Fluence Matrix Research Articles

A leaf sequencing algorithm for an orthogonal dual-layer multileaf collimator.

Purpose. Dual layer MLC (DMLC) has have been adopted in several commercial products and one major challenge in DMLC usage is leaf sequencing for intensity-modulated radiation therapy (IMRT). In this study we developed a leaf sequencing algorithm for IMRT with an orthogonal DMLC.Methods and Materials. This new algorithm is inspired by the algorithm proposed by Dai and Zhu for IMRT with single layer MLC (SMLC). It iterately determines a delivery segment intensity and corresponding segment shape for a given fluence matrix and leaves residual fluence matrix to following iterations. The segment intensity is determined according to complexities of residual fluence matrix when segment intensity varies from one to highest level in the matrix. The segment intensity and corresponding segment shape that result least complexity was selected. Although the algorithm framework is similar to Dai and Zhu's algorithm, this new algorithm develops complexity algorithms along with rules for determining segment leaf settings when delivered with orthogonal DMLC. This algorithm has been evaluated with 9 groups of randomly generated fluence matrices with various dimensions and intensity levels. Sixteen fluence matrices generated in Pinnacle system for two clinical IMRT examples were also used for evaluation. Statistical information of leaf sequences generated with this algorithm for both the random and clinical matrices were compared to the results of two typical algorithms for SMLC: that proposed by Dai and Zhu and that proposed by Bortfled.Results. Compared to the SMLC delivery sequences generated with Dai and Zhu's algorithm, the proposed algorithm for orthogonal DMLC delivery reduces the average number of segments by 27.7% ∼ 41.8% for 9 groups of randomly generated fluence matrices and 10.5% ∼ 41.7% for clinical ones. When comparing MU efficiency between different algorithms, it is observed that the proposed algorithm performs better than the optimal efficiency of SMLC delivery when dealing with simple fluence matrices, but slightly worse when handling complex ones.Conclusion. This new algorithm generates leaf sequences for orthogonal DMLC delivery with high delivery efficiency in terms of number of leaf segments. This algorithm has potential to work well with orthogonal DMLC for improving efficiency or quality of IMRT.

Impact of depth-dependent optical attenuation on wavelength selection for spectroscopic photoacoustic imaging

An optical wavelength selection method based on the stability of the absorption cross-section matrix to improve spectroscopic photoacoustic (sPA) imaging was recently introduced. However, spatially-varying chromophore concentrations cause the wavelength- and depth-dependent variations of the optical fluence, which degrades the accuracy of quantitative sPA imaging. This study introduces a depth-optimized method that determines an optimal wavelength set minimizing an inverse of the multiplication of absorption cross-section matrix and fluence matrix to minimize the errors in concentration estimation. This method assumes that the optical fluence distribution is known or can be attained otherwise. We used a Monte Carlo simulation of light propagation in tissue with various depths and concentrations of deoxy-/oxy-hemoglobin. We quantitatively compared the developed and current approaches, indicating that the choice of wavelength is critical and our approach is effective especially when quantifying deeper imaging targets.

Dose calculation for hypofractionated volumetric-modulated arc therapy: approximating continuous arc delivery and tongue-and-groove modeling.

Hypofractionated treatments generally increase the complexity of a treatment plan due to the more stringent constraints of normal tissues and target coverage. As a result, treatment plans contain more modulated MLC motions that may require extra efforts for accurate dose calculation. This study explores methods to minimize the differences between in‐house dose calculation and actual delivery of hypofractionated volumetric‐modulated arc therapy (VMAT), by focusing on arc approximation and tongue‐and‐groove (TG) modeling. For dose calculation, the continuous delivery arc is typically approximated by a series of static beams with an angular spacing of 2°. This causes significant error when there is large MLC movement from one beam to the next. While increasing the number of beams will minimize the dose error, calculation time will increase significantly. We propose a solution by inserting two additional apertures at each of the beam angle for dose calculation. These additional apertures were interpolated at two‐thirds’ degree before and after each beam. Effectively, there were a total of three MLC apertures at each beam angle, and the weighted average fluence from the three apertures was used for calculation. Because the number of beams was kept the same, calculation time was only increased by about 6%‐8%. For a lung plan, areas of high local dose differences (>4%) between film measurement and calculation with one aperture were significantly reduced in calculation with three apertures. Ion chamber measurement also showed similar results, where improvements were seen with calculations using additional apertures. Dose calculation accuracy was further improved for TG modeling by developing a sampling method for beam fluence matrix. Single element point sampling for fluence transmitted through MLC was used for our fluence matrix with 1 mm resolution. For Varian HDMLC, grid alignment can cause fluence sampling error. To correct this, transmission volume averaging was applied. For three paraspinal HDMLC cases, the average dose difference was greatly reduced in film and calculation comparisons with our new approach. The gamma (3%, 3 mm) pass rates have improved significantly from 74.1%, 90.0%, and 90.4% to 99.2%, 97.9%, and 97.3% for three cases, for calculation without volume averaging and calculation with volume averaging, respectively. Our results indicate that more accurate MLC leaf position and transmission sampling can improve accuracy and agreement between calculation and measurement, and are particularly important for hypofractionated VMAT that consists of large MLC movement.PACS number(s): 87.55.kd

