The variation in output of symmetric, asymmetric and irregularly shaped wedged radiotherapy fields

A model for calculating the variation in output of symmetric, asymmetric and irregularly shaped wedged radiotherapy fields is presented. The variation in output from the treatment head when a wedge is used is calculated by dividing the output into a primary component and one due to scattered radiation. The scatter component is then further subdivided into contributions from elements which have a 1 cm x 1 cm cross-sectional area at the isocentre. The scatter from each element is determined as the contribution from the head scatter component modified by the presence of the wedge and a contribution due to additional scattered radiation from the wedge. The relative intensity of the scattered radiation from the wedge is modelled using a simple first scatter approximation. In this approximation the magnitude of the scatter is given by a t exp(-mu t) function where t is the thickness of the wedge for the selected element. The magnitude of the primary component and the relative intensity of scatter from each element are then obtained by an iterative fit to measured data. The technique has been applied to two different internally mounted wedge designs, for a standard treatment head, two asymmetric treatment heads and two similar multileaf collimators, over a range of energies between 4 and 20 MV. Calculations agree with measured values over a range of field sizes and shapes to within 1.5%.     

Electron contamination in clinical high energy photon beams

The electron contamination in photon beams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a "clean" reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.     

Blocked field effects on collimator scatter factors

In routine dosimetry we assume separability of the collimator (Sc) and phantom (Sp) scatter components that together comprise the total scatter factor (Sc,p). In practice, the addition of blocking also affects the photon fluence attributable to the treatment head and flattening filter in a complicated way. The reduced aperture blocks out some of the head scatter contribution, while the block and tray add back secondary scatter. In the following we present techniques for directly measuring the aperture effect on Sc in air or in a full-scatter phantom. The change in Sc is found to be a scaleable quantity that can be modelled as a simple linear fit to the ratio of projected open-to-blocked equivalent square fields. Measurements have been made for 6, 18 and 24 MV photon beams on one Varian 2500 and two Varian 2100c accelerators. Results indicate a progressive loss of collimator scatter contribution with increased field blocking that is amplified with increasing energy. Block and tray scatter only contribute significantly to Sc for large fields and treatment distances of 80 cm or less. Application of these corrections in monitor unit calculations is presented.     

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.     

Measurement of photon beam backscatter from collimators to the beam monitor chamber using target-current-pulse-counting and telescope techniques

Backscattered radiation (BSR) arising from field-defining collimators and entering the beam monitor chamber (BMC) may contribute to observed variations in medical linear accelerator photon beam output with collimator setting. Measuring the magnitude of such contributions for particular accelerators under specified operating conditions is therefore important when attempting to understand and model accelerator head scatter. The present work was conducted to confirm some backscatter measurements for collimating jaws reported previously and to extend these to include other accelerators and a multileaf collimator (MLC). BSR reaching the BMC from the jaws of Clinac 600C, 2100C and 2300CD accelerators and from an MLC on the 2300CD was investigated using both target-current-pulse-counting and telescope methods. Our measurements show that for the Clinac 600C BSR-dependent output variations are negligible. However, for the 2100C and 2300CD BSR-dependent relative output increased in an almost linear fashion, by up to 2.4% for 15 and 18 MV beams, and by up to 1.7% for 6 MV beams, as the field size varied from 5 x 5 cm2 to 40 x 40 cm2. The magnitude of BSR dependent upon collimator location in the head, as expected, thereby contributing to the collimator exchange effect. An earlier study at our centre using the telescope method had reported higher BSR levels. This discrepancy was resolved when corrections for telescope block and room scatter, previously assumed negligible, were made.     

