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

Search for IMRT inverse plans with piecewise constant fluence maps using compressed sensing techniques

An intensity-modulated radiation therapy (IMRT) field is composed of a series of segmented beams. It is practically important to reduce the number of segments while maintaining the conformality of the final dose distribution. In this article, the authors quantify the complexity of an IMRT fluence map by introducing the concept of sparsity of fluence maps and formulate the inverse planning problem into a framework of compressing sensing. In this approach, the treatment planning is modeled as a multiobjective optimization problem, with one objective on the dose performance and the other on the sparsity of the resultant fluence maps. A Pareto frontier is calculated, and the achieved dose distributions associated with the Pareto efficient points are evaluated using clinical acceptance criteria. The clinically acceptable dose distribution with the smallest number of segments is chosen as the final solution. The method is demonstrated in the application of fixed-gantry IMRT on a prostate patient. The result shows that the total number of segments is greatly reduced while a satisfactory dose distribution is still achieved. With the focus on the sparsity of the optimal solution, the proposed method is distinct from the existing beamlet- or segment-based optimization algorithms.

Major reduction of monitor units with the avoidance of leaf-sequencing step by direct aperture based IMRT optimisation

BACKGROUND AND PURPOSE

Compared with conventional 3D conformal radiotherapy (3D-CRT) the use of intensity-modulated radiation therapy (IMRT) has increased monitor units (MUs) in the delivery of prescribed dose to the patient and thus a potential risk of radiation-induced secondary cancer. Due to the elimination of the leaf-sequencing step in direct aperture based IMRT optimisation (DABO) the MUs in the beam delivery can be reduced. We compared MUs calculated by DABO with other IMRT techniques and 3D-CRT.

MATERIAL AND METHODS

Treatment plans for five head and neck cancer patients using dynamic IMRT technique (DMLC) and step-and-shoot (SMLC) technique (Varian Helios Cadplan), 3D-CRT (Varian Eclipse) and a home-made DABO were produced. The total number of MUs, dose coverage and standard deviation of prescribed dose in planning target volume (PTV) between different techniques were compared.

RESULTS

In all patients the PTV coverage and sparing of critical structures between the DABO, Helios DMLC and SMLC IMRT techniques was equivalent. Average MUs for beam delivery were 883 MU, 683 MU, 379 MU and 411 MU for DMLC, SMLC, DABO and 3D-CRT, respectively.

CONCLUSIONS

The DABO IMRT technique is able to produce treatment plans equivalent in target coverage, dose uniformity and normal tissue sparing compared with the commercial IMRT techniques. The number of MUs with DABO were reduced with a factor of 2.3 (DMLC) and 1.8 (SMLC). The study suggests that with DABO IMRT technique the risk of secondary cancer can be reduced without compromise in the quality of treatment plans.

A comparison of volumetric modulated arc therapy and conventional intensity-modulated radiotherapy for frontal and temporal high-grade gliomas

PURPOSE

Volumetric modulated arc therapy (VMAT), the predecessor to Varian's RapidArc, is a novel extension of intensity-modulated radiotherapy (IMRT) wherein the dose is delivered in a single gantry rotation while the multileaf collimator leaves are in motion. Leaf positions and the weights of field samples along the arc are directly optimized, and a variable dose rate is used. This planning study compared seven-field coplanar IMRT (cIMRT) with VMAT for high-grade gliomas that had planning target volumes (PTVs) overlapping organs at risk (OARs).

METHODS AND MATERIALS

10 previously treated patients were replanned to 60 Gy in 30 fractions with cIMRT and VMAT using the following planning objectives: 98% of PTV covered by 95% isodose without violating OAR and hotspot dose constraints. Mean OAR doses were maximally decreased without reducing PTV coverage or violating hotspot constraints. We compared dose-volume histogram data, monitor units, and treatment times.

RESULTS

There was equivalent PTV coverage, homogeneity, and conformality. VMAT significantly reduced maximum and mean retinal, lens, and contralateral optic nerve doses compared with IMRT (p < 0.05). Brainstem, chiasm, and ipsilateral optic nerve doses were similar. For 2-Gy fractions, mean monitor units were as follows: cIMRT = 789 +/- 112 and VMAT = 363 +/- 45 (relative reduction 54%, p = 0.002), and mean treatment times (min) were as follows: cIMRT = 5.1 +/- 0.4 and VMAT = 1.8 +/- 0.1 (relative reduction 65%, p = 0.002).

CONCLUSIONS

Compared with cIMRT, VMAT achieved equal or better PTV coverage and OAR sparing while using fewer monitor units and less time to treat high-grade gliomas.

Volumetric modulated arc therapy: planning and evaluation for prostate cancer cases

PURPOSE

To develop an optimization method using volumetric modulated arc therapy (VMAT) and evaluate VMAT plans relative to the standard intensity-modulated radiotherapy (IMRT) approach in prostate cancer.

METHODS AND MATERIALS

A single gantry rotation was modeled using 177 equispaced beams. Multileaf collimator apertures and dose rates were optimized with respect to gantry angle subject to dose-volume-based objectives. Our VMAT implementation used conjugate gradient descent to optimize dose rate, and stochastic sampling to find optimal multileaf collimator leaf positions. A treatment planning study of 11 prostate cancer patients with a prescription dose of 86.4 Gy was performed to compare VMAT with a standard five-field IMRT approach. Plan evaluation statistics included the percentage of planning target volume (PTV) receiving 95% of prescribed dose (V95), dose to 95% of PTV (D95), mean PTV dose, tumor control probability, and dosimetric endpoints of normal organs, whereas monitor unit (MU) and delivery time were used to assess delivery efficiency.

