal direction, with 12 +/- 7 mm on average. Tumor motion between the two frames reconstructed at peak phases was not different in the craniocaudal and anterior-posterior directions but was systematically smaller in the left-right direction by 1 mm on average. The 3-dimensional distance between P(mean) and the tumor position in the mid-ventilation frame based on the internal tumor trajectory was 1.2 +/- 1 mm. Reconstruction of the mid-ventilation frame at the mean amplitude position of the external breathing signal resulted in tumor positions 2.0 +/- 1.1 mm distant from P(mean). Breathing-induced motion artifacts in mid-ventilation frames caused negligible changes in tumor volume and shape.

CONCLUSIONS

Direct reconstruction of the mid-ventilation frame and frames at peak phases based on the external breathing signal was reliable. This makes the reconstruction of only three 4D CT frames sufficient for application of the mid-ventilation technique in clinical practice.

Cumulative lung dose for several motion management strategies as a function of pretreatment patient parameters

PURPOSE

To evaluate patient parameters that may predict for relative differences in cumulative four-dimensional (4D) lung dose among several motion management strategies.

METHODS AND MATERIALS

Deformable image registration and dose accumulation were used to generate 4D treatment plans for 18 patients with 4D computed tomography scans. Three plans were generated to simulate breath hold at normal inspiration, target tracking with the beam aperture, and mid-ventilation aperture (control of the target at the mean daily position and application of an iteratively computed margin to compensate for respiration). The relative reduction in mean lung dose (MLD) between breath hold and mid-ventilation aperture (DeltaMLD(BH)) and between target tracking and mid-ventilation aperture (DeltaMLD(TT)) was calculated. Associations between these two variables and parameters of the lesion (excursion, size, location, and deformation) and dose distribution (local dose gradient near the target) were also calculated.

RESULTS

The largest absolute and percentage differences in MLD were 1.0 Gy and 21.5% between breath hold and mid-ventilation aperture. DeltaMLD(BH) was significantly associated (p < 0.05) with tumor excursion. The DeltaMLD(TT) was significantly associated with excursion, deformation, and local dose gradient. A linear model was constructed to represent DeltaMLD vs. excursion. For each 5 mm of excursion, target tracking reduced the MLD by 4% compared with the results of a mid-ventilation aperture plan. For breath hold, the reduction was 5% per 5 mm of excursion.

CONCLUSIONS

The relative difference in MLD among different motion management strategies varied with patient and tumor characteristics for a given dosimetric target coverage. Tumor excursion is useful to aid in stratifying patients according to appropriate motion management strategies.

Influence of increased target dose inhomogeneity on margins for breathing motion compensation in conformal stereotactic body radiotherapy

Background

Breathing motion should be considered for stereotactic body radiotherapy (SBRT) of lung tumors. Four-dimensional computer tomography (4D-CT) offers detailed information of tumor motion. The aim of this work is to evaluate the influence of inhomogeneous dose distributions in the presence of breathing induced target motion and to calculate margins for motion compensation.

Methods

Based on 4D-CT examinations, the probability density function of pulmonary tumors was generated for ten patients. The time-accumulated dose to the tumor was calculated using one-dimensional (1D) convolution simulations of a 'static' dose distribution and target probability density function (PDF). In analogy to stereotactic body radiotherapy (SBRT), different degrees of dose inhomogeneity were allowed in the target volume: minimum doses of 100% were prescribed to the edge of the target and maximum doses varied between 102% (P102) and 150% (P150). The dose loss due to breathing motion was quantified and margins were added until this loss was completely compensated.

Results

With the time-weighted mean tumor position as the isocentre, a close correlation with a quadratic relationship between the standard deviation of the PDF and the margin size was observed. Increased dose inhomogeneity in the target volume required smaller margins for motion compensation: margins of 2.5 mm, 2.4 mm and 1.3 mm were sufficient for compensation of 11.5 mm motion range and standard deviation of 3.9 mm in P105, P125 and P150, respectively. This effect of smaller margins for increased dose inhomogeneity was observed for all patients. Optimal sparing of the organ-at-risk surrounding the target was achieved for dose prescriptions P105 to P118. The internal target volume concept over-compensated breathing motion with higher than planned doses to the target and increased doses to the surrounding normal tissue.

Conclusion

Treatment planning with inhomogeneous dose distributions in the target volume required smaller margins for compensation of breathing induced target motion with the consequence of lower doses to the surrounding organs-at-risk.

Potential of image-guidance, gating and real-time tracking to improve accuracy in pulmonary stereotactic body radiotherapy

PURPOSE

To evaluate the potential of image-guidance, gating and real-time tumor tracking to improve accuracy in pulmonary stereotactic body radiotherapy (SBRT).