Evaluation of a new 3D in vivo dosimetry software using portal imaging for VMAT treatments of prostate: Preliminary results

Introduction/Objectives The control of VMAT treatments is a key issue in the Quality Assurance process. Current solutions are often cumbersome to implement and unsuitable for routine use. An innovative solution is proposed by the Dosimetry Check software (Math- Resolutions) allowing both to control pre-treatment as well as socalled transit dosimetry using the portal imager. Here we present the implementation and the evaluation of the software to control VMAT treatments of prostate. Material and methods Acquisition where made on a Clinac 2100C equipped with an IaS500. Images, acquired in continuous mode (@60 frames/rotation), CT scanner and associated structures and 3D dose matrix are imported in Dosimetry Check. Fluence matrix in air is obtained by filtered backprojection. The dose volume in the patient is calculated with Pencil Beam algorithm. Analysis is based on the comparison of point dose, dose profiles, and dose distributions in 2D and 3D. The calculation of Dose Volume Histogram and Gamma Index is also available. Results and conclusion The study focuses on 10 cases of prostate cancer treated in VMAT according to simultaneous integrated boost. The deviation of measured dose at the isocenter in pre-treatment and in transit mode is of the order of −4.0%(±1.3%) and 4.9%(±4.1%) respectively. These results improve significantly when corrected by the acquisition of an integrated image: +1.4%(±0.7%) in pre-treatment and −1.1%(±0.7%) in transit mode. Dosimetry Check software is a suitable tool to do pre-treatment control and in vivo dosimetry. The first results are satisfactory and we wish to test it on other sites, but weaknesses of Pencil Beam algorithm have to be taken into account.

Fixed number of segments in unidirectional decompositions of fluence matrices for step-and-shoot IMRT

The decomposition of a fluence matrix in step-and-shoot mode for intensity-modulated radiation therapy (IMRT) usually yields a large number of segments (NS) and, consequently, treatment time is substantially increased. In this paper, we propose a method for reducing the original NS in multileaf collimator segmentations to a user-specified quantity. The proposed method clusters original segments into the same number of groups as desired NS, and computes for each group an equivalent segment and an associated weight. In order to avoid important changes in dose–volume histograms (DVHs), equivalent segments and weights are computed taking into account the original fluence matrix and preserving the highest fluence zones, thus staying as close as possible to the original planned radiation. The method is applicable to unidirectional segmentations, where there is no backtracking of leaves, since this property facilitates the grouping of segments. The experiments showed that treatment times can be considerably reduced, while maintaining similar DVHs and dosimetric indexes. Furthermore, the algorithm achieved an excellent reduction/dose-quality ratio since the final NS was close to that reported for direct step-and-shoot solutions.

A minimum cost flow formulation for approximated MLC segmentation

Shape matrix decomposition is a subproblem in radiation therapy planning. A given fluence matrix A has to be written as a sum of shape matrices corresponding to homogeneous fields that can be shaped by a multileaf collimator. We solve the problem of finding an approximation B of A satisfying prescribed upper and lower bounds for each entry. The approximation B is determined such that the corresponding fluence can be realized with a prescribed delivery time using a multileaf collimator with an interleaf collision constraint, and under this condition the distance between A and B is minimized. © 2010 Wiley Periodicals, Inc. NETWORKS, Vol. 57(2), 135–140 2011