RESULTS

Patient-averaged PTV V95, D95, mean dose, and tumor control probability in VMAT plans were 96%, 82.6 Gy, 88.5 Gy, and 0.920, respectively, vs. 97%, 84.0 Gy, 88.9 Gy, and 0.929 in IMRT plans. All critical structure dose requirements were met. The VMAT plans presented better rectal wall sparing, with a reduction of 1.5% in normal tissue complication probability. An advantage of VMAT plans was that the average number of MUs (290 MU) was less than for IMRT plans (642 MU).

CONCLUSION

The VMAT technique can reduce beam on time by up to 55% while maintaining dosimetric quality comparable to that of the standard IMRT approach.

Parameter optimization in HN-IMRT for Elekta linacs

Planning and delivery in HN‐IMRT has been challenging for the Elekta linac because of numerous machine limitations. Direct aperture optimization (DAO) algorithms have had success in simplifying the planning process and improving plan quality. Commercial adaptations of DAO allow for widespread use in many clinics; however clinical validation of these methods is still needed. In this work we evaluated Pinnacle³ commercial software for HN‐IMRT on the Elekta linac. The purpose was to find a set of planning parameters that are applicable to most patients and optimal in terms of plan quality, delivery efficiency, and dosimetric accuracy. Four types of plans were created for each of 12 patients: ideal fluence optimization (FO), conventional two‐step optimization (TS), segment weight optimization (SW), and direct machine parameter optimization (DMPO). Maximum number of segments (NS) and minimum segment area (MSA) were varied in DMPO. Results showed DMPO plans have the best optimization scores and dosimetric indices, and the most consistent IMRT output among patients. At larger NS (≥80), plan quality decreases with increasing MSA as expected, except for MSA<8 cm2, suggesting presence of local minima in DMPO. Segment area and MUs can vary significantly between optimization methods and parameter settings; however, the quantity ‘integral MU’ remains constant. Irradiation time is linearly proportional to total plan segments, weakly dependent on MUs and independent of MSA. Dosimetric accuracy is independent of DMPO parameters. The superior quality of DMPO makes it the choice for HN‐IMRT on Elekta linacs and its consistency allows development of ‘class solutions’. However, planners should be aware of the local minima issue when pushing parameters to the limit such as NS<80 and MSA<8 cm2. The optimal set of parameters should be chosen to balance plan quality and delivery efficiency based on a systematic evaluation of the planning technique and system constraints.

PACS number: PACS: 87.55.D, 87.55.de

Planning and delivery of intensity-modulated radiation therapy

Intensity modulated radiation therapy (IMRT) is an advanced form of external beam radiation therapy. IMRT offers an additional dimension of freedom as compared with field shaping in three-dimensional conformal radiation therapy because the radiation intensities within a radiation field can be varied according to the preferences of locations within a given beam direction from which the radiation is directed to the tumor. This added freedom allows the treatment planning system to better shape the radiation doses to conform to the target volume while sparing surrounding normal structures. The resulting dosimetric advantage has shown to translate into clinical advantages of improving local and regional tumor control. It also offers a valuable mechanism for dose escalation to tumors while simultaneously reducing radiation toxicities to the surrounding normal tissue and sensitive structures. In less than a decade, IMRT has become common practice in radiation oncology. Looking forward, the authors wonder if IMRT has matured to such a point that the room for further improvement has diminished and so it is pertinent to ask what the future will hold for IMRT. This article attempts to look from the perspective of the current state of the technology to predict the immediate trends and the future directions. This article will (1) review the clinical experience of IMRT; (2) review what we learned in IMRT planning; (3) review different treatment delivery techniques; and finally, (4) predict the areas of advancements in the years to come.

Quality Control of Portal Imaging with PTW EPID QC PHANTOM

PURPOSE

Quality assurance (QA) and quality control (QC) of different electronic portal imaging devices (EPID) and portal images with the PTW EPID QC PHANTOM.

MATERIAL AND METHODS

Characteristic properties of images of different file formats were measured on Siemens OptiVue500aSi, Siemens BeamView Plus, Elekta iView, and Varian PortalVision and analyzed with the epidSoft 2.0 program in four radiation therapy centers. The portal images were taken with Kodak X-OMAT V and the Kodak Portal Localisation ReadyPack films and evaluated with the same program.

RESULTS

The optimal exposition both for EPIDs and portal films of different kind was determined. For double exposition, the 2+1 MU values can be recommended in the case of Siemens OptiVue500aSi Elekta iView and Kodak Portal Localisation ReadyPack films, while for Siemens BeamView Plus, Varian PortalVision and Kodak X-OMAT V film 7+7 MU is recommended.

CONCLUSION

The PTW EPID QC PHANTOM can be used not only for amorphous silicon EPIDs but also for images taken with a video-based system or by using an ionization chamber matrix or for portal film. For analysis of QC tests, a standardized format (used at the acceptance test) should be applied, as the results are dependent on the file format used.

Direct aperture optimization as a means of reducing the complexity of Intensity Modulated Radiation Therapy plans

Intensity Modulated Radiation Therapy (IMRT) is a means of delivering radiation therapy where the intensity of the beam is varied within the treatment field. This is done by dividing a large beam into many small beamlets. Dose constraints are assigned to both the target and sensitive structures and computerised inverse optimization is performed to find the individual weights of this large number of beamlets. The computer adjusts the intensities of these beamlets according to the required planning dose objectives. The optimized intensity patterns are then decomposed into a series of deliverable multi leaf collimator (MLC) shapes in the sequencing step.

