Enhancement of IMRT delivery through MLC rotation

Multileaf collimator (MLC) based intensity modulated radiation therapy (IMRT) techniques are well established but suffer several physical limitations. Dosimetric spatial resolution is limited by the MLC leaf width; interleaf leakage and tongue-and-groove effects degrade dosimetric accuracy and the range of leaf motion limits the maximum deliverable field size. Collimator rotation is used in standard radiation therapy to improve the conformity of the MLC shape to the target volume. Except for opposed orthogonal fields, collimator rotation has not been exploited in IMRT due to the complexity of deriving the MLC leaf configurations for rotated sub-fields. Here we report on a new way that MLC-based IMRT is delivered which incorporates collimator rotation, providing an extra degree of freedom in deriving leaf sequences for a desired fluence map. Specifically, we have developed a series of unique algorithms that are capable of determining rotated MLC segments. These IMRT fields may be delivered statically (with the collimator rotating to a new position in between sub-fields) or dynamically (with the collimator rotating and leaves moving simultaneously during irradiation). This introductory study provides an analysis of the rotating leaf motion calculation algorithms with focus on radiation efficiency, the range of collimator rotation and number of segments. We then evaluate the technique by characterizing the ability of the algorithms to generate rotating leaf sequences for desired fluence maps. Comparisons are also made between our method and conventional sliding window and step-and-shoot techniques. Results show improvements in spatial resolution, reduced interleaf effects and maximum deliverable field size over conventional techniques. Clinical application of these enhancements can be realized immediately with static rotational delivery although improved dosimetric modelling of the MLC will be required for dynamic delivery.     

The impact of fluctuations in intensity patterns on the number of monitor units and the quality and accuracy of intensity modulated radiotherapy

The purpose of this work is to examine the potential impact of the frequency and amplitude of fluctuations ("complexity") in intensity distributions on intensity-modulated radiotherapy (IMRT) dose distributions. The intensity-modulated beams are efficiently delivered using a multileaf collimator (MLC). Radiation may be delivered through a continuous (dynamic mode) or discrete (step-and-shoot) sequence of windows formed by the leaves. Algorithms and software that convert optimized intensity distributions into leaf trajectories apply approximate empirical corrections to account for the various effects associated with MLC characteristics, such as the rounded leaf tips, tongue-and-groove leaf design, leaf transmission, leaf scatter, and collimator scatter upstream from the MLC. Typically, the difference between inter- and intraleaf transmissions is ignored. In this paper, using a schematic example of IMRT for head and neck carcinomas, we demonstrate that complex anatomy and severe optimization constraints produce complex intensity patterns. Using idealized intensity patterns we also demonstrate that, for complex intensity patterns, the average window width tends to be smaller and, for the same dose received by the tumor, the number of MUs is larger. We found that as the complexity increases, so does the contribution of radiation transmitted through and scattered from the leaves ("indirect radiation") to the total delivered dose. As a consequence, the lowest deliverable intensity in complex intensity patterns may be significantly greater than that required to provide adequate protection for some normal tissues. Furthermore, since corrections for leaf transmission and scatter effects are approximate and the difference between inter- and intraleaf transmission is ignored, the accuracy of the delivered dose may be affected. Using the results of a simple experiment and a typical intensity-modulated beam for a head and neck case as examples, we show the effect of window width and complexity on the accuracy and deliverability of intensity patterns. Some possible strategies for improving the accuracy and for relaxing the lower limit on deliverable intensity are discussed.     

Implementing multiple static field delivery for intensity modulated beams.

A clinically oriented two-dimensional intensity-modulated beam delivery method is implemented using multiple static segmented fields, i.e., the "step-and-shoot" approach. Starting with a desired al" intensity distribution, it creates a multiple-level intensity approximation, and then constructs a sequence of segmented fields to deliver the multiple-level intensities using multileaf collimator (MLC) and independent backup jaws. The approach starts with a simple grouping of all the nonzero intensity values into a minimum number of clusters for a user specified deviation tolerance for the ideal plan. The k-means clustering algorithm is then employed to find the optimal levels of intensity that minimize the discrepancies between the ideal and the approximated intensities, without violating the user specified deviation tolerance. The multiple-level intensities are then decomposed into a sequence of machine deliverable segments. Apart from the first segment for each gantry angle, all the other segments are arranged to minimize the total travel distance of the leaves. The first segment covers the entire irradiated area and is used for treatment verification by electronic portal imaging. The implementation issues due to the physical constraints of the MLCs are also addressed.     

