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PubMed2001

Independent dose calculations for the PEACOCK System.

An independent dose calculation method has been developed to validate intensity-modulated radiation therapy (IMRT) plans from the NOMOS PEACOCK System. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle and each arc position. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The independent calculation uses the pencil beam data viz tissue maximum ratio (TMR) and beam profiles for a single 1 x 0.8-cm beamlet formed by the NOMOS multileaf intensity-modulating collimator (MIMiC) leaf. The pencil beam data were measured for the 6-MV photon beam from Siemens PRIMUS linear accelerator using film dosimetry. The dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for its beam intensity. Isodose distributions are generated using the independent dose calculations and compared to the CORVUS plans. Isodose distributions show good agreement with the CORVUS plans for a number of clinical cases. The independent dose calculation algorithm is described in this paper.

Algorithms↗
PubMed2001

Where goest the Peacock?

Since the treatment of the first patient in 1994, the Peacock system has maintained its presence as the dominant method of intensity-modulated radiation therapy (IMRT) delivery. Currently in use at nearly 80 institutions, over 8000 patients have been treated using the system. Peacock treatments have been delivered to sites throughout the body, including CNS, head & neck, prostate, liver, kidney, lung, mediastinum, and extremities. IMRT, however, is a young and rapidly evolving treatment methodology. As institutions have explored new ways of improving radiation therapy with intensity-modulated techniques, the requirements for the Peacock system have also expanded. More sophisticated planning algorithms have been implemented to satisfy these new requirements, as well as better tools for treatment verification and quality assurance. In addition, new delivery techniques are being examined to improve the ability of IMRT to increase target volume doses while limiting organ-at-risk doses. One such technique, using helical tomotherapy (Peacock is an example of sequential tomotherapy), is currently being evaluated at one institution. Both techniques use narrow, modulated delivery beams. However, helical tomotherapy requires continuous movement of the couch during radiation, similar to helical CT. This work reviews the development of tomotherapy with the Peacock system. It then looks at current IMRT treatment techniques using tomotherapy, and how the field has broadened since the first treatments were delivered. Finally, it looks at the future of tomotherapy techniques, and how these techniques will adapt to the changing requirements for radiation therapy.

Algorithms↗
PubMed2001

Abutment dosimetry for serial tomotherapy.

The dose distributions at the abutment region for serial tomotherapy are reviewed. While tomotherapy provides unparalleled dose distributions, precise couch motion and good patient immobilization are required because the dose in the abutment region changes by 25% for each millimeter of misalignment. The process of delivering intensity-modulated radiation therapy using sequentially delivered modulated arcs yields hot spots below and cold spots above the machine isocenter when arc angles of less than 360 degrees are used. The magnitude of the hot and cold spots increases significantly as the arc angle is reduced 180 degrees such as when limited by couch clearance restrictions. Placement of isocenter also significantly affects the dose heterogeneity in the abutment region, with the hot and cold spots increasing nearly linearly with off-axis distance in the vertical direction. Reduction of the magnitude of the abutment region dose heterogeneities is possible if helical delivery is provided by moving the couch during arc delivery. The dose heterogeneity can also be reduced by creating 2 treatment plans, each with slightly different abutment region positions, or by using multiple couch angles.

Head and Neck Neoplasms↗
PubMed2001

Commissioning and quality assurance for MLC-based IMRT.