MATERIALS AND METHODS

Safety margins for compensation of inter- and intra-fractional uncertainties of the target position were calculated based on SBRT treatments of 43 patients with pre- and post-treatment cone-beam CT imaging. Safety margins for compensation of breathing motion were evaluated for 17 pulmonary tumors using respiratory correlated CT, model-based segmentation of 4D-CT images and voxel-based dose accumulation; the target in the mid-ventilation position was the reference.

RESULTS

Because of large inter-fractional base-line shifts of the tumor, stereotactic patient positioning and image-guidance based on the bony anatomy required safety margins of 12 mm and 9 mm, respectively. Four-dimensional image-guidance targeting the tumor itself and intra-fractional tumor tracking reduced margins to <5 mm and <3 mm, respectively. Additional safety margins are required to compensate for breathing motion. A quadratic relationship between tumor motion and margins for motion compensation was observed: safety margins of 2.4mm and 6mm were calculated for compensation of 10 mm and 20 mm motion amplitudes in cranio-caudal direction, respectively.

CONCLUSION

Four-dimensional image-guidance with pre-treatment verification of the target position and online correction of errors reduced safety margins most effectively in pulmonary SBRT.

Tumor trailing strategy for intensity-modulated radiation therapy of moving targets

Internal organ motion during the course of radiation therapy of cancer affects the distribution of the delivered dose and, generally, reduces its conformality to the targeted volume. Previously proposed approaches aimed at mitigating the effect of internal motion in intensity-modulated radiation therapy (IMRT) included expansion of the target margins, motion-correlated delivery (e.g., respiratory gating, tumor tracking), and adaptive treatment plan optimization employing a probabilistic description of motion. We describe and test the tumor trailing strategy, which utilizes the synergy of motion-adaptive treatment planning and delivery methods. We regard the (rigid) target motion as a superposition of a relatively fast cyclic component (e.g., respiratory) and slow aperiodic trends (e.g., the drift of exhalation baseline). In the trailing approach, these two components of motion are decoupled and dealt with separately. Real-time motion monitoring is employed to identify the "slow" shifts, which are then corrected by applying setup adjustments. The delivery does not track the target position exactly, but trails the systematic trend due to the delay between the time a shift occurs, is reliably detected, and, subsequently, corrected. The "fast" cyclic motion is accounted for with a robust motion-adaptive treatment planning, which allows for variability in motion parameters (e.g., mean and extrema of the tidal volume, variable period of respiration, and expiratory duration). Motion-surrogate data from gated IMRT treatments were used to provide probability distribution data for motion-adaptive planning and to test algorithms that identified systematic trends in the character of motion. Sample IMRT fields were delivered on a clinical linear accelerator to a programmable moving phantom. Dose measurements were performed with a commercial two-dimensional ion-chamber array. The results indicate that by reducing intrafractional motion variability, the trailing strategy enhances relevance and applicability of motion-adaptive planning methods, and improves conformality of the delivered dose to the target in the presence of irregular motion. Trailing strategy can be applied to respiratory-gated treatments, in which the correction for the slow motion can increase the duty cycle, while robust probabilistic planning can improve management of the residual motion within the gate window. Similarly, trailing may improve the dose conformality in treatment of patients who exhibit detectable target motion of low amplitude, which is considered insufficient to provide a clinical indication for the use of respiratory-gated treatment (e.g., peak-to-peak motion of less than 10 mm). The mechanical limitations of implementing tumor trailing are less rigorous than those of real-time tracking, and the same technology could be used for both.

Image-guided radiotherapy for liver cancer using respiratory-correlated computed tomography and cone-beam computed tomography

PURPOSE

To evaluate a novel four-dimensional (4D) image-guided radiotherapy (IGRT) technique in stereotactic body RT for liver tumors.

METHODS AND MATERIALS

For 11 patients with 13 intrahepatic tumors, a respiratory-correlated 4D computed tomography (CT) scan was acquired at treatment planning. The target was defined using CT series reconstructed at end-inhalation and end-exhalation. The liver was delineated on these two CT series and served as a reference for image guidance. A cone-beam CT scan was acquired after patient positioning; the blurred diaphragm dome was interpreted as a probability density function showing the motion range of the liver. Manual contour matching of the liver structures from the planning 4D CT scan with the cone-beam CT scan was performed. Inter- and intrafractional uncertainties of target position and motion range were evaluated, and interobserver variability of the 4D-IGRT technique was tested.

RESULTS

The workflow of 4D-IGRT was successfully practiced in all patients. The absolute error in the liver position and error in relation to the bony anatomy was 8 +/- 4 mm and 5 +/- 2 mm (three-dimensional vector), respectively. Margins of 4-6 mm were calculated for compensation of the intrafractional drifts of the liver. The motion range of the diaphragm dome was reproducible within 5 mm for 11 of 13 lesions, and the interobserver variability of the 4D-IGRT technique was small (standard deviation, 1.5 mm). In 4 patients, the position of the intrahepatic lesion was directly verified using a mobile in-room CT scanner after application of intravenous contrast.

