Finite Leaf Width Research Articles

Toward the development of intrafraction tumor deformation tracking using a dynamic multi-leaf collimator.

Intrafraction deformation limits targeting accuracy in radiotherapy. Studies show tumor deformation of over 10 mm for both single tumor deformation and system deformation (due to differential motion between primary tumors and involved lymph nodes). Such deformation cannot be adapted to with current radiotherapy methods. The objective of this study was to develop and experimentally investigate the ability of a dynamic multi-leaf collimator (DMLC) tracking system to account for tumor deformation. To compensate for tumor deformation, the DMLC tracking strategy is to warp the planned beam aperture directly to conform to the new tumor shape based on real time tumor deformation input. Two deformable phantoms that correspond to a single tumor and a tumor system were developed. The planar deformations derived from the phantom images in beam's eye view were used to guide the aperture warping. An in-house deformable image registration software was developed to automatically trigger the registration once new target image was acquired and send the computed deformation to the DMLC tracking software. Because the registration speed is not fast enough to implement the experiment in real-time manner, the phantom deformation only proceeded to the next position until registration of the current deformation position was completed. The deformation tracking accuracy was evaluated by a geometric target coverage metric defined as the sum of the area incorrectly outside and inside the ideal aperture. The individual contributions from the deformable registration algorithm and the finite leaf width to the tracking uncertainty were analyzed. Clinical proof-of-principle experiment of deformation tracking using previously acquired MR images of a lung cancer patient was implemented to represent the MRI-Linac environment. Intensity-modulated radiation therapy (IMRT) treatment delivered with enabled deformation tracking was simulated and demonstrated. The first experimental investigation of adapting to tumor deformation has been performed using simple deformable phantoms. For the single tumor deformation, the A(u)+A(o) was reduced over 56% when deformation was larger than 2 mm. Overall, the total improvement was 82%. For the tumor system deformation, the A(u)+A(o) reductions were all above 75% and the total A(u)+A(o) improvement was 86%. Similar coverage improvement was also found in simulating deformation tracking during IMRT delivery. The deformable image registration algorithm was identified as the dominant contributor to the tracking error rather than the finite leaf width. The discrepancy between the warped beam shape and the ideal beam shape due to the deformable registration was observed to be partially compensated during leaf fitting due to the finite leaf width. The clinical proof-of-principle experiment demonstrated the feasibility of intrafraction deformable tracking for clinical scenarios. For the first time, we developed and demonstrated an experimental system that is capable of adapting the MLC aperture to account for tumor deformation. This work provides a potentially widely available management method to effectively account for intrafractional tumor deformation. This proof-of-principle study is the first experimental step toward the development of an image-guided radiotherapy system to treat deforming tumors in real-time.

SU-E-T-891: A Study of RapidArc SBRT Spine Treatment Plans

Purpose: The arc and modulating features of RapidArc treatments facilitate fast dose fall off and desirable dose distribution around the PTV and spinal cord in spine SBRT/SRS treatments. This is a study of the dosimetry characters of 11 RapidArc SBRT spine plans. Methods: All patients had treatment plans formulated on the Varian Eclipse system, and were treated on RapidArc upgraded Varian Trilogy unit, using 6MV photon beams. Patients were immobilized with the BodyFix system or the T‐frame mask system (cervical spine targets). All SBRT spine plans consisted of two arcs, each ∼350 degrees. To minimize MLC leakage and finite leaf width (0.5 cm) effects, we used collimator angles of 155 and 215 (Varian scale). Fraction doses ranged from 400 cGy to 1800 cGy. Spinal cord/cauda equina were contoured on CT myelogram or MRIimages; the spinal cord contour was expanded between 1–3 mm. Expanding the CTV by 2 mm generated the PTV, except where the PTV would intersect the spinal cord contour described above, in which case the PTV was restricted. Next, the original cord contour was again expanded by 5 mm, this time avoiding the PTV. The PTV, this second spinal cord expansion contour, as well as other OAR's were used for plan optimization. Results: This process allows for extremely rapid dose fall off. In particular, for 5 patients, the average minimum distances between 100% and 70%, and between 90% and 50% isodose lines were 2.5 mm and 3.4 mm respectively. 90% of the PTV received 100.0% of the prescription dose on average, while 1% of the PTV received 121.7% of dose. An average of 0.1 cc of spinal cord received 39.7% dose. Conclusions: We have been able to achieve improve SBRT plans for spine patients using this paradigm.