Beam characteristics and clinical possibilities of a new compact treatment unit design combining narrow pencil beam scanning and segmental multileaf collimation

Not until the last decade has flexible intensity modulated three-dimensional dose delivery techniques with photon beams become a clinical reality, first in the form of heavy metal transmission blocks and other beam compensators, then in dynamic and segmented multileaf collimation, and most recently by scanning high-energy narrow electron and photon beams. The merits of various treatment unit and bremsstrahlung target designs for high-energy photon therapy are investigated theoretically for two clinically relevant target sites, a cervix and a larynx cancer both in late stages. With an optimized bremsstrahlung target it is possible to generate photon beams with a half-width of about 3 cm at a source to axis distance (SAD) of 100 cm and an initial electron energy of 50 MeV. By making a more compact treatment head and shortening the SAD, it is possible to reduce the half-width even further to about 2 cm at a SAD of 70 cm and still have sufficient clearance between the collimator head and the patient. One advantage of a reduced SAD is that the divergence of the beam for a given field size on the patient is increased, and thus the exit dose is lowered by as much as 1%/cm of the patient cross section. A second advantage of a reduced SAD is that the electron beam on the patient surface will be only about 8 mm wide and very suitable for precision spot beam scanning. It may also be possible to reduce the beamwidth further by increasing the electron energy up to about 60 MeV to get a photon beam of around 15 mm half-width and an electron beam as narrow as 5 mm. The compact machine will be more efficient and easy to work with, due to the small gantry and the reduced isocentric height. For a given target volume and optimally selected static multileaf collimator, it is no surprise that the narrowest possible scanned elementary bremsstrahlung beam generates the best possible treatment outcome. In fact, by delivering a few static field segments with individually optimized scan patterns, it is possible to combine the advantage of being able to fine tune the fluence distribution by the scanning system with the steeper dose gradients that can be delivered by a few static multileaf collimator segments. It is demonstrated that in most cases a few collimator segments are sufficient and often a single segment per beam portal may suffice when narrow scanned photon beams are employed, and they can be delivered sequentially with a negligible time delay. A further advantage is the increase of therapeutically useful photons and improved patient protection, since the pencil beam is only scanned where the leaf collimator is open. Consequently, some of the problems associated with dynamic multileaf collimation such as the tongue and groove and edge leakage effects are significantly reduced. Fast scanning beam techniques combined with good treatment verification systems allow interesting future possibilities to counteract patient and internal organ motions in real time.     

A method for implementing dynamic photon beam intensity modulation using independent jaws and a multileaf collimator

A mathematical model is derived for digitally controlled linear accelerators to deliver a desired photon intensity distribution by combining collimator motion and machine dose rate variations. It shows that, at any instant, the quotient of the machine dose rate and the speed of collimator motion is proportional to the gradient of the desired in-air photon fluence distribution. The model is applicable for both independently controlled collimator jaws and multileaf collimators and can be implemented by controlling different parameters to accommodate linear accelerators from different manufactures. For independent jaws, each pair of jaws creates photon fluence variations along the direction of the jaw movement. For multileaf collimators, where each leaf is independently controlled, any two-dimensional (2D) photon fluence distribution can be delivered. The model has been implemented for wedged isodose distributions using independent jaws, and 2D intensity modulation using a multileaf collimator. One-dimensional (1D) wedged isodose distributions are created by moving an independent jaw at constant speed while varying machine dose rate. 2D intensity modulation has been implemented using a 'dynamic stepping' scheme, which controls the leaf progression during irradiation at constant machine dose rate. With this automated delivery scheme, the beam delivery time for dynamic intensity modulation, which depends on the complexity of the desired intensity distribution, approaches that of conventional beam modifiers. This paper shows the derivation of the model, its application, and our delivery scheme. Examples of 1D dynamic wedges and 2D intensity modulations will be given to illustrate the versatility of the model, the simplicity of its application, and the efficiency of beam delivery. These features make this approach practical for delivering conformal therapy treatments.     