Minimizing the number of segments in a delivery sequence for intensity-modulated radiation therapy with a multileaf collimator.

This paper proposes a sequencing algorithm for intensity-modulated radiation therapy with a multileaf collimator in the static mode. The algorithm aims to minimize the number of segments in a delivery sequence. For a machine with a long verification and recording overhead time (e.g., 15 s per segment), minimizing the number of segments is equivalent to minimizing the delivery time. The proposed new algorithm is based on checking numerous candidates for a segment and selecting the candidate that results in a residual intensity matrix with the least complexity. When there is more than one candidate resulting in the same complexity, the candidate with the largest size is selected. The complexity of an intensity matrix is measured in the new algorithm in terms of the number of segments in the delivery sequence obtained by using a published algorithm. The beam delivery efficiency of the proposed algorithm and the influence of different published algorithms used to calculate the complexity of an intensity matrix were tested with clinical intensity-modulated beams. The results show that no matter which published algorithm is used to calculate the complexity of an intensity matrix, the sequence generated by the algorithm proposed here is always more efficient than that generated by the published algorithm itself. The results also show that the algorithm used to calculate the complexity of an intensity matrix affects the efficiency of beam delivery. The delivery sequences are frequently most efficient when the algorithm of Bortfeld et al. is used to calculate the complexity of an intensity matrix. Because no single variation is most efficient for all beams tested, we suggest implementing multiple variations of our algorithm.     

Investigating the fundamentals of IMRT decomposition using ten simple collimator models

The fundamentals of IMRT collimation have been studied using ten conceptual collimators. Spanning a range of complexities from the LINAC jaws alone to a full multi-leaf collimator (MLC), these collimators were designed with two abilities in mind: (1) to be able to define arbitrary field shapes, and (2) to be able to irradiate multiple, disconnected regions in a single segment. The collimators were tested by finding decompositions of random and clinical intensity-modulated beams (IMBs), and collimator performance was measured using both the number of segments required to complete the IMB and the monitor-unit efficiency of the treatment. The decompositions were run on 10 x 10 IMBs with integer bixel values randomly between 1 and 10, and clinical IMBs of varying sizes from lung, head and neck, and pelvic patients taken from a Pinnacle treatment-planning system. Results confirmed that although treatment performance improves with increased collimator complexity, it is not solely dependent on the number of segment shapes deliverable by the collimator but instead on how well these shapes lend themselves to IMRT delivery.     

A new algorithm for determining collimator angles that favor efficiency in MLC based IMRT delivery

A new algorithm to determine collimator angles that favor delivery efficiency of intensity modulated radiotherapy plans was developed. It was found that the number of segments and monitor units (MUs) were largely reduced with the set of collimator angles determined with the new algorithm without compromising plan quality. The improvement of delivery efficiency using the new algorithm depends on the size and shape of the target(s), the number of modulation levels, and the type of leaf-sequencing algorithm. In a typical prostate case, when a sweeping leaf-sequencer is used for Varian 120 leaf (0.5 x 0.5 cm2 beamlet), 80 leaf (1 x 1 cm2 beamlet) and Elekta 40 leaf (1 x 1 cm2 beamlet), the number of segments was reduced by 42%, 29%, and 5%, respectively. The number of MUs was reduced by 41%, 35%, and 10%. For the Siemens MLC (IMFAST leaf sequencer, 1 x 1 cm2 beamlet) the segment reduction was 32% and the MU reduction was 14%. Comparison of the plans using the new and Brahme algorithms, in terms of target conformity index and dose volume histogram of the organs at risk, showed that the quality of the plans using the new algorithm was uncompromised. Similar results were obtained for a set of head and neck treatment plans.     

Leaf position error during conformal dynamic arc and intensity modulated arc treatments