One of the main problems of IMRT, which becomes even more apparent as the complexity of the IMRT plan increases, is the dramatic increase in the number of Monitor Units (MU) required to deliver a fractionated treatment. The difficulty with this increase in MU is its association with increased treatment times and a greater leakage of radiation from the MLCs increasing the total body dose and the risk of secondary cancers in patients. Therefore one attempts to find ways of reducing these MU without compromising plan quality.

The design of inverse planning systems where the beam is divided into small beamlets to produce the required intensity map automatically introduces complexity into IMRT treatment planning. Plan complexity is associated with many negative factors such as dosimetric uncertainty and delivery issues A large search space is required necessitating much computing power. However, the limitations of the delivery technology are not taken into consideration when designing the ideal intensity map therefore a further step termed the sequencing step is required to convert the ideal intensity map into a deliverable one.

Many approaches have been taken to reduce the complexity. These include setting intensity limits, putting penalties on the cost function and using smoothing filters Direct Aperture optimization (DAO) incorporates the limitations of the delivery technology at the initial design of the intensity map thereby eliminating the sequencing step. It also gives control over the number of segments and hence control over the complexity of the plan although the design of the segments is independent of the person preparing the plan.

Optimized removal of the tongue-and-groove underdose via constrained partial synchronization and variable depth recursion

Full synchronization (FS) leaf sequencing removes tongue-and-groove underdosages (TGU) but increases the number of segments. Constrained partial synchronization (CPS) uses a minimum tongue-and-groove ratio (TGR) to reduce the number of segments while achieving acceptable TGUs. TGR is the ratio of non-overlapping intensities that irradiate a common junction. For TGRs of 1, 1.5 and 2, the TGUs were measured as 18%, 4% and 0%, respectively, for a 6 MV beam and a Siemens 82 leaf MLC. The extraction and sweep processes of the variable depth recursion (VDR) leaf-sequencing algorithm were constrained to satisfy a minimum TGR. For a Siemens MLC and 15 clinical cases, VDR with a TGR = 1.5 produced 7% more segments than the unsynchronized VDR, while a fully synchronized sweeping window algorithm produces 62% more segments. For random intensity maps, VDR with CPS produced significantly fewer segments than an unsynchronized sweeping window. Similar results can be obtained for MLCs that interdigitate. This has implications for direct aperture optimization algorithms (DAO) that use the sweeping window as a starting point (Pinnacle), for which a significant TGU has been observed. The concept of CPS can be applied to DAO by choosing appropriate levels for each of the segments in DAO.

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.

A fiducial detection algorithm for real-time image guided IMRT based on simultaneous MV and kV imaging

The advantage of highly conformal dose techniques such as 3DCRT and IMRT is limited by intrafraction organ motion. A new approach to gain near real-time 3D positions of internally implanted fiducial markers is to analyze simultaneous onboard kV beam and treatment MV beam images (from fluoroscopic or electronic portal image devices). Before we can use this real-time image guidance for clinical 3DCRT and IMRT treatments, four outstanding issues need to be addressed. (1) How will fiducial motion blur the image and hinder tracking fiducials? kV and MV images are acquired while the tumor is moving at various speeds. We find that a fiducial can be successfully detected at a maximum linear speed of 1.6 cm/s. (2) How does MV beam scattering affect kV imaging? We investigate this by varying MV field size and kV source to imager distance, and find that common treatment MV beams do not hinder fiducial detection in simultaneous kV images. (3) How can one detect fiducials on images from 3DCRT and IMRT treatment beams when the MV fields are modified by a multileaf collimator (MLC)? The presented analysis is capable of segmenting a MV field from the blocking MLC and detecting visible fiducials. This enables the calculation of nearly real-time 3D positions of markers during a real treatment. (4) Is the analysis fast enough to track fiducials in nearly real time? Multiple methods are adopted to predict marker positions and reduce search regions. The average detection time per frame for three markers in a 1024 x 768 image was reduced to 0.1 s or less. Solving these four issues paves the way to tracking moving fiducial markers throughout a 3DCRT or IMRT treatment. Altogether, these four studies demonstrate that our algorithm can track fiducials in real time, on degraded kV images (MV scatter), in rapidly moving tumors (fiducial blurring), and even provide useful information in the case when some fiducials are blocked from view by the MLC. This technique can provide a gating signal or be used for intra-fractional tumor tracking on a Linac equipped with a kV imaging system. Any motion exceeding a preset threshold can warn the therapist to suspend a treatment session and reposition the patient.

Arc-modulated radiation therapy (AMRT): a single-arc form of intensity-modulated arc therapy

Arc-modulated radiation therapy (AMRT) is a novel rotational intensity-modulated radiation therapy (IMRT) technique developed for a clinical linear accelerator that aims to deliver highly conformal radiation treatment using just one arc of gantry rotation. Compared to fixed-gantry IMRT and the multiple-arc intensity-modulated arc therapy (IMAT) techniques, AMRT promises the same treatment quality with a single-arc delivery. In this paper, we present a treatment planning scheme for AMRT, which addresses the challenges in inverse planning, leaf sequencing and dose calculation. The feasibility and performance of this AMRT treatment planning scheme have been verified with multiple clinical cases of various sites on Varian linear accelerators.