SU-E-T-583: QA Verifications of RapidArc Type SBRT Treatment Plans

Purpose: RapidArc treatments combine features of IMRT and arc treatments. It may be a superior treatment technique for stereotactic body radiation therapy(SBRT) in both improved dose distribution as well as improved patient comfort and reduction in motion due to rapid treatmentdelivery. This is a study of the QA verification results on 15 spine and 9 lung patients treated with RapidArc SBRT since August 2009. Methods: Treatements were delivered on a Varian Trilogy with RapidArc upgrade. Plans generated used 6 MV photons utilizing Varian Eclipse software. Most plans consisted of a 2 arc technique. The 3D diode array is the Delta4 system from Scandidos. Delta4 has 1069 P type Si diodes on three wings, which are inserted to two planes in a cylindrical PMMA phantom. The three dimensional dose is transferred to the Delta4 by DICOM RT. To minimize the MLC leakage, and to reduce the finite leaf width (0.5 cm) effects, we used collimator angles of 155 and 215 (Varian scale). Fraction dose ranged from 400 cGy to 1800 cGy. For each plan, the planned dose distribution and measured dose distribution are compared by using 2D isodose display, profile comparisons, percentage dose deviation, and DTA and Gamma index. Results: The average percentage of diode with Gamma index (3% dose and 3 mm) 1 or less is 95.5% with standard deviation of 3.4% for spine SBRT plans, and average percentage of diode with Gamma index (3% dose and 3 mm) 1 or less is 97.4% with standard deviation of 2.0% for lungSBRT plans, Conclusions: All measurement results agree well with the dose distribution of all SBRT plans. Delta4 system is a good QA tool for RapidArc type SBRTtreatments.

Electromagnetic Real-Time Tumor Position Monitoring and Dynamic Multileaf Collimator Tracking Using a Siemens 160 MLC: Geometric and Dosimetric Accuracy of an Integrated System

Dynamic multileaf collimator tracking represents a promising method for high-precision radiotherapy to moving tumors. In the present study, we report on the integration of electromagnetic real-time tumor position monitoring into a multileaf collimator-based tracking system. The integrated system was characterized in terms of its geometric and radiologic accuracy. The former was assessed from portal images acquired during radiation delivery to a phantom in tracking mode. The tracking errors were calculated from the positions of the tracking field and of the phantom as extracted from the portal images. Radiologic accuracy was evaluated from film dosimetry performed for conformal and intensity-modulated radiotherapy applied to different phantoms moving on sinusoidal trajectories. A static radiation delivery to the nonmoving target served as a reference for the delivery to the moving phantom with and without tracking applied. Submillimeter tracking accuracy was observed for two-dimensional target motion despite the relatively large system latency of 500 ms. Film dosimetry yielded almost complete recovery of a circular dose distribution with tracking in two dimensions applied: 2%/2 mm gamma-failure rates could be reduced from 59.7% to 3.3%. For single-beam intensity-modulated radiotherapy delivery, accuracy was limited by the finite leaf width. A 2%/2 mm gamma-failure rate of 15.6% remained with tracking applied. The integrated system we have presented marks a major step toward the clinical implementation of high-precision dynamic multileaf collimator tracking. However, several challenges such as irregular motion traces or a thorough quality assurance still need to be addressed.

Real‐time tumor tracking: Automatic compensation of target motion using the Siemens 160 MLC

Advanced high quality radiation therapy techniques such as IMRT require an accurate delivery of precisely modulated radiation fields to the target volume. Interfractional and intrafractional motion of the patient's anatomy, however, may considerably deteriorate the accuracy of the delivered dose to the planned dose distributions. In order to compensate for these potential errors, a dynamic real-time capable MLC control system was designed. The newly developed adaptive MLC control system contains specialized algorithms which are capable of continuous optimization and correction of the aperture of the MLC according to the motion of the target volume during the dose delivery. The algorithms calculate the new leaf positions based on target information provided online to the system. The algorithms were implemented in a dynamic target tracking control system designed for a Siemens 160 MLC. To assess the quality of the new target tracking system in terms of dosimetric accuracy, experiments with various types of motion patterns using different phantom setups were performed. The phantoms were equipped with radiochromic films placed between solid water slabs. Dosimetric results of exemplary deliveries to moving targets with and without dynamic MLC tracking applied were compared in terms of the gamma criterion to the reference dose delivered to a static phantom. Our measurements indicated that dose errors for clinically relevant two-dimensional target motion can be compensated by the new control system during the dose delivery of open fields. For a clinical IMRT dose distribution, the gamma success rate was increased from 19% to 77% using the new tracking system. Similar improvements were achieved for the delivery of a complete IMRT treatment fraction to a moving lung phantom. However, dosimetric accuracy was limited by the system's latency of 400 ms and the finite leaf width of 5 mm in the isocenter plane. Different experimental setups representing different target tracking scenarios proved that the tracking concept, the new algorithms and the dynamic control system make it possible to effectively compensate for dose errors due to target motion in real-time. These early results indicate that the method is suited to increasing the accuracy and the quality of the treatment delivery for the irradiation of moving tumors.