Conformal dynamic arc (CD-ARC) and intensity modulated arc treatments (IMAT) are both treatment modalities where the multileaf collimator (MLC) can change leaf position dynamically during gantry rotation. These treatment techniques can be used to generate complex isodose distributions, similar to those used in fix-gantry intensity modulation. However, a beam-hold delay cannot be used during CD-ARC or IMAT treatments to reduce spatial error. Consequently, a certain amount of leaf position error will have to be accepted in order to make the treatment deliverable. Measurements of leaf position accuracy were taken with leaf velocities ranging from 0.3 to 3.0 cm/s. The average and maximum leaf position errors were measured, and a least-squares linear regression analysis was performed on the measured data to determine the MLC velocity error coefficient. The average position errors range from 0.03 to 0.21 cm, with the largest deviations occurring at the maximum achievable leaf velocity (3.0 cm/s). The measured MLC velocity error coefficient was 0.0674 s for a collimator rotation of 0 degrees and 0.0681 s for a collimator rotation of 90 degrees. The distribution in leaf position error between the 0 degrees and 90 degrees collimator rotations was within statistical uncertainty. A simple formula was developed based on these results for estimating the velocity-dependent dosimetric error. Using this technique, a dosimetric error index for plan evaluation can be calculated from the treatment time and the dynamic MLC leaf controller file.     

Comparison of algorithms for multileaf collimator field segmentation

In the "stop and shoot" method of intensity modulated radiation therapy, it is desirable to use an efficient multileaf collimator (MLC) field segmentation algorithm in the sense that it translates beam intensity maps into the least number of MLC field segments. In this work, we compare the performance of eight different algorithms, including the ones by Bortfeld et al., Galvin et al., Xia and Verhey, the Siemens IMFAST algorithm, and four other algorithms which have not been studied before. We find that the algorithm of Xia and Verhey is most frequently the algorithm that needs the least MLC field segments. However, no single algorithm is the most efficient for all clinical cases or intensity maps. This suggests that it is desirable to have multiple algorithms available in a clinical treatment planning system which will search through all algorithms automatically and find the most efficient delivery sequence for a given treatment. Each intensity map in a treatment could be delivered by a different algorithm, whichever is the most efficient for that map. It is pointed out that when the background intensity level is not zero, it is not always efficient to deliver a segment to bring the background level down to zero.     

Algorithms for optimal sequencing of dynamic multileaf collimators

Dynamic multileaf collimator (DMLC) intensity modulated radiation therapy (IMRT) is used to deliver intensity modulated beams using a multileaf collimator (MLC), with the leaves in motion. DMLC-IMRT requires the conversion of a radiation intensity map into a leaf sequence file that controls the movement of the MLC while the beam is on. It is imperative that the intensity map delivered using the leaf sequence file be as close as possible to the intensity map generated by the dose optimization algorithm, while satisfying hardware constraints of the delivery system. Optimization of the leaf-sequencing algorithm has been the subject of several recent investigations. In this work, we present a systematic study of the optimization of leaf-sequencing algorithms for dynamic multileaf collimator beam delivery and provide rigorous mathematical proofs of optimized leaf sequence settings in terms of monitor unit (MU) efficiency under the most common leaf movement constraints that include leaf interdigitation constraint. Our analytical analysis shows that leaf sequencing based on unidirectional movement of the MLC leaves is as MU efficient as bi-directional movement of the MLC leaves.     

A leaf sequencing algorithm to enlarge treatment field length in IMRT

With MLC-based IMRT, the maximum usable field size is often smaller than the maximum field size for conventional treatments. This is due to the constraints of the overtravel distances of MLC leaves and/or jaws. Using a new leaf sequencing algorithm, the usable IMRT field length (perpendicular to the MLC motion) can be mostly made equal to the full length of the MLC field without violating the upper jaw overtravel limit. For any given intensity pattern, a criterion was proposed to assess whether an intensity pattern can be delivered without violation of the jaw position constraints. If the criterion is met, the new algorithm will consider the jaw position constraints during the segmentation for the step and shoot delivery method. The strategy employed by the algorithm is to connect the intensity elements outside the jaw overtravel limits with those inside the jaw overtravel limits. Several methods were used to establish these connections during segmentation by modifying a previously published algorithm (areal algorithm), including changing the intensity level, alternating the leaf-sequencing direction, or limiting the segment field size. The algorithm was tested with 1000 random intensity patterns with dimensions of 21 x 27 cm2, 800 intensity patterns with higher intensity outside the jaw overtravel limit, and three different types of clinical treatment plans that were undeliverable using a segmentation method from a commercial treatment planning system. The new algorithm achieved a success rate of 100% with these test patterns. For the 1,000 random patterns, the new algorithm yields a similar average number of segments of 36.9 +/- 2.9 in comparison to 36.6 +/- 1.3 when using the areal algorithm. For the 800 patterns with higher intensities outside the jaw overtravel limits, the new algorithm results in an increase of 25% in the average number of segments compared to the areal algorithm. However, the areal algorithm fails to create deliverable segments for 90% of these patterns. Using a single isocenter, the new algorithm provides a solution to extend the usable IMRT field length from 21 to 27 cm for IMRT on a commercial linear accelerator using the step and shoot delivery method.     