The commissioning and quality assurance (QA) associated with the implementation of linear accelerator multileaf collimator (MLC)-based intensity-modulated radiation therapy (IMRT) at the University of Nebraska Medical Center are described. Our MLC-based IMRT is implemented using the PRIMUS linear accelerator interface through the IMPAC record and verification system to the CORVUS treatment planning system. The "step-and-shoot" technique is used for this MLC-based IMRT. Commissioning process requires the verification of predefined parameters available on the CORVUS and the collection of some machine data. The machine data required are output factor in air and output factor in phantom, and percent depth dose for a number of field sizes. In addition, inplane and crossplane dose profiles of 4 x 4 cm and 20 x 20 cm field sizes and diagonal dose profiles of a large field size have to be measured. Validation of connectivity and dose model includes the use of uniform intensity bar strips, triangular-shaped nonuniform intensity bar strip, and N-shaped target. QA procedure follows the recommendation of the AAPM Task Group No. 40 report. In addition, the leaf position accuracy and reproducibility of the MLC should be checked at regular intervals. The dose validation is implemented through the hybrid plan where the patient beam parameters are applied to a flat phantom. Independent dose calculation method is used to confirm the dose delivery plan and data input to the CORVUS.

Equipment Design↗
PubMed2001

Intensity-modulated radiosurgery/radiotherapy using a micromultileaf collimator.

Intensity modulation with inverse treatment planning for 3 clinical stereotactic radiotherapy cases were directly compared against forward planning techniques using beam modification by enhanced dynamic wedge. Dose-volume histogram (DVH) analysis demonstrated that a significant reduction in dose to neighboring critical structures can-be achieved through intensity modulation patterns determined from inverse planning, while a marginal change is achieved in the target volume dose uniformity. This study also demonstrates that the intensity modulated dose patterns generated from inverse planning may differ significantly from the intuitive beam modified patterns developed in the forward planning model. These results suggest that one advantage of intensity modulated radiosurgery/radiotherapy with inverse planning is the significant reduction in dose to normal tissue and critical structures, with its coincident implications for dose escalation studies.

Brain Neoplasms↗
PubMed1999

Comparison of three-dimensional conformal radiation therapy and intensity-modulated radiation therapy systems.

The use of three-dimensional conformal radiation therapy (3DCRT) has now become common practice in radiation oncology departments around the world. Using beam's eye viewing of volumes defined on a treatment planning computed tomography scan, beam directions and beam shapes can be selected to conform to the shape of the projected target and minimize dose to critical normal structures. Intensity-modulated radiation therapy (IMRT) can yield dose distributions that conform closely to the three-dimensional shape of the target volume while still minimizing dose to normal structures by allowing the beam intensity to vary across those shaped fields. Predicted dose distributions for patients with tumors of the prostate, nasopharynx, and paraspinal region are compared between plans made with 3DCRT programs and those with inverse-planned IMRT programs. The IMRT plans are calculated for either static or dynamic beam delivery methods using multileaf collimators. Results of these comparisons indicate that IMRT can yield significantly better dose distributions in some situations at the expense of additional time and resources. New technologies are being developed that should significantly reduce the time needed to plan, implement, and verify these treatments. Current research should help define the future role of IMRT in clinical practice.

Costs and Cost Analysis↗
PubMed1999

Tomotherapy.

Tomotherapy is delivery of intensity-modulated, rotational radiation therapy using a fan-beam delivery. The NOMOS (Sewickley, PA) Peacock system is an example of sequential (or serial) tomotherapy that uses a fast-moving, actuator-driven multileaf collimator attached to a conventional C-arm gantry to modulate the beam intensity. In helical tomotherapy, the patient is continuously translated through a ring gantry as the fan beam rotates. The beam delivery geometry is similar to that of helical computed tomography (CT) and requires the use of slip rings to transmit power and data. A ring gantry provides a stable and accurate platform to perform tomographic verification using an unmodulated megavoltage beam. Moreover, megavoltage tomograms have adequate tissue contrast and resolution to provide setup verification. Assuming only translational and rotational offset errors, it is also possible to determine the offsets directly from tomographic projections, avoiding the time-consuming image reconstruction operation. The offsets can be used to modify the leaf delivery pattern to match the beam to the patient's anatomy on each day of a course of treatment. If tomographic representations of the patient are generated, this information can also be used to perform dose reconstruction. In this way, the actual dose distribution delivered can be superimposed onto the tomographic representation of the patient obtained at the time of treatment. The results can be compared with the planned isodose on the planning CT. This comparison may be used as an accurate basis for adaptive radiotherapy whereby the optimized delivery is modified before subsequent fractions. The verification afforded tomotherapy allows more precise conformal therapy. It also enables conformal avoidance radiotherapy, the complement to conformal therapy, for cases in which the tumor volume is ill-defined, but the locations of sensitive structures are adequately determined. A clinical tomotherapy unit is under construction at the University of Wisconsin.