Intensity modulated radiation therapy with irregular multileaf collimated field: A dosimetric study on the penumbra region with different leaf stepping patterns

Using a Varian 21 EX linear accelerator with a multileaf collimator (MLC) of 120 leaves, the penumbra regions of beam profiles within an irregular multileaf collimated fields were studied. MLC fields with different leaf stepping angles from 21.8 degrees to 68.2 degrees were used. Beam profiles in different directions: (1) along the cross-line and in-line axis, (2) along the leaf stepping edges of the field, and (3) parallel to the stepping edges but in the middle of the field, were measured and calculated using Kodak XV radiographic film and Pinnacle3 treatment planning system version 7.4f. These beam profiles were measured and calculated at source to axis distance= 100 cm with 5 cm of solid water slab on top. On the one hand, for both cross-line and in-line beam profiles, the penumbra widths of 20%-80% did not vary with the leaf stepping angles and were about 0.4 cm. On the other hand, the penumbra widths of 10%-90% of the above two profiles varied with the stepping angles and had maximum widths of about 1.9 cm (cross-line) and 1.65 cm (in-line) for stepping angles of 38.7 degrees and 51.3 degrees , respectively. For profiles crossing the "rippled" stepping edges of the field, the penumbra widths (10%-90%) at the regions between two opposite leaves (i.e., profile end at the Y1/Y2 jaw position) decreased with the stepping angles. At the penumbra regions between two leaf edges with the tongue-and-groove structure of the same bank (i.e., profile end at the X1/X2 jaw position), the penumbra widths increased with the stepping angles. When the penumbra widths were measured between two opposite leaf edges and at corners between two leaves, the widths first decreased with the stepping angles and then increased beyond the minimum width point at stepping angle of 45 degrees. The penumbra width (10%-90%) measured at the leaf edge was larger than that at the corner. For the beam profiles calculated using Pinnacle3, it is found that the results agreed well with the measurements along the cross-line and in-line axis, while there was a deviation for the profiles along the leaf stepping edge of the field compared to the film measurements. The measured results in this study can help us to understand the dosimetric effect of the leaf stepping (due to finite leaf width), tongue-and-groove and rounded leaf end structure in the penumbra region of an irregular MLC field. A more dedicated penumbra model can be developed for the treatment planning system.

Virtual micro MLC commissioning

The resolution of multileaf collimators (MLCs) is limited by their finite leaf width. A commercial package (HD‐270™) uses 3D couch translation and leaf adjustments to emulate smaller leaf widths. In this paper, we report on the commissioning of this feature using software testing, dial gauge measurements, and film dosimetry. We also identify a variety of limitations: software bugs and truncation artifacts, MLC leaf positioning uncertainties (random variations, systematic gantry dependence and backlash), and uncertainties in couch positioning. These reduce the capabilities of this implementation below that achievable theoretically.PACS numbers: 87.53.Kn, 87.53.Uv, 87.53.Xd

Improving the resolution of dynamic intensity modulated radiation therapy delivery by reducing the multileaf collimator sampling distance.

The conformality of a dose distribution delivered by a multileaf collimator (MLC) for intensity modulated radiation therapy (IMRT) is limited in the direction perpendicular to leaf motion by the finite leaf width. Two methods of improving the resolution of IMRT intensity maps in this direction were investigated. In the first, the desired fluence distribution is considered to be sampled by the MLC, with the sampling distance being the center-to-center distance between the MLC leaves. The sampling distance is reduced below the leaf width by combining separate irradiations with a couch shift between them. This has been applied to static field therapy [Galvin et al., Int. J. Radiat. Oncol., Biol., Phys. 35, 89-94 (1996)], and was proposed for IMRT by Bortfeld et al. [Med. Phys. 27, 2494-2502 (2000)]. In the second method, two MLC component fluences, with leaf width L = 2deltay and offset by deltay, are combined to reproduce desired intensity bins with deltay width. The effect of MLC leaf sampling distance on dose resolution was quantified for both 1.0 and 0.5 cm MLC leaf widths, utilizing a high resolution bar-pattern fluence, an annular shaped fluence, and an intensity step-edge. Improvement in resolution was found for the 1.0 cm leaf width at a sampling distance of 0.5 cm, with only a small benefit for further reduction. For the 0.5 cm leaf width, a sampling distance of 0.25 cm resulted in a dose resolution that was nearly independent of direction.