Hardware-sensitive optimization for intensity modulated radiotherapy

The multileaf collimator (MLC) hardware constraints are usually neglected in the process of intensity-modulated beam optimization. Consequently, it is not always possible to deliver planned beam modulation using dynamic MLC. Beam optimization is significantly diminished if the results must be approximated due to limitations imposed by the delivery device. To overcome this problem, an inverse beam optimization method which incorporates the hardware constraints has been developed. The hardware constraints, including the leaf velocity, the dose rate and the minimum required gap between opposing and adjacent leaves, were considered. An iterative search for feasible modulation was conducted alternately in the dosimetric space and the MLC position-time space. The optimization algorithm was designed for a unidirectional leaf trajectory and a constant dose rate. A scheme to reduce tongue-and-groove underdosage during optimization was also implemented. Comparisons were made between the solutions produced by this method and conventional optimization disregarding the hardware restrictions. The beam profiles generated by the conventional method were modified to satisfy the hardware specifications. The results indicate that inclusion of MLC constraints during optimization can improve the degree of conformity that is deliverable.     

Incorporating multi-leaf collimator leaf sequencing into iterative IMRT optimization

Intensity modulated radiation therapy (IMRT) treatment planning typically considers beam optimization and beam delivery as separate tasks. Following optimization, a multi-leaf collimator (MLC) or other beam delivery device is used to generate fluence patterns for patient treatment delivery. Due to limitations and characteristics of the MLC, the deliverable intensity distributions often differ from those produced by the optimizer, leading to differences between the delivered and the optimized doses. Objective function parameters are then adjusted empirically, and the plan is reoptimized to achieve a desired deliverable dose distribution. The resulting plan, though usually acceptable, may not be the best achievable. A method has been developed to incorporate the MLC restrictions into the optimization process. Our in-house IMRT system has been modified to include the calculation of the deliverable intensity into the optimizer. In this process, prior to dose calculation, the MLC leaf sequencer is used to convert intensities to dynamic MLC sequences, from which the deliverable intensities are then determined. All other optimization steps remain the same. To evaluate the effectiveness of deliverable-based optimization, 17 patient cases have been studied. Compared with standard optimization plus conversion to deliverable beams, deliverable-based optimization results show improved isodose coverage and a reduced dose to critical structures. Deliverable-based optimization results are close to the original nondeliverable optimization results, suggesting that IMRT can overcome the MLC limitations by adjusting individual beamlets. The use of deliverable-based optimization may reduce the need for empirical adjustment of objective function parameters and reoptimization of a plan to achieve desired results.     

Genetic algorithm based deliverable segments optimization for static intensity-modulated radiotherapy

The static delivery technique (also called step-and-shoot technique) has been widely used in intensity-modulated radiotherapy (IMRT) because of the simple delivery and easy quality assurance. Conventional static IMRT consists of two steps: first to calculate the intensity-modulated beam profiles using an inverse planning algorithm, and then to translate these profiles into a series of uniform segments using a leaf-sequencing tool. In order to simplify the procedure and shorten the treatment time of the static mode, an efficient technique, called genetic algorithm based deliverable segments optimization (GADSO), is developed in our work, which combines these two steps into one. Taking the pre-defined beams and the total number of segments per treatment as input, the number of segments for each beam, the segment shapes and weights are determined automatically. A group of interim modulated beam profiles quickly calculated using a conjugate gradient (CG) method are used to determine the segment number for each beam and to initialize segment shapes. A modified genetic algorithm based on a two-dimensional binary coding scheme is used to optimize the segment shapes, and a CG method is used to optimize the segment weights. The physical characters of a multileaf collimator, such as the leaves interdigitation limitation and leaves maximum over-travel distance, are incorporated into the optimization. The algorithm is applied to some examples and the results demonstrate that GADSO is able to produce highly conformal dose distributions using 20-30 deliverable segments per treatment within a clinically acceptable computation time.     