Dose Fractionation, Radiation↗
PubMed1995

Dynamic beam modulation by using a single computer-controlled absorber.

The authors have developed an apparatus able to generate ID intensity-modulated beams, using only one moving absorber within the irradiation field. A procedure for deriving optimized absorber-speed profiles in order to produce the desired fluence/dose profiles has been suggested. Experimental tests show that the system should be sufficiently reliable in reproducing modulated beam profiles of different shape: expected relative doses against measured relative doses have been found to be in agreement in a number of situations within 3% using a nonfocused device. A better agreement should be expected using a focused apparatus (which is currently being developed). Beam modulation by single absorber cannot modulate the beam fluence in any was one wishes, due to physical constraints, which depend on the absorber and field widths and on the shape of the desired fluence profile. However, the authors show that this simple and low-cost tool could offer, with a sufficient degree of accuracy, the possibility of modulating the beam fluence with a high degree of versatility. In particular, a procedure or performing tissue-missing compensation by single-absorber dynamic beam modulation is suggested. Moreover, 'strongly' modulated beam profiles can be created, showing that this simple technique could also have some interesting applications in the field of conformal radiotherapy by non-uniform dose delivery.

Algorithms↗
PubMed1996

A method to study the characteristics of 3D dose distributions created by superposition of many intensity-modulated beams delivered via a slit aperture with multiple absorbing vanes.

Highly conformal dose distributions can be created by the superposition of many radiation fields from different directions, each with its intensity spatially modulated by the method known as tomotherapy. At the planning stage, the intensity of radiation of each beam element (or bixel) is determined by working out the effect of superposing the radiation through all bixels with the elemental dose distribution specified as that from a single bixel with all its neighbours closed (the 'independent-vane' (IV) model). However, at treatment-delivery stage, neighbouring bixels may not be closed. Instead the slit beam is delivered with parts of the beam closed for different periods of time to create the intensity modulation. As a result, the 3D dose distribution actually delivered will differ from that determined at the planning stage if the elemental beams do not obey the superposition principle. The purpose of this paper is to present a method to investigate and quantify the relation between planned and delivered 3D dose distributions. Two modes of inverse planning have been performed: (i) with a fit to the measured elemental dose distribution and (ii) with a 'stretched fit' obeying the superposition principle as in the PEACOCK 3D planning system. The actual delivery has been modelled as a series of component deliveries (CDs). The algorithm for determining the component intensities and the appropriate collimation conditions is specified. The elemental beam from the NOMOS MIMiC collimator is too narrow to obey the superposition principle although it can be 'stretched' and fitted to a superposition function. Hence there are differences between the IV plans made using modes (i) and (ii) and the raw and the stretched elemental beam, and also differences with CD delivery. This study shows that the differences between IV and CD dose distributions are smaller for mode (ii) inverse planning than for mode (i), somewhat justifying the way planning is done within PEACOCK. Using a stretched elemental beam is a useful adjustment to improve the accuracy of inverse planning but the 3D dose distribution actually delivered will display characteristics of the collimation.

Humans↗
PubMed1998

The effects of intra-fraction organ motion on the delivery of dynamic intensity modulation.