Reducing the number of segments in unidirectional MLC segmentations

In intensity-modulated radiation therapy (IMRT), fluence matrices obtained from a treatment planning system are usually delivered by a linear accelerator equipped with a multileaf collimator (MLC). A segmentation method is needed for decomposing these fluence matrices into segments suitable for the MLC, and the number of segments used is an important factor for treatment time. In this work, an algorithm for reduction of the number of segments (NS) is presented for unidirectional segmentations, where there is no backtracking of the MLC leaves. It uses a geometrical representation of the segmentation output for searching the key values in a fluence matrix that complicate its decomposition. The NS reduction is achieved by performing minor modifications in these values, under the conditions of avoiding substantial modifications of the dose–volume histogram, and does not increase in average the total number of monitor units delivered. The proposed method was tested using two clinical cases planned with the PCRT 3D® treatment planning system.

An energy fluence‐convolution model for amorphous silicon EPID dose prediction

In this work, an amorphous silicon electronic portal imaging device (a-Si EPID) dose prediction model based on the energy fluence model of the Pinnacle treatment planning system Version 7 (Philips Medical Systems, Madison, WI) is developed. An energy fluence matrix at very high resolution (< 1 mm) is used to incorporate multileaf collimator (MLC) leaf effects in the predicted EPID images. The primary dose deposited in the EPID is calculated from the energy fluence using experimentally derived radially dependent EPID interaction coefficients. Separate coefficients are used for the open beam energy fluence component and the component of the energy fluence transmitted through closed MLC leaves to each EPID pixel. A spatially invariant EPID dose deposition kernel that describes both radiative dose deposition, central axis EPID backscatter, and optical glare is convolved with the primary dose. The kernel is further optimized to give accurate EPID penumbra prediction and EPID scatter factor with changing MLC field size. An EPID calibration method was developed to reduce the effect of nonuniform backscatter from the support arm (E-arm) in a calibrated EPID image. This method removes the backscatter component from the pixel sensitivity (flood field) correction matrix retaining only field-specific backscatter in the images. The model was compared to EPID images for jaw and MLC defined open fields and eight head and neck intensity modulated radiotherapy (IMRT) fields. For the head and neck IMRT fields with 2%, 2 mm criteria 97.6 +/- 0.6% (mean +/- 1 standard deviation) of points passed with a gamma index less than 1, and for 3%, 3 mm 99.4 +/- 0.4% of points were within the criteria. For these fields, the 2%, 2 mm pass score reduced to 96.0 +/- 1.5% when backscatter was present in the pixel sensitivity correction matrix. The model incorporates the effect of MLC leaf transmission, EPID response to open and MLC leakage dose components, and accurately predicts EPID images of IMRT fields. Removing the backscatter component of the pixel sensitivity matrix correction reduces the effect of nonuniform E-arm backscatter.

The complexity of minimizing the number of shape matrices subject to minimal beam-on time in multileaf collimator field decomposition with bounded fluence

The use of multileaf collimators (MLCs) is a modern way to realize intensity modulated fields in radiotherapy. An important step in the treatment planning is the shape matrix decomposition: the desired fluence distribution, given by an integer matrix, has to be decomposed into a small number shape matrices, i.e. ( 0 , 1 ) -matrices corresponding to the field shapes that can be delivered by the MLC used. The two main objectives are to minimize the total irradiation time, and the number of shape matrices. Assuming that the entries of the fluence matrix are bounded by a constant, we prove that a shape matrix decomposition with minimal number of shape matrices under the condition that the total irradiation time is minimal, can be determined in time polynomial in the matrix dimensions. The results of our algorithm are compared with Engel’s [K. Engel, A new algorithm for optimal multileaf collimator field segmentation, Discrete Appl. Math. 152 (1–3) (2005) 35–51.] heuristic for the reduction of the number of shape matrices.

SU‐GG‐T‐129: A High Resolution Fluence‐Convolution Model for Prediction of Amorphous Silicon EPID IMRT Images