Virtual micro-intensity modulated radiation therapy

Virtual micro-intensity modulated radiation therapy (VMIMRT) combines a 10 x 5 mm2 intensity map with a 5 x 10 mm2 intensity map, delivered at orthogonal collimator settings. The superposition of these component maps (CM) yields a 5 x 5 mm2 virtual micro-intensity map (VMIM) that can be delivered with a 1 cm leaf width MLC. A pair of CMs with optimal delivery efficiency and quality must be chosen, since a given VMIM can be delivered using several different pairs. This is possible since, for each group of four VMIM cells that can be covered by an MLC leaf in either collimator orientation, the minimum intensity can be delivered from either collimator setting. By varying the proportions of the minimum values that go into each CM, one can simultaneously minimize the number of potential junction effects and the number of segments required to deliver the VMIM. The minimization is achieved by reducing high leaf direction gradients in the CMs. Several pseudoclinical and random VMIMs were studied to determine the applicability of this new technique. A nine level boost map was also studied to investigate dosimetric and spatial resolution issues. Finally, clinical issues for this technique are discussed.     

Using total-variation regularization for intensity modulated radiation therapy inverse planning with field-specific numbers of segments

Currently, there are two types of treatment planning algorithms for intensity modulated radiation therapy (IMRT). The beamlet-based algorithm generates beamlet intensity maps with high complexity, resulting in large numbers of segments in the delivery after a leaf-sequencing algorithm is applied. The segment-based direct aperture optimization (DAO) algorithm includes the physical constraints of the deliverable apertures in the calculation, and achieves a conformal dose distribution using a small number of segments. However, the number of segments is pre-fixed in most of the DAO approaches, and the typical random search scheme in the optimization is computationally intensive. A regularization-based algorithm is proposed to overcome the drawbacks of the DAO method. Instead of smoothing the beamlet intensity maps as in many existing methods, we include a total-variation term in the optimization objective function to reduce the number of signal levels of the beam intensity maps. An aperture rectification algorithm is then applied to generate a significantly reduced number of deliverable apertures. As compared to the DAO algorithm, our method has an efficient form of quadratic optimization, with an additional advantage of optimizing field-specific numbers of segments based on the modulation complexity. The proposed approach is evaluated using two clinical cases. Under the condition that the clinical acceptance criteria of the treatment plan are satisfied, for the prostate patient, the total number of segments for five fields is reduced from 61 using the Eclipse planning system to 35 using the proposed algorithm; for the head and neck patient, the total number of segments for seven fields is reduced from 107 to 28. The head and neck result is also compared to that using an equal number of four segments for each field. The comparison shows that using field-specific numbers of segments achieves a much improved dose distribution.     

Optimal leaf sequencing with elimination of tongue-and-groove underdosage

The individual leaves of a multileaf collimator (MLC) have a tongue-and-groove or stepped-edge design to minimize leakage radiation between adjacent leaves. This design element has a drawback in that it creates areas of underdosages in intensity-modulated photon beams unless a leaf trajectory is specifically designed such that for any two adjacent leaf pairs, the direct exposure under the tongue-and-groove is equal to the lower of the direct exposures of the leaf pairs. In this work, we present a systematic study of the optimization of a leaf sequencing algorithm for segmental multileaf collimator beam delivery that completely eliminates areas of underdosages due to tongue-and-groove or stepped-edge design of the MLC. Simultaneous elimination of tongue-and-groove effect and leaf interdigitation is also studied. This is an extension of our previous work (Kamath et al 2003a Phys. Med. Biol. 48 307) in which we described a leaf sequencing algorithm that is optimal for monitor unit (MU) efficiency under most common leaf movement constraints that include minimum leaf separation. Compared to our previously published algorithm (without constraints), the new algorithms increase the number of sub-fields by approximately 21% and 25%, respectively, but are optimal in MU efficiency for unidirectional schedules.     

Direct aperture optimization: a turnkey solution for step-and-shoot IMRT

IMRT treatment plans for step-and-shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf-sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach "direct aperture optimization." This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf-sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step-and-shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.     

Continuous intensity map optimization (CIMO): a novel approach to leaf sequencing in step and shoot IMRT