Adaptive diffusion smoothing: a diffusion-based method to reduce IMRT field complexity

Inverse-planned intensity modulated radiation therapy (IMRT) is often able to achieve complex treatment planning goals that are unattainable with forward three-dimensional (3D) conformal planning. However, the common use of IMRT has introduced several new challenges. The potentially high degree of modulation in IMRT beams risks the loss of some advantages of 3D planning, such as excellent target coverage and high delivery efficiency. Previous attempts to reduce beam complexity by smoothing often result in plan degradation because the smoothing algorithm cannot distinguish between areas of desirable and undesirable modulation. The purpose of this work is to introduce and evaluate adaptive diffusion smoothing (ADS), a novel procedure designed to preferentially reduce IMRT beam complexity. In this method, a discrete diffusion equation is used to smooth IMRT beams using diffusion coefficients, automatically defined for each beamlet, that dictate the degree of smoothing allowed for each beamlet. This yields a method that can distinguish between areas of desirable and undesirable modulation. The ADS method has been incorporated into our optimization system as a weighted cost function penalty, with two diffusion coefficient definitions designed to promote: (1) uniform smoothing everywhere or (2) smoothing based on cost function gradients with respect to the plan beamlet intensities. The ADS method (with both coefficient types) has been tested in a phantom and in two clinical examples (prostate and head/neck). Both types of diffusion coefficients produce plans with reduced modulation and minimal dosimetric impact, but the cost function gradient-based coefficients show more potential for reducing beam modulation without affecting dosimetric plan quality. In summary, adaptive diffusion smoothing is a promising tool for ensuring that only the necessary amount of beam modulation is used, promoting more efficient and accurate IMRT planning, QA, and delivery.

Effect of radiation protraction in intensity-modulated radiation therapy with direct aperture optimization: a phantom study

The effect of radiation protraction in step-and-shoot IMRT is investigated for treatment plans created with the help of direct aperture optimization. The latter approach can be used during inverse planning for all clinical linear accelerators with conventional MLC. Direct aperture optimization significantly shortens fraction time for IMRT plans as compared to that for plans obtained by using the conventional inverse planning approach. By analyzing several IMRT plans obtained with direct aperture optimization we found that for alpha/beta ratio of 10 Gy (characteristic of fast growing tumors) the protraction effect is probably clinically insignificant for both conventional and large fraction sizes of 1.9 Gy and 5.7 Gy, respectively. For small alpha/beta of 1-1.5 Gy and conventional fraction size the effect of protraction is still small; however, this effect can be significant for hypofractionated treatments. Based on the obtained results it is recommended that, when possible, IMRT for slow growing prostate cancers be performed with small number of beams (e.g., 5) and small number of segments (e.g., 5-7 segments per beam) to reduce delivery time and, as a result, the associated effect of radiation protraction.

The effect of statistical noise on IMRT plan quality and convergence for MC-based and MC-correction-based optimized treatment plans

Monte Carlo (MC) is rarely used for IMRT plan optimization outside of research centres due to the extensive computational resources or long computation times required to complete the process. Time can be reduced by degrading the statistical precision of the MC dose calculation used within the optimization loop. However, this eventually introduces optimization convergence errors (OCEs). This study determines the statistical noise levels tolerated during MC-IMRT optimization under the condition that the optimized plan has OCEs <100 cGy (1.5% of the prescription dose) for MC-optimized IMRT treatment plans.Seven-field prostate IMRT treatment plans for 10 prostate patients are used in this study. Pre-optimization is performed for deliverable beams with a pencil-beam (PB) dose algorithm. Further deliverable-based optimization proceeds using: (1) MC-based optimization, where dose is recomputed with MC after each intensity update or (2) a once-corrected (OC) MC-hybrid optimization, where a MC dose computation defines beam-by-beam dose correction matrices that are used during a PB-based optimization. Optimizations are performed with nominal per beam MC statistical precisions of 2, 5, 8, 10, 15, and 20%. Following optimizer convergence, beams are re-computed with MC using 2% per beam nominal statistical precision and the 2 PTV and 10 OAR dose indices used in the optimization objective function are tallied. For both the MC-optimization and OC-optimization methods, statistical equivalence tests found that OCEs are less than 1.5% of the prescription dose for plans optimized with nominal statistical uncertainties of up to 10% per beam. The achieved statistical uncertainty in the patient for the 10% per beam simulations from the combination of the 7 beams is ~3% with respect to maximum dose for voxels with D>0.5D(max). The MC dose computation time for the OC-optimization is only 6.2 minutes on a single 3 Ghz processor with results clinically equivalent to high precision MC computations.

Multibeam tomotherapy: a new treatment unit devised for multileaf collimation, intensity-modulated radiation therapy