A dual source photon beam model used in convolution/superposition dose calculations for clinical megavoltage x-ray beams

A realistic photon beam model based on Monte Carlo simulation of clinical linear accelerators was implemented in a convolution/superposition dose calculation algorithm. A primary and an extra-focal sources were used in this beam model to represent the direct photons from the target and the scattered photons from other head structures, respectively. The effect of the finite size of the extra-focal source was modeled by a convolution of the source fluence distribution with the collimator aperture function. Relative photon output in air (Sc) and in phantom (Scp) were computed using the convolution method with this new photon beam model. Our results showed that in a 10 MV photon beam, the Sc, Sp (phantom scatter factor), and Scp factors increased by 11%, 10%, and 22%, respectively, as the field size changed from 3 x 3 cm2 to 40 x 40 cm2. The variation of the Sc factor was contributed mostly by an increase of the extra-focal radiation with field size. The radiation backscattered into the monitor chamber inside the accelerator head affected the Sc by about 2% in the same field range. The output factors in elongated fields, asymmetric fields, and blocked fields were also investigated in this study. Our results showed that if the effect of the backscattered radiation was taken into account, output factors in these treatment fields can be predicted accurately by our convolution algorithm using the dual source photon beam model.     

Clinical implementation of a two-component x-ray source model for calculation of head-scatter factors

A calculation method is described, which presumes that linac output has two components. One component is direct x rays from the target and the remaining component is an extra-focal, distributed source of radiation that is scattered from the flattening filter, primary collimator, and the jaws of the moveable collimator. This calculation method gives values for output factors that differ from measured values by no more than 0.5% for field widths from 4 to 40 cm, rectangular fields with long to short axis ratios as great as 10, symmetric and asymmetric fields, and source-to-axis distances from 65 to 360 cm. While this calculation method has great accuracy and flexibility, a minimal amount of input data is required: (1) measured output factors at a source-to-axis distance of 100 cm for square fields; (2) positions of the collimator with respect to the x-ray source target; and (3) output factors measured at various source-to-axis distances for a 10 cm x 10 cm field.     

Clinical use of a simulation-multileaf collimator

BACKGROUND: At the University of Lübeck, radiotherapy is delivered by a 6/18-MV linear accelerator. Using the integrated multileaf collimator, irradiation of individually shaped treatment fields is possible in place of alloy blocks. Due to unsatisfactory pretherapeutic review of the radiation-field-specific multileaf collimator (MLC) configuration, we developed a simulation-multileaf collimator (SMLC) and assessed its feasibility at different tumor sites. MATERIAL AND METHODS: The SMLC is made of a perspex carrier with 52 horizontal sliding leaves. The position of each leaf is calculated by a 3D treatment-planning computer. The technician manually adjusts the leaves according to the beams-eye-view plot of the planning computer. Consequently, the SMLC is mounted on the therapy simulator at a distance of 64.8 cm from the focus. The treatment fields and the position of the leaves are documented by X-ray films. RESULTS: Using the SMLC, radiation oncologists are able to review exactly the leaf configuration of each MLC-shaped radiation field and to correlate the MLC-shaped radiation field with the treated volume, the organs at risk and the port films acquired by the Portal Vision system. CONCLUSION: The SMLC is a new tool to review radiation planning that uses an MLC in daily routine. The use of the SMLC improves the documentation and the quality assurance. It accelerates the treatment field review at the linear accelerator by comparing the SMLC simulator films with the portal images.     

Field margin reduction using intensity-modulated x-ray beams formed with a multileaf collimator