A new leaf-sequencing approach has been developed that is designed to reduce the number of required beam segments for step-and-shoot intensity modulated radiation therapy (IMRT). This approach to leaf sequencing is called continuous-intensity-map-optimization (CIMO). Using a simulated annealing algorithm, CIMO seeks to minimize differences between the optimized and sequenced intensity maps. Two distinguishing features of the CIMO algorithm are (1) CIMO does not require that each optimized intensity map be clustered into discrete levels and (2) CIMO is not rule-based but rather simultaneously optimizes both the aperture shapes and weights. To test the CIMO algorithm, ten IMRT patient cases were selected (four head-and-neck, two pancreas, two prostate, one brain, and one pelvis). For each case, the optimized intensity maps were extracted from the Pinnacle3 treatment planning system. The CIMO algorithm was applied, and the optimized aperture shapes and weights were loaded back into Pinnacle. A final dose calculation was performed using Pinnacle's convolution/superposition based dose calculation. On average, the CIMO algorithm provided a 54% reduction in the number of beam segments as compared with Pinnacle's leaf sequencer. The plans sequenced using the CIMO algorithm also provided improved target dose uniformity and a reduced discrepancy between the optimized and sequenced intensity maps. For ten clinical intensity maps, comparisons were performed between the CIMO algorithm and the power-of-two reduction algorithm of Xia and Verhey [Med. Phys. 25(8), 1424-1434 (1998)]. When the constraints of a Varian Millennium multileaf collimator were applied, the CIMO algorithm resulted in a 26% reduction in the number of segments. For an Elekta multileaf collimator, the CIMO algorithm resulted in a 67% reduction in the number of segments. An average leaf sequencing time of less than one minute per beam was observed.     

Effect of MLC leaf width on the planning and delivery of SMLC IMRT using the CORVUS inverse treatment planning system

This study investigates the influence of multileaf collimator (MLC) leaf width on intensity modulated radiation therapy (IMRT) plans delivered via the segmented multileaf collimator (SMLC) technique. IMRT plans were calculated using the Corvus treatment planning system for three brain, three prostate, and three pancreas cases using leaf widths of 0.5 and 1 cm. Resulting differences in plan quality and complexity are presented here. Plans calculated using a 1 cm leaf width were chosen over the 0.5 cm leaf width plans in seven out of nine cases based on clinical judgment. Conversely, optimization results revealed a superior objective function result for the 0.5 cm leaf width plans in seven out of the nine comparisons. The 1 cm leaf width objective function result was superior only for very large target volumes, indicating that expanding the solution space for plan optimization by using narrower leaves may result in a decreased probability of finding the global minimum. In the remaining cases, we can conclude that we are often not utilizing the objective function as proficiently as possible to meet our clinical goals. There was often no apparent clinically significant difference between the two plans, and in such cases the issue becomes one of plan complexity. A comparison of plan complexity revealed that the average 1 cm leaf width plan required roughly 60% fewer segments and over 40% fewer monitor units than required by 0.5 cm leaf width plans. This allows a significant decrease in whole body dose and total treatment time. For very complex IMRT plans, the treatment delivery time may affect the biologically effective dose. A clinically significant improvement in plan quality from using narrower leaves was evident only in cases with very small target volumes or those with concavities that are small with respect to the MLC leaf width. For the remaining cases investigated in this study, there was no clinical advantage to reducing the MLC leaf width from 1 to 0.5 cm. In such cases, there is no justification for the increased treatment time and whole body dose associated with the narrower MLC leaf width.     

Validation of dynamic MLC-controller log files using a two-dimensional diode array

Intensity-modulated radiation therapy (IMRT) delivered with multi-leaf collimator (MLC) in the step-and-shoot mode uses multiple static MLC segments to achieve intensity modulation. For typical IMRT treatment plans, significant numbers of segments are delivered with monitor units (MUs) of much less than 10. Verification of the ability of the linear accelerator (linac) to deliver small MU segments accurately is an important step in the IMRT commissioning and quality assurance (QA) process. Recent studies have reported large discrepancies between the intended and delivered segment MUs. These discrepancies could potentially cause large errors in the delivered patient dose. We have undertaken a systematic study to evaluate the accuracy of the dynamic MLC log files, which are created automatically by our commercial MLC workstation after each delivery, in recording the fractional MU delivered in the step-and-shoot mode. Two linac models were evaluated with simple-geometry leaf sequences and delivered with different total MUs and different nominal dose rates. A commercial two-dimensional diode array was used for the measurement. Large discrepancies between the intended and delivered segment MUs were found. The discrepancies were larger for small MU segments at higher dose rate, with some small MU segments completely undelivered. The recorded fractional MUs in the log files were found to agree with what was delivered within the limits of our experimental uncertainty. Our results indicate that it is important to verify the delivery accuracy of small MU segments that could potentially occur in a patient treatment and that the log files are useful in checking the integrity of the linac delivery once validated. Thus validated log files can be used as a QA tool for general IMRT delivery and patient-specific plan verification.