A fully integrated system for treatment planning, application, and verification for automated multileaf collimator (MLC) based, intensity-modulated, image-guided, and adaptive radiation therapy (IMRT, IGRT and ART, respectively) is proposed. Patient comfort, which was the major development goal, will be achieved through a new unit design and short treatment times. Our device for photon beam therapy will consist of a new dual energy linac with five fixed treatment heads positioned evenly along one plane but one electron beam generator only. A minimum of moving parts increases technical reliability and reduces motion times to a minimum. Motion is allowed solely for the MLCs, the robotic patient table, and the small angle gantry rotation of +/- 36 degrees. Besides sophisticated electron beam guidance, this compact setup can be built using existing modules. The flattening-filter-free treatment heads are characterized by reduced beam-on time and contain apertures restricted in one dimension to the area of maximum primary fluence output. In the case of longer targets, this leads to a topographic intensity modulation, thanks to the combination of "step and shoot" MLC delivery and discrete patient couch motion. Owing to the limited number of beam directions, this multislice cone beam serial tomotherapy is referred to as "multibeam tomotherapy." Every patient slice is irradiated by one treatment head at any given moment but for one subfield only. The electron beam is then guided to the next head ready for delivery, while the other heads are preparing their leaves for the next segment. The "Multifocal MLC-positioning" algorithm was programmed to enable treatment planning and optimize treatment time. We developed an overlap strategy for the longitudinally adjacent fields of every beam direction, in doing so minimizing the field match problem and the effects of possible table step errors. Clinical case studies show for the same or better planning target volume coverage, better organ-at-risk sparing, and comparable mean integral dose to the normal tissue a reduction in treatment time by more than 50% to only a few minutes in comparison to high-quality 3-D conformal and IMRT treatments. As a result, it will be possible to incorporate features for better patient positioning and image guidance, while sustaining reasonable overall treatment times at the same time. The virtual multibeam tomotherapy design study TOM'5-CT contains a dedicated electron beam CT (TOM'AGE) and an objective optical topometric patient positioning system (TOPOS). Thanks to the wide gantry bore of 120 cm and slim gantry depths of 70 cm, patients can be treated very comfortably, in all cases tumor-isocentrically, as well as with noncoplanar beam arrangements as in stereotactic radiosurgery with a couch rotation of up to +/- 54 degrees. The TOM'5 treatment unit on which this theoretical concept is based has a stand-alone depth of 40 cm and an outer diameter of 245 cm; the focus-isocenter distance of the heads is 100 cm with a field size of 40 cm x 7 cm and 0.5 cm leaves, which operate perpendicular to the axis of table motion.

Optimal starting gantry angles using equiangular-spaced beams with intensity modulated radiation therapy for prostate cancer on RTOG 0126: A clinical study of 5 and 7 fields

BACKGROUND AND PURPOSE

To investigate the effects of starting gantry angle and number of equiangular-spaced beams for prostate cancer radiotherapy on the Radiation Therapy Oncology Group (RTOG) 0126 protocol using intensity-modulated radiation therapy (IMRT).

MATERIALS AND METHODS

Ten localized prostate cancer patients were prescribed to 79.2Gy in 44 fractions. Static IMRT plans using five and seven equiangular-spaced beams were generated. The starting gantry angles were incremented by 5 degrees resulting in 15 (5 beams) and 11 (7 beams) plans per patient. Constant target coverage was ensured for all plans in order to isolate the variation in the rectal and bladder metrics as a function of starting gantry angle.

RESULTS

The variation with starting gantry angle in rectal metrics using 5 beams was statistically significant (p<0.001) with dosimetric importance. The 5-beam rectal V 75Gy and V 70Gy demonstrated a class solution with a characteristic 'W' pattern and two optimal starting gantry angles near 20 degrees and 50 degrees . Statistically insignificant differences were observed for the bladder metrics using 5 beams. There was little dosimetric variation in the rectal and bladder metrics with 7 beams. Nearly equivalent rectal V 75Gy was achieved between 5 optimal equiangular-spaced beams starting at 20 degrees (class solution) and 7 equiangular-spaced beams starting at 0 degrees for most patients.

CONCLUSIONS

The use of an optimal starting gantry angle for 5 equiangular-spaced beams, as indicated by a class solution in this study, will facilitate rectal sparing and can produce plans that are equivalent to those employing 7 equiangular-spaced beams.

Comparison of intensity modulated x-ray therapy and intensity modulated proton therapy for selective subvolume boosting: a phantom study

Selective subvolume boosting can theoretically improve tumour control probability while maintaining normal tissue complication probabilities similar to those of uniform dose distributions. In this work the abilities of intensity-modulated x-ray therapy (IMXT) and intensity-modulated proton therapy (IMPT) to deliver boosts to multiple subvolumes of varying size and proximities are compared in a thorough phantom study. IMXT plans were created using the step-and-shoot (IMXT-SAS) and helical tomotherapy (IMXT-HT) methods. IMPT plans were created with the spot scanning (IMPT-SS) and distal gradient tracking (IMPT-DGT) methods. IMPT-DGT is a generalization of the distal edge tracking method designed to reduce the number of proton beam spots required to deliver non-uniform dose distributions relative to IMPT-SS. The IMPT methods were delivered over both 180 degrees and 360 degrees arcs. The IMXT-SAS and IMPT-SS methods optimally satisfied the non-uniform dose prescriptions the least and the most, respectively. The IMPT delivery methods reduced the normal tissue integral dose by a factor of about 2 relative to the IMXT delivery methods, regardless of the delivery arc. The IMPT-DGT method reduced the number of proton beam spots by a factor of about 3 relative to the IMPT-SS method.

Comparison of direct machine parameter optimization versus fluence optimization with sequential sequencing in IMRT of hypopharyngeal carcinoma

Background

To evaluate the effects of direct machine parameter optimization in the treatment planning of intensity-modulated radiation therapy (IMRT) for hypopharyngeal cancer as compared to subsequent leaf sequencing in Oncentra Masterplan v1.5.

Methods

For 10 hypopharyngeal cancer patients IMRT plans were generated in Oncentra Masterplan v1.5 (Nucletron BV, Veenendal, the Netherlands) for a Siemens Primus linear accelerator.

For optimization the dose volume objectives (DVO) for the planning target volume (PTV) were set to 53 Gy minimum dose and 59 Gy maximum dose, in order to reach a dose of 56 Gy to the average of the PTV. For the parotids a median dose of 22 Gy was allowed and for the spinal cord a maximum dose of 35 Gy. The maximum DVO to the external contour of the patient was set to 59 Gy. The treatment plans were optimized with the direct machine parameter optimization ("Direct Step & Shoot", DSS, Raysearch Laboratories, Sweden) newly implemented in Masterplan v1.5 and the fluence modulation technique ("Intensity Modulation", IM) which was available in previous versions of Masterplan already. The two techniques were compared with regard to compliance to the DVO, plan quality, and number of monitor units (MU) required per fraction dose.