PURPOSE: In axial, coplanar treatments with multiple fields, the superior and inferior ends of a planning target volume (PTV) are at risk to get underdosed due to the overlapping penumbras of all treatment fields. We have investigated a technique using intensity modulated x-ray beams that allows the use of small margins for definition of the superior and inferior field borders while still reaching a minimum PTV-dose of 95% of the isocenter dose. METHODS AND MATERIALS: The applied intensity modulated beams, generated with a multileaf collimator, include narrow (1.1-1.6 cm) boost fields to increase the dose in the superior and inferior ends of the PTV. The benefits of this technique have been assessed using 3D treatment plans for 10 prostate cancer patients. Treatment planning was performed with the Cadplan 3D planning system (Varian-Dosetek). Dose calculations for the narrow boost fields have been compared with measurements. The application of the boost fields has been tested on the MM50 Racetrack Microtron (Scanditronix Medical AB), which allows fully computer-controlled setup of all involved treatment fields. RESULTS: Compared to our standard technique, the superior-inferior field length can be reduced by 1.6 cm, generally yielding smaller volumes of rectum and bladder in the high dose region. For the narrow boost fields, calculated relative dose distributions agree within 2% or 0.2 cm with measured dose distributions. For accurate monitor unit calculations, the phantom scatter table used in the Cadplan system had to be modified using measured data for square fields smaller than 4 x 4 cm2. The extra time needed at the MM50 for the setup and delivery of the boost fields is usually about 1 min. CONCLUSION: The proposed use of intensity modulated beams yields improved conformal dose distributions for treatment of prostate cancer patients with a superior-inferior field size reduction of 1.6 cm. Treatments of other tumor sites can also benefit from the application of the boost fields.     

An equivalent square field formula for determining head scatter factors of rectangular fields

A simple formula is derived for the calculation of an equivalent square field that gives the same head scatter factor as a given rectangular field. This formula is based strictly on the configuration of a medical linear accelerator treatment head. The geometric parameters used are the distances between the target and the top of each field-defining aperture. The formula accounts for both the effect of field elongation and the collimator exchange effect. This method predicts the output to within 1% accuracy for both open and wedged fields and does not require any new measured data other than the field size dependence of head scatter for a range of square field sizes. Interestingly, the formula we derived has the same format as the formula that was empirically obtained by Vadash and Bj?rngard [Med. Phys. 20, 733-734 (1993)].     

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.     

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.     

Multileaf collimator leaf sequencing algorithm for intensity modulated beams with multiple static segments

The "stop and shoot" method of producing intensity modulation using combinations of static multileaf collimator (MLC) segments has a number of advantages including precise dose delivery, easy verification, and general availability. However, due to the potential limitation of prolonged treatment time, it is essential to keep the number of required segments to a reasonable number. We propose an algorithm to minimize the number of segments for an intensity modulated field. In this algorithm, the sequence of delivery intensity is proposed to be a series of powers of 2, depending on the maximum intensity level in the matrix. The MLC leaf position sequence is designed directly on the two-dimensional intensity matrix to irradiate the largest possible area in each segment. The algorithm can be applied directly to MLC systems with different motion constraints. This algorithm has been evaluated by generating 1000 random 15 x 15 cm intensity matrices, each having from 3 to 16 intensity levels. Five clinical intensity modulated fields generated from the NOMOS CORVUS planning system for a complex clinical head and neck case were also tested with this and two other algorithms. The results of both the statistical and clinical studies showed that for all the intensity matrices tested, the proposed algorithm results in the smallest number of segments with a moderately increased monitor units. Thus it is well-suited for use in static MLC intensity modulation beam delivery. For MLC systems with interleaf motion constraint, we prove mathematically that this constraint reduces the tongue and groove effect at the expense of an increase of 25% in the number of segments.     

Theoretical and experimental determination of phantom scatter factors for photon fields with different radial energy variation

The output factor used for monitor unit determination in radiotherapy can be divided into two factors: the head scatter factor and the phantom scatter factor. Theoretical and experimental phantom scatter factors have been compared for different beam qualities between 4 MV and 50 MV and field sizes from 5 cm x 5 cm to 30 cm x 30 cm. The theoretical data were obtained through a convolution method based on Monte Carlo calculated energy spectra and dose kernels. The calculations have been performed both for accelerators with a rather large energy variation within the field and for accelerators with a constant energy distribution in the field. Deviations between theoretical and experimental data were found to be less than 1%.     