CONCLUSION

The results of our study have shown that 4D image guidance using liver contour matching between respiratory-correlated CT and cone-beam CT scans increased the accuracy compared with stereotactic positioning and compared with IGRT without consideration of breathing motion.

Comparison of different strategies to use four-dimensional computed tomography in treatment planning for lung cancer patients

PURPOSE

To discuss planning target volumes (PTVs) based on internal target volume (PTVITV), exhale-gated radiotherapy (PTVGating), and a new proposed midposition (PTVMidP; time-weighted mean tumor position) and compare them with the conventional free-breathing CT scan PTV (PTVConv).

METHODS AND MATERIALS

Respiratory motion induces systematic and random geometric uncertainties. Their contribution to the clinical target volume (CTV)-to-PTV margins differs for each PTV approach. The uncertainty margins were calculated using a dose-probability-based margin recipe (based on patient statistics). Tumor motion in four-dimensional CT scans was determined using a local rigid registration of the tumor. Geometric uncertainties for interfractional setup errors and tumor baseline variation were included. For PTVGating, the residual motion within a 30% gating (time) window was determined. The concepts were evaluated in terms of required CTV-to-PTV margin and PTV volume for 45 patients.

RESULTS

Over the patient group, the PTVITV was on average larger (+6%) and the PTVGating and PTVMidP smaller (-10%) than the PTVConv using an off-line (bony anatomy) setup correction protocol. With an on-line (soft tissue) protocol the differences in PTV compared with PTVConv were +33%, -4%, and 0, respectively.

CONCLUSIONS

The internal target volume method resulted in a significantly larger PTV than conventional CT scanning. The exhale-gated and mid-position approaches were comparable in terms of PTV. However, mid-position (or mid-ventilation) is easier to use in the clinic because it only affects the planning part of treatment and not the delivery.

Mean position tracking of respiratory motion

Modeling and predicting tumor motion caused by respiration is challenging due to temporal variations in breathing patterns. Treatment approaches such as gating or adaptive bed adjustment/ alignment may not require full knowledge of instantaneous position, but might benefit from tracking the general trend of the motion. One simple method for tracking mean tumor position is to apply moving average filters with window sizes corresponding to the breathing periods. Yet respiratory motion is only semiperiodic, so such methods require reliable phase estimation, which is difficult in the presence of noise. This article describes a robust method to track the mean position of respiratory motion without explicitly estimating instantaneous phase. We form a state vector from the respiration signal values at the current instant and at a previous time, and fit an ellipse model to training data. Ellipse eccentricity and orientation potentially capture hysteresis in respiratory motion. Furthermore, we provide two recursive online algorithms for real time mean position tracking: a windowed version with an adaptive window size and another one with temporal discounting. We test the proposed method with simulated breathing traces, as well as with real time-displacement (RPM, Varian) signals. Estimation traces are compared with retrospectively generated moving average results to illustrate the performance of the proposed approach.

On-line target position localization in the presence of respiration: a comparison of two methods

PURPOSE

To compare two "four-dimensional" methods for image-guided target localization in the presence of respiration.

METHODS AND MATERIALS

Four-dimensional image guidance was performed with two methods. A respiration-correlated computed tomography (RCCT) was acquired on a CT simulator, and an average CT (AVG-CT) image was generated from the RCCT. A respiration-correlated cone-beam CT (RC-CBCT) and a free-breathing cone-beam CT (FB-CBCT) were acquired. The "RCCT method" consisted of calculating the mean target position on both the RCCT and RC-CBCT, registering the RCCT to the RC-CBCT, and determining the shift in the mean target position from the planned mean position. The "AVG-CT method" consisted of registering the AVG-CT to the FB-CBCT. The ability of each to measure the shift in the mean target position was compared, both in a respiratory phantom and in 8 patients.

RESULTS

In phantom, the RCCT and AVG-CT methods were able to measure the true mean target position to within 0.15 cm and 0.10 cm, respectively. In the patient study, the mean error between the methods was 0.13 cm (left-right), 0.14 cm (anterior-posterior), and 0.10 cm (cranio-caudal). The error was not observed to vary with tumor position or magnitude of tumor motion.

CONCLUSIONS

Respiration may impact the on-line image guidance process. The RCCT method enables localization of the mean tumor position and measurement of changes in the motion pattern, whereas the AVG-CT method is simple, fast, and easily implemented. We found the methods to be nearly equivalent in detecting shifts in the mean tumor position.