Results

The plans optimized with the DSS technique met the DVO for the PTV significantly better than the plans optimized with IM (p = 0.007 for the min DVO and p < 0.0005 for the max DVO). No significant difference could be observed for compliance to the DVO for the organs at risk (OAR) (p > 0.05). Plan quality, target coverage and dose homogeneity inside the PTV were superior for the plans optimized with DSS for similar dose to the spinal cord and lower dose to the normal tissue. The mean dose to the parotids was lower for the plans optimized with IM. Treatment plan efficiency was higher for the DSS plans with (901 ± 160) MU compared to (1151 ± 157) MU for IM (p-value < 0.05).

Renormalization of the IM plans to the mean of the dose to 95% of the PTV (D₉₅) of the DSS plans, resulted in similar target coverage and dose to the parotids for both strategies, at the cost of a significantly higher dose to the normal tissue and maximum dose to the target. The relative volume of the PTV receiving 107% or more of the prescription dose V₁₀₇ increased to 35.5% ± 20.0% for the IM plan as compared to a mean of 0.9% ± 0.9% for the DSS plan.

Conclusion

The direct machine parameter optimization is a major improvement compared to the fluence modulation with subsequent leaf sequencing in Oncentra Masterplan v1.5. The resulting dose distribution complies better with the DVO and better plan quality is achieved for identical specification of DVO. An additional asset is the reduced number of MU as compared to IM.

Development of an optimization concept for arc-modulated cone beam therapy

In this paper, we propose an optimization concept for a rotation therapy technique which is referred to as arc-modulated cone beam therapy (AMCBT). The aim is a reduction of the treatment time while achieving a treatment plan quality equal to or better than that of IMRT. Therefore, the complete dose is delivered in one single gantry rotation and the beam is modulated by a multileaf collimator. The degrees of freedom are the field shapes and weights for a predefined number of beam directions. In the new optimization loop, the beam weights are determined by a gradient algorithm and the field shapes by a tabu search algorithm. We present treatment plans for AMCBT for two clinical cases. In comparison to step-and-shoot IMRT treatment plans, it was possible by AMCBT to achieve dose distributions with a better dose conformity to the target and a lower mean dose for the most relevant organ at risk. Furthermore, the number of applied monitor units was reduced for AMCBT in comparison to IMRT treatment plans.

Postoperative Irradiation of Gynecologic Malignancies: Improving Treatment Delivery Using Aperture-Based Intensity-Modulated Radiotherapy

PURPOSE

To evaluate dosimetric and treatment delivery advantages of aperture-based intensity-modulated radiotherapy (AB-IMRT) for the treatment of patients receiving whole pelvic radiotherapy for gynecologic malignancies.

METHODS AND MATERIALS

Nineteen patients undergoing pelvic radiotherapy after resection of endometrial cancers were selected. A 45-Gy dose was prescribed to the target volume delineated on a planning CT scan. An in-house inverse planning system, Ballista, was used to develop a treatment plan using aperture-based multileaf collimator segments. This approach was compared with conventional four-field, enlarged four-field, and static beamlet-based IMRT (BB-IMRT) techniques in terms of target coverage, dose-volume histogram statistics for surrounding normal tissues, and numbers of segments and monitor units (MU).

RESULTS

Three quarters (76.4%) of the planning target volume received the prescription dose with conventional four-field plans. With adequate target coverage, the Ballista plans significantly reduced the volume of bowel and bladder irradiated at the prescribed dose (p < 0.001), whereas the two approaches provided equivalent results for the rectum (p = 0.5). On the other hand, AB-IMRT and BB-IMRT plans showed only small differences in dose-volume histogram statistics of unknown clinical impact, whereas Ballista plan delivery required on average 73% and 59% fewer segments and MU, respectively.

CONCLUSION

With respect to conventional techniques, AB-IMRT for the treatment of gynecologic malignancies provides dosimetric advantages similar to those with BB-IMRT but with clear treatment delivery improvements.

Direct aperture optimization for online adaptive radiation therapy

This paper is the first investigation of using direct aperture optimization (DAO) for online adaptive radiation therapy (ART). A geometrical model representing the anatomy of a typical prostate case was created. To simulate interfractional deformations, four different anatomical deformations were created by systematically deforming the original anatomy by various amounts (0.25, 0.50, 0.75, and 1.00 cm). We describe a series of techniques where the original treatment plan was adapted in order to correct for the deterioration of dose distribution quality caused by the anatomical deformations. We found that the average time needed to adapt the original plan to arrive at a clinically acceptable plan is roughly half of the time needed for a complete plan regeneration, for all four anatomical deformations. Furthermore, through modification of the DAO algorithm the optimization search space was reduced and the plan adaptation was significantly accelerated. For the first anatomical deformation (0.25 cm), the plan adaptation was six times more efficient than the complete plan regeneration. For the 0.50 and 0.75 cm deformations, the optimization efficiency was increased by a factor of roughly 3 compared to the complete plan regeneration. However, for the anatomical deformation of 1.00 cm, the reduction of the optimization search space during plan adaptation did not result in any efficiency improvement over the original (nonmodified) plan adaptation. The anatomical deformation of 1.00 cm demonstrates the limit of this approach. We propose an innovative approach to online ART in which the plan adaptation and radiation delivery are merged together and performed concurrently-adaptive radiation delivery (ARD). A fundamental advantage of ARD is the fact that radiation delivery can start almost immediately after image acquisition and evaluation. Most of the original plan adaptation is done during the radiation delivery, so the time spent adapting the original plan does not increase the overall time the patient has to spend on the treatment couch. As a consequence, the effective time allotted for plan adaptation is drastically reduced. For the 0.25, 0.5, and 0.75 cm anatomical deformations, the treatment time was increased by only 2, 4, and 6 s, respectively, as compared to no plan adaptation. For the anatomical deformation of 1.0 cm the time increase was substantially larger. The anatomical deformation of 1.0 cm represents an extreme case, which is rarely observed for the prostate, and again demonstrates the limit of this approach. ARD shows great potential for an online adaptive method with minimal extension of treatment time.