Computer-optimized treatment plans, aimed to enhance tumour control and reduce normal tissue complication, generally require non-uniform beam intensities. One of the techniques for delivering intensity-modulated beams is the use of dynamic multileaf collimation, where the beam aperture moves and the field shape changes during irradiation. When intensity-modulated beams are delivered with dynamic collimation, the problem of intra-fraction organ motion can cause distortions to the desired beam intensities. Unlike static field treatments, where intra-fraction organ motion only affects the boundaries creating broad dose penumbra, the interplay of the movement of the beam aperture and the movement of the patient anatomy can create 'hot' and 'cold' spots throughout the field. The mechanism for creating these effects is not well understood. This paper provides a simple analytical model which illustrates the fundamental mechanism for creating the dosimetric variations in the target when both the beam aperture and the target move during irradiation. Numerical analysis was carried out which calculates the cumulative primary photon fluence, or beam intensity, received by each point in the target, for a given pattern of motion. The results show that, for clinically realistic parameters, the magnitude of intensity variations in the target can be greater than 100% of the desired beam intensity. The magnitude of the photon intensity variations is strongly dependent on the speed of the beam aperture relative to the speed of the target motion, and the width of the scanning beam relative to the amplitude of target motion. The effects of fractionation as well as methods of minimizing and eliminating the dosimetric effects of intra-fraction organ motion are discussed.

Biophysical Phenomena↗
PubMed1998

Configuration options for intensity-modulated radiation therapy using multiple static fields shaped by a multileaf collimator.

One-dimensional (1D) intensity-modulated beams (IMBs) can be generated by multiple static fields (MSFs) created by a multileaf collimator (MLC) with the radiation switched off between field re-settings (Bortfeld-Boyer method). Each component irradiation is of equal fluence. This paper presents and analyses the formulae for the number of physically allowed combinations of leaf settings which generate any given IMB. The formulae are general to an IMB with any number of local minima and extend from the well-known N! formula for a single-peaked IMB with N left-leaf (L-leaf) and N right-leaf (R-leaf) positions. A 'combination' is a set of N L-leaf and R-leaf pairings. A 'physically allowed combination' is one in which no L-leaf is paired with an R-leaf to its left. The physically allowed combinations are grouped by specific properties into classes in which the well-known techniques of 'leaf-sweep' and 'close-in' are just two members. Consideration of these properties leads to a new suggestion of the 'forced-baseline' configuration in which the first intensity increment is delivered for the full field width and there remain choices concerning the delivery of the rest of the IMB within which two different possibilities are 'one-out-of-sync leaf-sweep' and 'minimum leaf travel'. The extension to 2D is briefly introduced.

Models, Theoretical↗
PubMed1998

Configuration options for intensity-modulated radiation therapy using multiple static fields shaped by a multileaf collimator. II: constraints and limitations on 2D modulation.

This paper addresses the technique of using multiple static multileaf-collimator-shaped field components to create a two-dimensional intensity-modulated beam (2D IMB). It addresses the physical constraints on the problem of determining the optimum field-component leaf configurations under the circumstances that (i) the static field components are shaped by leaves alone and (ii) the 2D intensity distribution is delivered by exactly N field components when there are N rising-intensity equal-fluence increments in the 1D channel containing the maximum fluence in the 2D IMB. This corresponds to the least inefficient delivery. In general it is noted that an optimum solution (set of field-component leaf configurations) with zero tongue-and-groove underdose may not exist (depending on the distribution) and an exhaustive search for the set of leaf configurations with the minimum tongue-and-groove underdose is impossible for realistically sized problems. Against this background iterative methods to examine a limited search space are shown to yield an optimum solution with zero tongue-and-groove underdose for certain intensity distributions. These searches are not robust and can be defeated. The problem of finding an optimum solution may be generally insoluble for some 2D IMBs under the conditions (i) and (ii). If, however, a larger number of field components is permitted and/or the accelerator jaws may also be used, in addition to the multileaves, then an optimum solution with zero tongue-and-groove underdose can always be found with lower efficiency.

Biophysical Phenomena↗
PubMed2000

A new concept of multileaf collimator (the shuttling MLC)--an interpreter for high-efficiency IMRT.

