F Lerma

Real‐time tumor tracking with preprogrammed dynamic multileaf‐collimator motion and adaptive dose‐rate regulation

The authors have developed a new method for real-time tumor tracking with dynamic multileaf-collimator (MLC) motion under condition of free breathing. Unlike other previously proposed tumor-tracking methods, their new method uses a preprogrammed dynamic MLC sequence in combination with real-time dose-rate control. This new scheme circumvents the technical challenge in MLC-based tumor tracking of having to control the MLC motion in real time, based on real-time detected tumor motion. With their new method, the movement of the tumor, as a function of breathing phase, amplitude, or tidal volume, is reflected in the preprogrammed MLC sequence. The irregularity of breathing during treatment is handled by real-time regulation of the machine dose rate, which effectively speeds up or slows down the delivery of radiation as needed. This method is based on the fact that all of the parameters in dynamic radiation delivery, including MLC motion, are enslaved to the cumulative dose, which, in turn, can be accelerated or decelerated by varying the dose rate. Because commercially available MLC systems do not allow the MLC delivery sequence to be modified in real time based on the patients breathing signal, previously proposed tumor-tracking techniques using a MLC cannot be readily implemented in the clinic today. By using a preprogrammed MLC sequence to handle the required motion, the task for real-time control is greatly simplified. With their new scheme, which they call dose-rate-regulated tracking (DRRT), it is possible to use existing linear accelerators that have dynamic MLC capability to achieve real-time tumor tracking, provided that the beam dose rate can be controlled externally. Tracking-error evaluation for 13 patients out of 14 resulted in a tracking error of less than 1 mm (1 sigma), if the effect of the response time of the treatment machine on the dose-rate modulation can be neglected. Film measurements on a moving phantom with variable breathing patterns and DRRT delivery showed that 97% of the measurement points have gamma values less than 1 (for 3% and 2-mm criteria), while non-DRRT delivery showed only 87%. This study shows that real-time tracking is feasible with DRRT even when the patient breathing frequency is irregular. Effects of the variation of breathing amplitude and of base line drift on the tracking error with DRRT are discussed; pending further study, a criterion is suggested for patient selection in the application of this new technique in the clinic.

Role of image-guided patient repositioning and online planning in localized prostate cancer IMRT

PURPOSE To determine the expected benefit of image-guided online replanning over image-guided repositioning of localized prostate cancer intensity-modulated radiotherapy (IMRT). MATERIALS AND METHODS On 10 to 11 CT scans of each of 10 early-stage prostate cancer patients, the prostate, bladder and rectum are manually segmented. Using a 3-mm PTV margin expansion from the CTV, an IMRT plan is made on the first CT scan of each patient. Online repositioning is simulated by recalculating the IMRT plan from the initial CT scan on the subsequent CT scans of each patient. For online replanning, IMRT is replanned twice on all CT scans, using 0-mm and 3-mm margins. The doses from subsequent CT images of each patient are then deformed to the initial CT anatomy using a mesh-based thin-plate B-spline deformation method and are accumulated for DVH and isodose review. RESULTS Paired t-tests show that online replanning with 3-mm margins significantly increases the prostate volume receiving the prescribed dose over replanning with 0-mm margins (p-value 0.004); gives marginally better target coverage than repositioning with 3-mm margins(p-value 0.06-0.343), and reduces variations in target coverage over repositioning. Fractional volumes of rectum and bladder receiving 75%, 80%, 85%, 90%, and 95% (V75, V80, V85, V90, and V95) of the prescription dose are evaluated. V90 and V95 values for the rectum are 1.6% and 0.7 % for 3-mm margin replanning and 1% and 0.4 % for 0-mm margin replanning, with p-values of 0.010-0.011. No significant differences between repositioning and replanning with 3-mm margins are found for both the rectum and the bladder. CONCLUSIONS Image-guided replanning using 3-mm margins reduces target coverage variations, and maintains comparable rectum and bladder sparing to patient repositioning in localized prostate cancer IMRT. Marginal reductions in doses to rectum and bladder are possible when planning margins are eliminated in the online replanning scenario. However, further reduction in treatment planning margins is not recommended.