Simplifying intensity-modulated radiotherapy plans with fewer beam angles for the treatment of oropharyngeal carcinoma

The first aim of the present study was to investigate the feasibility of using fewer beam angles to improve delivery efficiency for the treatment of oropharyngeal cancer (OPC) with inverse‐planned intensity‐modulated radiation therapy (IP‐IMRT). A secondary aim was to evaluate whether the simplified IP‐IMRT plans could reduce the indirect radiation dose. The treatment plans for 5 consecutive OPC patients previously treated with a forward‐planned IMRT (FP‐IMRT) technique were selected as benchmarks for this study. The initial treatment goal for these patients was to deliver 70 Gy to ≥95% of the planning gross tumor volume (PTV‐70) and 59.4 Gy to ≥95% of the planning clinical tumor volume (PTV‐59.4) simultaneously. Each case was re‐planned using IP‐IMRT with multiple beam‐angle arrangements, including four complex IP‐IMRT plans using 7 or more beam angles, and one simple IMRT plan using 5 beam angles. The complex IP‐IMRT plans and simple IP‐IMRT plans were compared to each other and to the FP‐IMRT plans by analyzing the dose coverage of the target volumes, the plan homogeneity, the dose–volume histograms of critical structures, and the treatment delivery parameters including delivery time and the total number of monitor units (MUs). When comparing the plans, we found no significant difference between the complex IP‐IMRT, simple IP‐IMRT, and FP‐IMRT plans for tumor target coverage (PTV‐70: p=0.56; PTV‐59.4: p=0.20). The plan homogeneity, measured by the mean percentage isodose, did not significantly differ between the IP‐IMRT and FP‐IMRT plans (p=0.08), although we observed a trend toward greater inhomogeneity of dose in the simple IP‐IMRT plans. All IP‐IMRT plans either met or exceeded the quality of the FP‐IMRT plans in terms of dose to adjacent critical structures, including the parotids, spinal cord, and brainstem. As compared with the complex IP‐IMRT plans, the simple IP‐IMRT plans significantly reduced the mean treatment time (maximum probability for four pairwise comparisons: p=0.0003). In conclusion, our study demonstrates that, as compared with complex IP‐IMRT, simple IP‐IMRT can significantly improve treatment delivery efficiency while maintaining similar target coverage and sparing of critical structures. However, the improved efficiency does not significantly reduce the total number of MUs nor the indirect radiation dose.

PACS number: 87.53.tf

A feasibility study of using conventional jaws to deliver IMRT plans in the treatment of prostate cancer

The aim of this study is to investigate the feasibility of using conventional jaws to deliver inverse planned intensity-modulated radiotherapy (IMRT) plans for patients with prostate cancer. For ten patients, each had one three-dimensional conformal plan (3D plan) and seven inverse IMRT plans using direct aperture optimization. For IMRT plans using conventional jaws (JO plans), the number of apertures per beam angle was set from two to seven while three apertures per beam angle were set for the multi-leaf collimator (MLC) plans. To evaluate each planning method, we compared average dose volume histograms (DVH), the conformal index (COIN), total number of segments and total number of monitor units. Among the JO plans with the number of apertures per beam angle varying from two to seven, no difference was observed in the average DVHs, and the plan conformal index became saturated after four apertures per beam angle. Subsequently, JO plans with four apertures per beam angle (JO-4A) were compared with 3D and MLC plans. Based on the average DVHs, no difference was found among 3D, JO-4A and MLC plans with regard to the planning target volume and rectum, but the DVHs for the bladder and penile bulb were significantly better with inverse IMRT plans than those with 3D plans. When compared with the plan conformity, the average COIN values for 3D, JO-4A and MLC plans were 0.61 +/- 0.07, 0.73 +/- 0.05 and 0.83 +/- 0.05, respectively. In conclusion, inverse IMRT plans using conventional jaws are clinically feasible, achieving better plan quality than 3D-CRT plans.

Maximizing the potential of direct aperture optimization through collimator rotation

Intensity-modulated radiation therapy (IMRT) treatment plans are conventionally produced by the optimization of fluence maps followed by a leaf sequencing step. An alternative to fluence based inverse planning is to optimize directly the leaf positions and field weights of multileaf collimator (MLC) apertures. This approach is typically referred to as direct aperture optimization (DAO). It has been shown that equivalent dose distributions may be generated that have substantially fewer monitor units (MU) and number of apertures compared to fluence based optimization techniques. Here we introduce a DAO technique with rotated apertures that we call rotating aperture optimization (RAO). The advantages of collimator rotation in IMRT have been shown previously and include higher fluence spatial resolution, increased flexibility in the generation of aperture shapes and less interleaf effects. We have tested our RAO algorithm on a complex C-shaped target, seven nasopharynx cancer recurrences, and one multitarget nasopharynx carcinoma patient. A study was performed in order to assess the capabilities of RAO as compared to fixed collimator angle DAO. The accuracy of fixed and rotated collimator aperture delivery was also verified. An analysis of the optimized treatment plans indicates that plans generated with RAO are as good as or better than DAO while maintaining a smaller number of apertures and MU than fluence based IMRT. Delivery verification results show that RAO is less sensitive to tongue and groove effects than DAO. Delivery time is currently increased due to the collimator rotation speed although this is a mechanical limitation that can be eliminated in the future.