This paper proposes a radically new concept for a multileaf collimator for a photon linear accelerator for delivering IMRT with high monitor-unit efficiency. The concept is to consider each M (rows) x N (columns) two-dimensional intensity-modulated beam (2D IMB) as a set of N/2 M (rows) x 2 (columns) areas of modulation. Each area is then delivered by a set of M shuttling attenuating elements (called here the shuttling MLC) with a very high monitor-unit efficiency. The elements shuttle between each of the two columns comprising the M x 2 area and the modulation is provided by the variation in dwell time of the elements. The principles of this shuttling multileaf collimator are discussed and examples illustrating its operation are given. The main achievement reported in this paper is the development and robust testing of an interpreter which describes the position-time course of movement of the elements as a function of monitor units. This interpreter fully accounts for leakage transmission through the elements. It completely avoids the across-the-rows tongue-and-groove underdose. A large number of ID and 2D IMBs have been subjected to this interpreter and it is shown that for random patterns of fluence the SMLC is more monitor-unit efficient than the Bortfeld-Boyer technique (the most efficient with a conventional MLC) when the modulation is highly structured.

Algorithms↗
PubMed2000

Dynamic splitting of large intensity-modulated fields.

The aims of this paper are to describe a method of splitting large intensity-modulated fields that cannot be delivered as a single field and to verify the accuracy of our method. Some multi-leaf collimators may be operated in the dynamic mode to deliver intensity-modulated radiation treatments (IMRT) using the 'sliding window' technique. In this technique each pair of leaves sweeps over the treatment field while the beam is on. However, there are limitations on the width of the field that can be treated due to the limited length of the leaves. For instance, the leaf length of the Varian MLC is 14.5 cm. Since each leaf pair must travel from the left boundary to the right boundary of the beam aperture, the maximum width of the field aperture that can be accommodated in one sweep of leaves is also limited to 14.5 cm, in fact to a slightly smaller value. It has been shown that IMRT is more efficient when used to plan and deliver the large and boost fields simultaneously. In such situations, the fields must be large enough to cover simultaneously the volumes of the gross tumour, microscopic disease and electively treated regions. Such field sizes are often larger than 14.5 cm wide. In this paper, we present a dynamic 'feathering' technique to split the large intensity-modulated fields into smaller fields. In this technique, the component beams overlap each other by a small amount, and the intensity in the overlap region gradually decreases for one field component and increases for the other. The sum of intensities remains the same as for the original field. This method eliminates the field matching problems associated with the conventional step 'break' for static fields. The splitting process is integrated into the IMRT treatment procedure and the entire planning process is automated. Comparison of dose distributions calculated and measured in a phantom showed good agreement. Such a method can be applied to the 'step and shoot' technique as well. IMRT fields of widths up to 25 cm can be delivered by splitting only once, which is adequate for most treatments.

Algorithms↗
PubMed2000

Intensity modulated arc therapy (IMAT) with centrally blocked rotational fields.

A new technique for intensity-modulated beam (IMB) delivery that combines the features of intensity modulated arc therapy (IMAT) with the use of 'classical blocks' is proposed. The role of the blocks is to realize the high-gradient modulation of the intensity profile corresponding to the region to be protected within the body contour, while the MLC leaves or the secondary collimator defines the rest of the field and delivers intensity-modulated multiple rotational segments. The centrally blocked radiation fields are applied sequentially, in several rotations. Each rotation of the gantry is responsible for delivering one segment of the optimal intensity profile. The new IMAT technique is applied for a treatment geometry represented by an annular target volume centrally located within a circular body contour. The annulus encompasses a circular critical structure, which is to be protected. The beam opening and corresponding weight of each segment are determined in two ways. The first method applies a linear optimization algorithm to precalculated centrally blocked radial dose profiles. These radial profiles are calculated for a set of beam openings, ranging from the largest field that covers the whole planning target volume (PTV) to the smallest, which is 1 cm larger than the width of the central block. The optimization is subjected to dose homogeneity constraints imposed on a linear combination of these profiles and finally delivers the dimensions and weights of the rotational beams to be used in combination. The second method decomposes into several subfields the fluence profile of a rotational beam known to deliver a constant dose level to PTV. This fluence profile is determined by using the analytical method proposed by Brahme for the case of the annular PTV and the concentric organ at risk (OAR). The proper segmentation of this intensity profile provides the field sizes and corresponding weights of the subfields to be used in combination. Both methods show that for this particular treatment geometry, three to seven segments are sufficient to cover the PTV with the 95% dose level and to keep the dose level to the central critical structure under 30% of the maximum dose. These results were verified by experimentally delivering the calculated segments to radiotherapy verification films sandwiched between two cylindrical pieces of a pressed-wood phantom. The total beam time for a three-field irradiation was 77 s. The predicted and experimental dose profiles along the radius of the phantom agreed to within 5%. Generalization of this technique to real-patient treatment geometry and advantages over other conformal radiotherapy techniques are also discussed.