Verification of MLC based real‐time tumor tracking using an electronic portal imaging device

PURPOSE The authors have developed a novel technique using an electronic portal imaging device (EPID) to verify the geometrical accuracy of delivery of dose-rate-regulated tracking (DRRT). This technique, called verification of real-time tracking with EPID (VORTE), can potentially be used for both on-line and off-line quality assurance (QA) of MLC-based dynamic tumor tracking. METHODS The shape and position of target as a function of time, which is assumed to be known, is projected onto the EPID plane. This projected sequence of apertures as a function of time (target motion) is then used as the reference. The accuracy of dynamic MLC tracking can then be assessed by how well the delivered beam follows this projected target motion without the use of a physical moving phantom. The beam apertures controlled by DRRT (aperture motion) is detected by the EPID as a function of time. The aperture motion is compared to the target motion to evaluate tracking error introduced by DRRT. The accuracy of VORTE was measured using film measurements of ten static fields. The VORTE for dynamic tumor tracking was tested with several target motions, including (1) rigid-body two-dimensional (2-D) cyclic motion in the superior-inferior direction with various period and amplitude; (2) the above 2-D cyclic motion plus cyclic deformation; and (3) 2-D cyclic motion with both deformation and rotation. For each target motion, the controlled aperture motion resulting from DRRT was acquired at ∼8Hz using EPID in the continuous-acquisition mode. Leaf positions in all captured frames were measured from the EPID and compared to their expected positions. The passing rate of 2 mm criteria for all leaves from all frames was calculated for each of the four patterns of tumor motion. Additionally, the root-mean-square (RMS) deviations of the centroid of the apertures between the designed and delivered beams were calculated for all three cases. RESULTS The accuracy of MLC-leaf position determination by VORTE is 0.5 mm (1 standard deviation) by comparison to film measurements. With DRRT, the passing rates using the 2 mm criteria for all acquired frames are 100% for the 2-D displacement, 99% for the 2-D displacement with deformation, and 88% for the 2-D displacement combined with both deformation and rotation. The RMS deviations are 0.6 mm for the 2-D displacement, 1.0 mm for the 2-D displacement with deformation, and 1.1 mm for the 2-D displacement combined with both deformation and rotation. CONCLUSIONS The VORTE can measure the accuracy of MLC-based tumor tracking without the necessity of employing a moving phantom. Moreover, it can be used for complex target motion (i.e., 2-D displacement combined with deformation and rotation) that is difficult to create with physical moving phantoms. Therefore, the VORTE and the novel QA process illustrated by this study have a great potential for verifying real-time tumor tracking.

Lung dose for minimally moving thoracic lesions treated with respiration gating.

PURPOSE To evaluate incidental doses to benign lung tissue for patients with minimally moving lung lesions treated with respiratory gating. METHODS AND MATERIALS Seventeen lung patient plans were studied retrospectively. Tumor motion was less than 5 mm in all cases. For each patient, mid-ventilation (MidVen) and mid-inhalation (MidInh) breathing phases were reconstructed. The MidInh phase was centered on the end-of-inhale (EOI) phase within a 30% gating window. Planning target volumes, heart, and spinal cord were delineated on the MidVen phase and transferred to the MidInh phase. Lungs were contoured separately on each phase. Intensity-modulated radiotherapy plans were generated on the MidVen phases. The plans were transferred to the MidInh phase, and doses were recomputed. The evaluation metric was based on dose indices, volume indices, generalized equivalent uniform doses, and mass indices for targets and critical structures. Statistical tests were used to establish the significance of the differences between the reference (MidVen) and compared (MidInh) dose distributions. RESULTS Statistical tests demonstrated that the indices evaluated for targets, cord, and heart differed by within 2.3%. The index differences in the lungs, however, are in excess of 6%, indicating the potentially achievable lung sparing and/or dose escalation. CONCLUSIONS Respiratory gating is a clinical option for patients with minimally moving lung lesions treated at EOI. Gating will be more beneficial for larger tumors, since dose escalation in those cases will result in a larger increase in the tumor control probability.

Usefulness of guided breathing for dose rate-regulated tracking.

PURPOSE To evaluate the usefulness of guided breathing for dose rate-regulated tracking (DRRT), a new technique to compensate for intrafraction tumor motion. METHODS AND MATERIALS DRRT uses a preprogrammed multileaf collimator sequence that tracks the tumor motion derived from four-dimensional computed tomography and the corresponding breathing signals measured before treatment. Because the multileaf collimator speed can be controlled by adjusting the dose rate, the multileaf collimator positions are adjusted in real time during treatment by dose rate regulation, thereby maintaining synchrony with the tumor motion. DRRT treatment was simulated with free, audio-guided, and audiovisual-guided breathing signals acquired from 23 lung cancer patients. The tracking error and duty cycle for each patient were determined as a function of the system time delay (range, 0-1.0 s). RESULTS The tracking error and duty cycle averaged for all 23 patients was 1.9 +/- 0.8 mm and 92% +/- 5%, 1.9 +/- 1.0 mm and 93% +/- 6%, and 1.8 +/- 0.7 mm and 92% +/- 6% for the free, audio-guided, and audiovisual-guided breathing, respectively, for a time delay of 0.35 s. The small differences in both the tracking error and the duty cycle with guided breathing were not statistically significant. CONCLUSION DRRT by its nature adapts well to variations in breathing frequency, which is also the motivation for guided-breathing techniques. Because of this redundancy, guided breathing does not result in significant improvements for either the tracking error or the duty cycle when DRRT is used for real-time tumor tracking.