In this work an amorphous silicon electronic portal imaging device (a‐Si EPID) dose prediction model based on the fluence model of the Pinnacle treatment planning system Version 7 (Philips Medical Systems, Madison, WI, USA) is developed. A fluence matrix at very high resolution (0.5 mm) is used to incorporate multileaf collimator (MLC) leaf transmission effects in the predicted EPID images. The primary dose deposited in the EPID is calculated from the fluence using experimentally derived radially dependent EPID interaction coefficients for the open and MLC transmitted fluence components. A spatially invariant EPID dose deposition kernel that describes both radiative dose deposition and optical scatter is convolved with the primary dose. The kernel is further optimised to give accurate EPID scatter factor with changing MLC field size. Model predictions were compared to a‐Si EPID images corrected for pixel sensitivity variation, support‐arm backscatter and calibrated to dose for various static jaw defined and MLC defined fields and a step and shoot intensity modulated radiotherapy (IMRT) field. For the static fields the model predicts EPID off‐axis ratio, penumbral shape as well as inter‐leaf leakage. For the IMRT field with a Gamma criteria of 2% and 2 mm, 97.2% of points had a Gamma index less than 1. We found that incorporating the difference in EPID response to open and MLC transmission did not improve the accuracy of the prediction for the IMRT field. The developed model incorporates the effects of MLC design on the dose and therefore should improve the verification of IMRT treatments with EPIDs.

WE‐C‐M100F‐08: Use of Adaptive Filtering and Relative Intensity Limits in Deliverable IMRT Optimization to Improve Delivery Efficiency

Purpose: To investigate the effect of adaptive filtering, relative intensity limits, and fluence matrix resolution in deliverable IMRT optimization on MU efficiency and plan quality for dynamic MLC delivery. Method and Materials: An adaptive filtering method that filters the optimized intensities as needed in combination with different relative intensity limits were used to reduce MUs needed to deliver the beams. The adaptive filtering parameters were adjusted so that it achieved the optimal results. A deliverable‐based optimization that incorporates beam‐delivery constraints directly into the IMRT optimization process was used. H&amp;N and prostate cases were used to evaluate the quality and complexity of each plan. Each case was optimized with and without adaptive filtering and in combination with a series of relative intensity limits for fluence matrix resolution of 1.5 mm to 6 mm. The IMRT plans were evaluated in terms of dose‐volume statistics, MUs, and the complexity of fluence matrix resolution. Results: Up to 47.5% reductions in MUs were achieved using adaptive filtering in combination with relative intensity limits. When the adaptive filtering is on, the use of a relative intensity limit of 1.5 further reduced the total MUs by 6.3% and 15.9% for a fluence matrix resolution of 3‐ and 6mm respectively. Adaptive filtering in combination with relative intensity limits was able to reduce the total MUs without significantly changing the prostate and nodal PTV coverages. The changes in critical structure doses were negligible. The similar results have been obtained for the H&amp;N case included in the study. Conclusion: The adaptive filtering was able to significantly reduce total MUs without compromising the plan quality for deliverable IMRT optimization process. The use of relative intensity limits in combination with adaptive filtering did not have a significant effect on MU efficiency as compared to the use of adaptive filtering alone.

SU‐CC‐J‐6C‐06: A Generalized Field Splitting Algorithm for Optimal IMRT Delivery Efficiency

Purpose: The purpose of this work is to develop an algorithm for optimally splitting a large intensity‐modulated field for delivery with a MLC into two or more adjacent subfields that maximizes MU efficiency, and to provide mathematical proofs that the algorithm is optimal in the most general cases. Method and Materials: The field split can be stated as the following mathematical problem: given a fluence matrix which exceeds the largest field size limitation of the delivery system, find two or three subfields, each of which satisfies the field size limitation constraint, that combine to give the original fluence map, and with the additional constraint that the sum of the delivered MUs of the subfields be minimized. In this general formalism, the subfields can overlap and the field split does not have to be in a straight line. We first construct an optimal MU leaf sequence for the large field ignoring the width constraint. The optimal field split is then generated by appropriate partitioning of the optimal leaf sequence into leaf sequences for the subfields. The overlapping region of the subfields creates a natural feathering area which is clinically desirable. Results: Compared to a simple field splitting that cuts through the center of a fluence map, our algorithm showed an average decrease in total MU of about 19% on 32 clinical fluence maps with the largest decrease in total MU of 45%. In many cases, the total MU of the split fields does not increase from the MU of the original sequence when the width constraint is ignored. Conclusion: We have developed an algorithm that solves the most general version of the field splitting problem for large intensity modulated fields. We provide rigorous mathematical proofs that the proposed algorithm for field splitting is optimal in MU efficiency.