Algorithms↗
PubMed2001

Concepts for shuttling multileaf collimators for intensity-modulated radiation therapy.

Shuttling multileaf collimators (SMLCs) can increase the MU efficiency of intensity-modulated radiation therapy compared with the multiple-static-field (MSF-MLC) technique or dynamic MLC (DMLC) technique with conventional MLCs. In a previous paper (Phys. Med. Biol. 45 3343-58) a particular SMLC was shown, for highly modulated intensity distributions, to increase the MU efficiency compared with the MSF-MLC technique. In this companion paper, two new arrangements similar to that described in the earlier paper, but with less mechanical complexity, are shown to be constructionally simpler but less MU efficient. Additionally another new concept of SMLC is shown which also increases the MU efficiency compared with the MSF-MLC technique and often improves the MU efficiency compared with the previously reported SMLC for highly modulated intensity distributions. It also leads to zero tongue-and-groove underdose in the direction orthogonal to that of the shuttling elements (so-called across-the-rows).

Equipment Design↗
PubMed2000

Dependence of linac output on the switch rate of an intensity-modulated tomotherapy collimator.

The electro-mechanical, multivane intensity modulated collimator ("MIMiC") slit collimator with 40 vanes has been applied in the delivery of inversely planned sequential tomotherapy to over 4,000 patients. The collimator is binary in that each vane switches between fully open or closed status. Resulting beamlet patterns provide the intensity distributions imparting dose to the patient. The bouncing and damping of vanes at the two ends of their travel cause transient dose perturbations near and at the borders of the treatment field. These perturbations are not explicitly modeled by the planning system. Clinical beamlet profiles and output factors may then differ from those in the planning system and as a function of the vane switch period. A mechanical model of vane switching was developed to describe this dependency. Dose output and distribution of seven simple vane patterns with different switch times were measured with ionization chambers and radiographic films in polystyrene and anthropomorphic phantoms. Linac output dependence on switch time relative to vane open time was determined for four intensity modulated radiotherapy (IMRT) patients from measurements of an ionization chamber embedded in a cylindrical polystyrene phantom. Results demonstrate output dependence on switch time and, accordingly, on the servo mechanism for monitor units, arc length, dose rate, and gantry speed. In conclusion, the output dependence borders on clinical significance-improvements to collimator, dose calculation, commissioning, and quality assurance (QA) are suggested.

Humans↗
PubMed1997

The optimum intensities for multiple static multileaf collimator field compensation.

A method of determining the optimum beam intensities for compensation using multiple static multileaf collimator fields is presented. In this method a histogram of the number of beam pixels against beam intensity is generated for the intensity-modulated beam (IMB). The intensity of each beam to be used is chosen to minimize the mean square deviation between each bin in the histogram and the closest beam intensity. This method has been applied to sample IMBs possessing one maximum and two maxima. For both cases, the use of uniform beam intensity increments is shown to be close to optimal. In the case with two maxima, the efficacy of irradiating both peaks simultaneously, rather than separately, has been studied and shown to be of potential benefit. The optimum intensities for an IMB for breast radiotherapy are also presented.

Humans↗