The accuracy of dose-rate-regulated tracking: a parametric study

Dose-rate-regulated tracking (DRRT) is a novel tumor-tracking technique based on a preprogrammed multileaf-collimator (MLC) sequence and dose-rate modulation. We have performed a parametric study on how limitations of the DRRT system and breathing irregularities affect the tracking error and the duty cycle of DRRT. The time delay and the allowed dose-rate increment (continuous-, discrete-increment or beam switching) were used as two parameters for the DRRT system limitation. The breathing irregularity was quantified in terms of three variables, namely, breathing period variation, variation of peak-to-peak amplitude and baseline drift. DRRT treatments were simulated using 2126 breathing cycles obtained from 24 lung-cancer patients. Tracking errors and duty cycles from all 24 patients were combined to evaluate their dependence on each parameter or variable. The tracking error and the duty cycle show a modest difference among the three dose-rate-increment cases. Time delay, breathing peak-to-peak variation and baseline drift are the main factors affecting tracking error. The duty cycle is affected mostly by the allowed dose-rate increment, peak-to-peak variation and baseline drift.

MO-F-BRC-05: Minimization of the Total Inter-Segment Time for the Real-Time Tumor Tracking with Step & Shoot IMRT

Purpose: We developed an algorithm to minimize total inter‐segment time (TIST) for the MLC‐based real‐time tumor tracking with step‐and‐shoot IMRT. This algorithm optimizes a starting phase of tumor motion for each segment to minimize TIST. Methods: The optimizing algorithm consists of four steps: (1) implementation of feathering motion for the closed leaves that will be opened at the next segment, (2) calculation of inter‐segment time for all segments, (3) reordering segments to minimize TIST, and (4) optimization of the starting phase of tumor motion for each segment to minimize TIST. Thirty step‐and‐shoot IMRT fields from five patients with lung and abdominal cancer were used to test the algorithm. Tumor motion was varied with a period (2.0 to 4.0 s) and a peak‐to‐peak distance (0.5 to 4.0 cm). TIST and duty cycle for each field were compared to those from the strategy of starting each segment at end‐of‐exhale. Results: The TIST was reduced by 54.0% on average (from 30.2 ± 16.9 to 13.9 ± 10.6 s) and, the effective duty cycle was increased from 32 ± 10% to 52 ± 15% for a tumor motion with 4 s and 1.0 cm peak‐to‐peak. More reduction in the TIST was observed from 45.1 to 72.1% with an increase of the period from 2 to 8 s; effect of reduction was degraded by 54.5 to 46.2% when the peak‐to‐peak increased from 0.5 to 4.0 cm. The TIST increased when a field size formed by x‐jaws increased (correlation coefficient: 0.7). Conclusions: : Total treatment time was reduced noticeably with the algorithm presented in this study so that real‐time tumor tracking can be delivered with step‐and‐shoot IMRT with an increased duty cycle. This research was supported in part by a NIH grant 1R01CA133539‐01A2 and in part by the Intramural Research Program of the NIH, NCI.

Relation Between Tumor Size and Range of Motion in IMRT Treatment Planning for Thoracic Lesions