Direct aperture optimization–based intensity-modulated radiotherapy for whole breast irradiation

PURPOSE

To investigate the technical and dosimetric advantages and the efficacy of direct aperture optimized intensity-modulated radiation therapy (DAO-IMRT) over standard (e.g., beamlet optimized) IMRT and conventional three-dimensional conformal radiotherapy (3D-CRT) for whole breast irradiation in supine and prone positions.

METHODS AND MATERIALS

We retrospectively designed DAO-IMRT plans for 15 breast cancer patients in supine (10 patients) and prone (5 patients) positions with a goal of uniform dose coverage of the whole breast. These DAO-IMRT plans were compared with standard IMRT using beamlet optimization and conventional 3D-CRT plans using wedges. All plans used opposed tangential beam arrangements.

RESULTS

In all cases, the DAO-IMRT plans were equal to or better than those generated with 3D-CRT and standard beamlet-IMRT. For supine cases, DAO-IMRT provided higher uniformity index (UI, defined as the ratio of the dose to 95% of breast volume to the maximum dose) than either 3D-CRT (0.88 vs. 0.82; p = 0.026) or beamlet-IMRT (0.89 vs. 0.85; p = 0.003). Direct aperture optimized IMRT also gave lower lung doses than either 3D-CRT (V20 = 7.9% vs. 8.6%; p = 0.024) or beamlet-IMRT (V20 = 8.4% vs. 9.7%; p = 0.0008) for supine patients. For prone patients, DAO-IMRT provided higher UI than either 3D-CRT (0.89 vs. 0.83; p = 0.027) or beamlet-IMRT (0.89 vs. 0.85; p = 0.003). The planning time for DAO-IMRT was approximately 75% less than that of 3D-CRT. The monitor units for DAO-IMRT were approximately 60% less than those of beamlet-IMRT.

CONCLUSION

Direct aperture optimized IMRT improved the overall quality of dose distributions as well as the planning and delivery efficiency for treating whole breast in both supine and prone positions.

Variable depth recursion algorithm for leaf sequencing

The processes of extraction and sweep are basic segmentation steps that are used in leaf sequencing algorithms. A modified version of a commercial leaf sequencer changed the way that the extracts are selected and expanded the search space, but the modification maintained the basic search paradigm of evaluating multiple solutions, each one consisting of up to 12 extracts and a sweep sequence. While it generated the best solutions compared to other published algorithms, it used more computation time. A new, faster algorithm selects one extract at a time but calls itself as an evaluation function a user-specified number of times, after which it uses the bidirectional sweeping window algorithm as the final evaluation function. To achieve a performance comparable to that of the modified commercial leaf sequencer, 2-3 calls were needed, and in all test cases, there were only slight improvements beyond two calls. For the 13 clinical test maps, computation speeds improved by a factor between 12 and 43, depending on the constraints, namely the ability to interdigitate and the avoidance of the tongue-and-groove under dose. The new algorithm was compared to the original and modified versions of the commercial leaf sequencer. It was also compared to other published algorithms for 1400, random, 15 X 15, test maps with 3-16 intensity levels. In every single case the new algorithm provided the best solution.

Reduction of IMRT beam complexity through the use of beam modulation penalties in the objective function

Inverse planned intensity modulated radiation therapy (IMRT) has become commonplace in treatment centers across the world. Due to the implications of beam complexity on treatment planning, delivery, and quality assurance, several methods have been proposed to reduce the complexity. These methods include beamlet intensity restrictions, smoothing procedures, and direct aperture optimization. Many of these methods typically sacrifice target coverage and/or normal tissue sparing in return for increased beam smoothness and delivery efficiency. In the present work, we penalize beam modulation in the inverse planning cost function to reduce beam complexity and increase delivery efficiency, while maintaining dosimetric quality. Three modulation penalties were tested: two that penalized deviation from Savitzky-Golay filtered versions of the optimized beams, and one that penalized the plan intensity map variation (a measure of overall beam modulation). The modulation penalties were applied at varying weights in a weighted sum objective (or cost) function to investigate their ability to reduce beam complexity while preserving IMRT plan quality. The behavior of the penalties was characterized on a CT phantom, and then clinical optimization comparisons were performed in the brain, prostate, and head/neck. Comparisons were made between (i) plans with a baseline cost function (ii) plans with a baseline cost function employing maximum beamlet intensity limits, and (iii) plans with each of the modulation penalties added to the baseline cost function. Plan analysis was based upon dose-volume histograms, relevant dose metrics, beam modulation, and monitor units required for step and shoot delivery. Each of the techniques yielded improvements over a baseline cost function in terms of MU reduction. In most cases, this was achieved with minimal change to the plan DVHs and metrics. In all cases, an acceptable plan was reached with each of the methods while reducing MU substantially. Each individual method has merit as a tool for reducing IMRT beam complexity and could be easily applied in the clinic to improve overall inverse plan quality. However, the penalty based upon the plan intensity map variation consistently produced the most delivery-efficient plans with the fewest computations.