An experimental investigation of the tongue and groove effect for the Philips multileaf collimator

The tongue and groove effect is an underdosing effect which can occur in certain applications of multileaf collimators. It results from the need to overlap adjacent leaves of a multileaf collimator in order to limit leakage between leaves. The applications in which the effect can occur are the abutment of fields where the beam edges are defined by the leaf edge and the production of intensity-modulated fields by dynamic collimation. The effect has been measured for the 'worst case' when just two MLC fields are matched along leaf edges which have overlapping steps. Measurements of the dose have been made at d(max) and also at a more clinically relevant depth of 87 mm in Perspex for beam energies of 6 MV, 8 MV and 20 MV on two Philips SL series accelerators. Dose distributions were recorded on radiographic film which was subsequently digitized for analysis. The dose reduction of the tongue and groove effect was found to be 15-28% and spread over a width of 3.8 to 4.2 mm. This is somewhat shallower and wider than would be expected from a simple, idealized model of the effect which would predict a dose reduction of 80% over a width of 1 mm.     

The potential for increased sparing of normal tissue in parallel opposed techniques with a multileaf collimator

A multileaf collimator (MLC) can be used in parallel opposed techniques as a direct replacement for standard-shaped beam blocks. However, improved shielding is possible if the MLC field is designed to fit a target rather than to mimic a straight-edged block. This study has compared the treatment areas produced by the MLC and by conventionally blocked fields with the target area for 43 parallel opposed treatments. It was found in every case that the MLC treated less than 10% excess tissue, and, in over 70% of patients, the excess was less than 5%. The conventional fields, however, treated more than 10% excess tissue in 70% of patients. The effect of MLC orientation and the benefits of using an MLC are discussed.     

Characteristics of scattered electron beams shaped with a multileaf collimator

Characteristics of dual-foil scattered electron beams shaped with a multileaf collimator (MLC) (instead of an applicator system) were studied. The electron beams, with energies between 10 and 25 MeV, were produced by a racetrack microtron using a dual-foil scattering system. For a range of field sizes, depth dose curves, profiles, penumbra width, angular spread in air, and effective and virtual source positions were compared. Measurements were made when the MLC alone provided collimation and when an applicator provided collimation. Identical penumbra widths were obtained at a source-to-surface distance of 85 cm for the MLC and 110 cm for the applicator. The MLC-shaped beams had characteristics similar to other machines which use trimmers or applicators to collimate scanned or scattered electron beams. Values of the effective source position and the angular spread parameter for the MLC beams were similar to those of the dual-foil scattered beams of the Varian Clinac 2100 CD and the scanned beams of the Sagittaire linear accelerators. A model, based on Fermi-Eyges multiple scattering theory, was adapted and applied successfully to predict penumbra width as a function of collimator-surface distance.     

Verification of the omni wedge technique

The optimal field shape achieved using a multileaf collimator (MLC) often requires collimator rotation to minimize the adverse effects of the scalloped dose distribution the leaf steps produce. However, treatment machines are designed to deliver wedged fields parallel or perpendicular to the direction of the leaves. An analysis of cases from our clinic showed that for 25% of the wedged fields used to treat brain and lung tumors, the wedge direction and optimal MLC orientation differed by 20 degrees or more. The recently published omni wedge technique provides the capability of producing a wedged field with orientation independent of the orientation of the collimator. This paper presents a comparison of the three-dimensional (3D) dose distributions of the omni wedged field with distributions of wedged fields produced using both the universal and dynamic wedge techniques. All measurements were performed using film dosimetry techniques. The omni wedge generated fields closely matched the conventional wedged fields. Throughout 95% of the irradiated volume (excluding the penubra), the dose distribution of the omni wedged field ranged from +5.5 to -3.5 +/- 1.5% of that of the conventionally wedged fields. Calculation of the omni wedged field is as accurate as conventional wedged field calculation when using a 3D treatment planning systems. For two-dimensional treatment planning systems, where one must assume that the omni wedged field is identical to a conventional field, the calculated field and the delivered field differs by a small amount.