Purpose: To evaluate the relation between tumor size/volume, tumor range of motion, and healthy lung volume in light of radiotherapy motion management paradigm. Materials and Methods: Four patient data sets were considered in this investigation. Each patient underwent time resolved (4D) CT data scan. Mid-ventilation CT data sets, with nominal lung volumes ranging from ~3000 cm3 to ~6000 cm3, were considered for treatment planning. Spheres with pre-specified radii were auto-contoured in the left lower lobes as simulated planning target volumes (PTVs) for each patient. Motion in superior-inferior direction was superimposed on the simulated spherical PTVs, such that motion-inclusive ITVs were generated. Nine-field IMRT treatment plans were created for all lung volumes, different combinations of simulated PTV spherical size and ranges of motion. Three dose levels of 60 Gy, 70 Gy, and 80 Gy were utilized. The doses were prescribed to 95% of the ITV. Simulated PTV sizes and ranges of motion were varied until prescriptions were met, given that organs at risk (OARs) were spared. The OAR constraints were: 40 Gy to 1% of the cord and 30% of the heart, as well as 20 Gy and 30 Gy to 30% and 20% of benign lung, respectively. These constraints, representative for 2 Gy per fraction fractionation schemes, are commonly used clinically. The treatment plans were deemed clinically acceptable when standard deviation of the dose across the ITV was less than 3% of the prescription dose in addition to fulfillment of the OAR constraints. Results: For each nominal lung volume three look-up curves, corresponding to the prescription dose levels were generated. The plots related the PTV sphere sizes with its range of motion. In addition, correlation between the absolute tumor volume and its range of motion was also established and presented in graphical format. Conclusions: The motion management threshold of 0.5 cm found in the literature is reasonable. However, in some cases, depending on the tumor size, tumor range of motion, and nominal lung volumes, it might be too restrictive. In the determination of the most appropriate individualized treatment planning approach all factors such as tumor and lung volumes, tumor range of motion and patient tolerance toward the treatment technique need to be assessed.

MO-FF-A1-01: Fluoroscopic Verification of Intensity-Modulated Rotational Arc Therapy Delivery

Purpose: Intensity modulated rotational arc technique requires verification of leaf positions, gantry angle and dose rate in the entire arc. This study shows how to achieve this with a detailed verification of Varian RapidArc using a fluoroscopic electronic portal imaging device(EPID).Materials and Methods: Three Rapid Arc plans (prostate 1, whole pelvis 1, and head and neck 1) are delivered on a Triology linac (Varian Medical Systems, CA). During delivery, approximately 600 fluoroscopic portal images are acquired (∼8 images/s) per arc with a PV‐aS1000 EPID, without use of secondary phantoms or blocks. Each leaf position of each gantry angle is calculated from the acquired EPIDimages offline. Gantry angle information of each portal image is acquired from the dynalog file generated during beam delivery. Leaf positions from the dynalog file are compared to scheduled positions from the DICOM RT plan file. Results: Online EPIDimage acquisition of Rapid Arc delivery is prompt, involving extension of the EPID system and beam delivery time. The measurement error depends on the displacement of EPID system relatively to the center of rotation, which is only 1mm–1.5 mm. Offline analyses show the accuracy of leaf positions for static leaf and gantry field are better than 1 mm. More than 98.5% of leaf sequences exhibit less than 3mm deviations, 83 % show 2mm and 56% for 1mm. Conclusions: Position of each leaf of each gantry angle for Rapid Arc delivery is verified within 1 mm accuracy with fluoroscopic portal images. Use of fluoroscopic EPIDimages can be considered as a practical QA tool for the verification of the Rapid Arc delivery. This study is partially supported by Varian Medical Systems.

SU‐FF‐J‐132: A Novel Technique for Closed‐Loop Feedback Real‐Time Tracking by Controlling Dose Rate : A Parametric Study

Purpose: We have developed a new technique called dose‐rate‐modulated tracking (DRMT) for closed‐feedback real‐time tumor tracking by changing the dose rate. Method and Materials: DRMT uses pre‐programmed MLC sequences that are generated using measured data for tumor motion obtained at an earlier time. Since the leaves move, their positions are programmed with schedules that are a function of the accumulated dose. The leaf trajectory on the time axis then can be changed during treatment by changing the dose rate. DRMT changes the dose rate to minimize the discrepancy between the scheduled MLC position and the target position or breathing signal on the day of treatment. If the monitored breathing is slower than that observed at simulation, the dose rate is lowered to slow down the movement of the MLC and vice versa, thereby maintaining synchrony. DRMT tumor‐tracking power was tested with sinusoidalbreathing functions and patient‐breathing signals (RPM, Varian). The tracking error (2σ) for each breathing signal was derived as a function of the system reaction time and the dose‐rate correction period. Results and Conclusions: DRMT simulation showed that for the sinusoidal‐breathing signal with 2‐cm peak‐to‐peak tumor motion, less than 0.2 cm tracking error was obtained if the system can react to a detected mismatch in less than 0.3 s and the dose rate can be adjusted in 0.36 s. Fourteen out of 26 patients were eligible for DRMT. The selection criteria were: (1) peak‐to‐peak breathing motion greater than 0.5 cm and (2) amplitude variation less than 20 % during the DRMT simulation. The tracking error for the patient data is expressed as a percentage of the motion amplitude. A tracking error of less than 20 % was achieved for 13 out of the 14 patients if the system can respond in 0 s.