A dosimetric leaf-setting strategy for shaping radiation fields using a multileaf collimator.

A dosimetric leaf-setting strategy of using multileaf collimators (MLC) for shaping radiation fields has been developed. Existing MLC leaf-setting strategies are all based upon geometric criteria. This new approach, however, matches a prescribed field contour with a MLC using clinically consistent dosimetric criteria. The leaf positions are determined using an iterative optimization algorithm. An empirical dose model was developed to compare the dosimetric-based leaf-setting strategy with the geometric-based leaf-setting strategies. Differences up to half a centimeter in the leaf positions and isodose lines were found between setting the MLC geometrically and setting the MLC dosimetrically. The dosimetric leaf-setting strategy provides the ability to achieve better dose conformation for a clinically desired isodose line. Since the desired isodose line that covers a treatment volume is typically higher than 50% of the maximum dose, the scalloping effects due to the finite leaf width at the leaf edge or 50% isodose lines are much reduced. Another benefit of the dosimetric leaf-setting is that it separates the leaf-setting process from the treatment planning process, and this frees the treatment planning vendors from developing detailed dose models for various existing types and future upgrades of MLC systems.

A method of improving the spatial resolution of treatments that involve a multileaf collimator

In this paper we present a novel method of reducing the dosimetric effects of the finite leaf width of a multileaf collimator (MLC) in conformal and intensity modulated radiotherapy (IMRT). This is achieved by decomposing the required high-resolution fluence distribution into two orthogonal components, which are delivered with two leaf sweeps with head-twists differing by 90°. Before the decomposition stage, a filter is applied to the required beam to force it to have a constant gradient in the two delivery directions. The component deliveries were found to be very spiky in nature, resulting in very inefficient delivery with the scanning leaves of our MLC. This method was evaluated using film dosimetry of four idealized beams: a 45° edge, a circle, a hemispherical intensity modulated beam (IMB) and a sinc-like IMB. The measurements showed that this method had significantly reduced the effects of the 1 cm leaf width of our MLC at the 50% isodose level, but there was significant overdosage at the edge of the field and immediately inside the field edge. This method shows promise but further work is required before it may find clinical utility.

Comparison of a multi-leaf collimator with conformal blocks for the delivery of stereotactically guided conformal radiotherapy

Stereotactically-guided conformal radiotherapy is a practical technique for irradiating irregular lesions in the brain. The shaping of the conformal fields may be achieved using lead alloy blocks, a conventional multi-leaf collimator (MLC) or a mini/micro-MLC. Although the former gives more precise shaping, it is labour intensive. The latter methods are more practical as both mould room and treatment room times are reduced, but the shaping is limited by the finite leaf-width. This study compares treatment plans, in terms of normal tissue doses and tumour coverage, for fields shaped using conformal blocks and a conventional MLC in two series of geometrical shapes and nine patient tumours. For the range of tumour sizes considered (volumes 14–264 cm 3, minimum dimension 30 mm, maximum 102 mm), the MLC treats, on average, 14% (range 3–34%) and 17% (range 0–36%) more normal brain tissue than conformal blocks to >50% and >80% of the prescription dose, respectively. The large variability is due to strong dependence on tumour shape and the presence of partial leaf-widths in the MLC fit. It is therefore important to consider both of these effects when deciding whether the MLC is appropriate for a particular target volume.

Multifeaf collimation versus alloy blocks: Analysis of geometric accuracy

To compare the misalignment error due to the fabrication of custom lead alloy blocks with the displacement error introduced by the finite resolution of a multileaf collimator relative to the prescribed smooth apertures. Treatment field apertures for randomly selected patients for four clinical sites were obtained at various stages of the block fabrication process. These apertures and the corresponding multileaf collimator (MLC) apertures for each field were superimposed with the smooth apertures prescribed by physicians. The deviations from the prescribed apertures were measured at 10 degrees intervals. Comparisons of the magnitude and frequency of errors from block fabrication with those from the geometric displacements introduced by the finite leaf width of the multileaf collimator were made. The degree of conformity of the multileaf collimator is treatment-site dependent as, in general, are the shapes of fields. For three of the four sites examined, the multileaf collimator apertures track the prescribed apertures at least as accurately as custom blocking when the block design, construction, mounting, and alignment on the treatment machine are considered. The geometric conformality of multileaf collimation is comparable to, and in some cases superior to, that of custom blocks.