SU-FF-T-76: Implementation and Initial Testing of a Monte Carlo Based Algorithm for IMRT Inverse Treatment Planning

Purpose: To report on the implementation of a Monte Carlo (MC) based algorithm and to compare this system with a convolution/superposition-based algorithm (CS) for IMRT inverse planning. Method and Materials: The DPM MC code was modified using a fluence matrix approach to perform beamlet calculations for IMRT planning. The code was integrated within our in-house inverse treatment planning system and compared with the TPS (CS) algorithm. Initial testing involved the computation of 6 MV beamlet depth doses for 1×1, 2×2 and 10×10 (100, 1 cm beamlets) in a water phantom. MC and CS calculations were then performed for an example lung treatment plan to examine dosimetric differences between these algorithms. MC statistical uncertainties were on average less than 2% (in the depth doses) for all beamlet calculations. Optimization of beamlet doses is carried out using simulated annealing with quadratic cost functions derived from our clinical protocols. Results: Beamlet depth doses calculated with MC and CS are in good absolute agreement for field sizes larger than 2×2 cm2. Significant differences exist for 1×1 beamlets because CS is unable to accurately model lateral electron transport. For the example lung plan, much smaller differences were found. This is likely due to the fact that with larger field sizes (∼10×10 cm in the example), effects of lateral electron scattering are much less pronounced. Conclusion: We have implemented a fluence matrix method to perform MC-based beamlet calculations for IMRT planning. Initial testing for an example lung plan and field sizes larger than 2×2, revealed good agreement between MC and CS. However, larger differences were found for 1×1 beamlets due to lateral electron transport issues. Testing is currently being performed for a variety of treatment plans, spanning a range of field sizes to thoroughly investigate dosimetric differences between MC and CS in IMRT planning. Supported by NIH P01-CA59827

233 poster Gaussian source convolution with non planar fluence matrix for photon output factor in air calculations

233 poster Gaussian source convolution with non planar fluence matrix for photon output factor in air calculations

Experimental verification of convolution/superposition photon dose calculations for radiotherapy treatment planning

This work describes an experimental verification of the two-photon dose calculation engines available on the Helax-TMS (version 6.1) commercial radiotherapy treatment planning system. The performance of the pencil beam convolution and the collapsed cone superposition algorithms was examined for 4, 6, 15 MV beams, under a range of clinically relevant irradiation geometries. Comparisons against measurements were carried out in terms of absolute dose, thus assessment of the accuracy of monitor unit (MU) calculations was also carried out. Results show that both algorithms agree with measurement to acceptable tolerance levels in most cases in homogeneous water-equivalent media irradiated under full scatter conditions. The collapsed cone algorithm slightly overestimates the penumbra width and this is mainly due to discretization effects of the fluence matrix. The accuracy of this algorithm strongly depends on the resolution of the patient density matrix. It is recommended that the density matrix voxel size used for dose calculations is less than 5 × 5 × 5 mm3. The dose in media irradiated under missing tissue geometry, or in the presence of low or high-density heterogeneities, is modelled best with the collapsed cone algorithm. This is of particular clinical interest in treatment planning of the breast and of the thorax. For these treatment sites, a retrospective study of treatment plans indicated in certain cases significant overestimation of the dose to the planning target volume when using the pencil beam convolution model.

Development and application of a multimodality inverse treatment planning system (in English)

The introduction of intensity-modulated radiation therapy(IMRT) opened new possibilities for the treatment planning process. Most of the existing treatment planning (TP) tools for IMRT are limited to photontreatments. To achieve a very high computational speed, simplified dose calculation algorithms are often used. Furthermore, approximations are made in the calculation of the fluence matrix. In this thesis an IMRT planning tool is developed that can be used with any available radiation modality and makes use of improved dose calculation algorithms. For the dose calculation engine, a method was implemented in which the dose deposited in every voxel by all beams is precalculated and, for later use, stored in a dose matrix. This enables the use of advanced dose calculation algorithms and the simultaneous optimization of different radiation modalities. The new TP system was applied to clinical patient data for proton and photonIMRT plans. For the delivery of the protonIMRT plans, two promising delivery techniques, “distal edge tracking” (DET) and a fully three-dimensional (3D) technique, were compared with each other. By using the DET technique dose distributions as good as for the 3D technique were achieved but the optimization time and the expected delivery time were reduced by a factor 10. No significant differences in the effect of organ motion and particle range uncertainties were observed for the techniques. The effect of the number of beams on the protonIMRTdose distribution was also studied. It was found that even with only five evenly spaced beams the DET dose distribution is significantly better than the photon distribution, and almost indistinguishable from the seven-beam DET plan.