Four-Dimensional Treatment Planning for Stereotactic Body Radiotherapy

PURPOSE

To investigate the influence of tumor motion on the calculation of four-dimensional (4D) dose distributions of the gross tumor volume (GTV) in pulmonary stereotactic body radiotherapy.

METHODS AND MATERIALS

For 7 patients with eight pulmonary tumors, a respiratory-correlated 4D-computed tomography study was acquired. The internal target volume was the sum of all tumor positions in the planning 4D-computed tomography study, and a 5-mm margin was used for generation of the planning target volume. Three-dimensional (3D) treatment plans were generated with a dose prescription of 3 x 12.5 Gy to the planning target volume enclosing the 65% and 80% isodose. After model-based nonrigid image registration, the 4D dose distributions were calculated.

RESULTS

No significant difference was found in the dose to the GTV with the tumor in the end-exhalation, end-inhalation, or mid-ventilation phase of the breathing cycle. The high-dose region was confined to the solid tumor, and lower doses were delivered to the surrounding pulmonary tissue of lower density. This nonstatic, variant dose distribution increased the 4D dose to the GTV by 6.2%, on average, compared with calculations using on a static dose distribution during the breathing cycle. The 4D accumulation resulted in a biologic effective dose (BED) of 143 +/- 8 Gy and 106 +/- 4 Gy to the GTV in the plan-65% and plan-80%, respectively. The dose to the ipsilateral lung was not different between the 3D and 4D dose calculations or between plan-65% and plan-80%.

CONCLUSIONS

In this study, the dose to the GTV was not decreased or blurred in the 4D plan compared with the 3D plan. The 3D doses to the GTV, internal target volume, and dose at the isocenter were good approximations of the 4D dose calculations. The 3D dose at the planning target volume margin underestimated the 4D dose significantly.

Intra- and interfraction breathing variations during curative radiotherapy for lung cancer

BACKGROUND AND PURPOSE

This study aimed at quantifying the breathing variations among lung cancer patients over full courses of fractionated radiotherapy. The intention was to relate these variations to the margins assigned to lung tumours, to account for respiratory motion, in fractionated radiotherapy.

MATERIALS AND METHODS

Eleven lung cancer patients were included in the study. The patients' chest wall motions were monitored as a surrogate measure for breathing motion during each fraction of radiotherapy by use of an external optical marker. The exhale level variations were evaluated with respect to exhale points and fraction-baseline, defined for intra- and interfraction variations respectively. The breathing amplitude was evaluated as breathing cycle amplitudes and fraction-max-amplitudes defined for intra- and interfraction breathing, respectively.

RESULTS

The breathing variations over a full treatment course, including both intra- and interfraction variations, were 15.2mm (median over the patient population), range 5.5-26.7mm, with the variations in exhale level as the major contributing factor. The median interfraction span in exhale level was 14.8mm, whereas the median fraction-max-amplitude was 6.1mm (median of patient individual SD 1.4). The median intrafraction span in exhale level was 1.6mm, and the median breathing cycle amplitude was 4.0mm (median of patient individual SD 1.4).

CONCLUSIONS

The variations in externally measured exhale levels are larger than variations in breathing amplitude. The interfraction variations in exhale level are in general are up to 10 times larger than intrafraction variations. Margins to account for respiratory motion cannot safely be based on one planning session, especially not if relying on measuring external marker motion. Margins for lung tumours should include interfraction variations in breathing.

Positioning accuracy of cone-beam computed tomography in combination with a HexaPOD robot treatment table

PURPOSE

To scrutinize the positioning accuracy and reproducibility of a commercial hexapod robot treatment table (HRTT) in combination with a commercial cone-beam computed tomography system for image-guided radiotherapy (IGRT).

METHODS AND MATERIALS

The mechanical stability of the X-ray volume imaging (XVI) system was tested in terms of reproducibility and with a focus on the moveable parts, i.e., the influence of kV panel and the source arm on the reproducibility and accuracy of both bone and gray value registration using a head-and-neck phantom. In consecutive measurements the accuracy of the HRTT for translational, rotational, and a combination of translational and rotational corrections was investigated. The operational range of the HRTT was also determined and analyzed.

RESULTS

The system performance of the XVI system alone was very stable with mean translational and rotational errors of below 0.2 mm and below 0.2 degrees , respectively. The mean positioning accuracy of the HRTT in combination with the XVI system summarized over all measurements was below 0.3 mm and below 0.3 degrees for translational and rotational corrections, respectively. The gray value match was more accurate than the bone match.

CONCLUSION

The XVI image acquisition and registration procedure were highly reproducible. Both translational and rotational positioning errors can be corrected very precisely with the HRTT. The HRTT is therefore well suited to complement cone-beam computed tomography to take full advantage of position correction in six degrees of freedom for IGRT. The combination of XVI and the HRTT has the potential to improve the accuracy of high-precision treatments.