Dosimetric validation for multileaf collimator-based intensity-modulated radiotherapy: a review

Medical Dosimetry, Vol. 26, No. 2, pp. 179 –188, 2001 Copyright © 2001 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/01/$–see front matter

PII: S0958-3947(01)00058-9

DOSIMETRIC VALIDATION FOR MULTILEAF COLLIMATOR-BASED INTENSITY-MODULATED RADIOTHERAPY: A REVIEW MARK R. ARNFIELD, QIUWEN WU, SHIDONG TONG, and RADHE MOHAN Department of Radiation Oncology, Medical College of Virginia at Virginia Commonwealth University, Richmond, VA; and McGuire VA Medical Center, Richmond, VA ( Accepted 8 March 2001)

Abstract—The creation of intricate dose distributions produced by intensity-modulated radiotherapy (IMRT) depends on complex planning systems and specialized mechanical devices. The many possible sources of inaccuracy and the complexity of the dose maps themselves require that a substantial effort be made to ensure that calculated and delivered dose distributions agree. This review provides an overview of the current status of the validation of dose predictions of IMRT planning systems by comparisons with measurements. Emphasis is placed on multileaf collimator- (MLC) based IMRT. Discrepancies between calculations and measurements may be due to any of 3 causes: errors and uncertainties in the dose calculation algorithm, in measurements, or in beam delivery by the accelerator/MLC combination. Some of the factors affecting dosimetry include: the technique employed for modulating the fluence, the dose calculation algorithm and other aspects of the planning system, mechanical limitations of the MLC hardware, dosimetric characteristics of the MLC, such as MLC leakage and rounded leaf ends, the choice of dosimeter, and the measurement geometry and technique. The advantages and drawbacks of various dosimeters including film, ion chambers, thermoluminescent dosimetry, and electronic portal imaging devices are discussed. The steps involved in validating dosimetrically a planning system are outlined, including the various fields that need to be measured, the phantoms that may be used, and measurement techniques. The achievable accuracy of dosimetry for IMRT is discussed. © 2001 American Association of Medical Dosimetrists. Key Words: Multileaf collimator, Intensity-modulated radiotherapy, Dosimetry, Quality assurance .

INTRODUCTION

FACTORS AFFECTING DOSIMETRY

Intensity-modulated radiotherapy (IMRT) involves the modulation of radiation beams to conform to target volumes and spare surrounding critical structures and normal tissues. Before introducing IMRT to the clinic, one of the questions that need to be answered is: how can we know that the intensity-modulated dose will be delivered accurately? The complexity of IMRT dose patterns makes the verification of the match between planned and delivered doses considerably more difficult than for conventional beams. Furthermore, the complex planning systems and beam delivery hardware needed for the creation of the distributions introduce potential errors and uncertainties in dosimetry. Thus, the accuracy of delivered dose is a critical issue both in the commissioning phase and for ongoing quality assurance in an IMRT program. This review provides an overview of the considerations in establishing the dosimetric accuracy of IMRT delivery/planning systems. The different technical approaches to IMRT include tomotherapy1,2 and multileaf collimator- (MLC) based IMRT.3–11 This paper concentrates on dosimetry for MLC-based IMRT.

Dosimetric accuracy of a treatment planning system is established when, for a field defined by a specific setting of the treatment machine, the planned and measured doses for that field agree within some reasonable tolerance level. Such agreement must be confirmed for a sufficient variety of fields to ensure, with reasonable confidence, that it is true in general. For this to occur, both dose calculations and measurements must be accurate. The “deliverable dose” in IMRT may be defined as the dose due to the fluence pattern that is achievable, given the physical limitations of the particular MLC. This fluence pattern may be somewhat different than the desired (optimized) fluence. The planning system produces an MLC leaf position file corresponding to the deliverable fluence distribution. For dynamic IMRT treatments, dosimetric accuracy includes the requirements of calculation and measurement accuracy, as well as an additional requirement not applicable to conventional planning systems. The beam delivery hardware, including the multileaf collimator control mechanism, must accurately reproduce the intended leaf motion control file. Thus, the accuracy of dose delivery depends in part on how accurately the MLC/accelerator combination is able to reproduce the planned monitor unit vs. position

Reprint requests to: Mark Arnfield, Ph.D., Department of Radiation Oncology, Medical College of Virginia, Virginia Commonwealth University, 401 College Street, Richmond, VA 23298. E-mail: [email protected] 179

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Table 1. Selected design and dosimetric characteristics of some commercial MLCs Siemens MLC32,74 Leaf configuration

Average leakage for 10 ⫻ 10 cm2 field Focus/leaf-end design MLC/jaw configuration

Scanditronix MLC75

Varian Mark II MLC19,21,76,77

Varian Millennium MLC21

27 inner leaf pairs of 1.0-cm width, 2 outer pairs of 6.5-cm width 6 MV: 0.9–1.25%

32 leaf pairs of 1.25-cm width 20 MV: approx. 1.6%

40 leaf pairs of 1.0-cm 40 inner leaf pairs of 0.5-cm width, 20 width (earlier model: outer pairs of 26 leaf pairs76,77) 1.0-cm width 6 MV: 1.7–1.9% 6 MV: 1.6%

Double focus, straight end MLC replaces lower jaws

Double focus, straight end MLC replaces lower jaws

Single focus, rounded end MLC mounted below lower jaws

Elekta/Philips MLC16,22,78 40 leaf pairs of 1.0-cm width 6 MV: 1.8–2.5%*

Single focus, rounded Single focus, rounded end end MLC mounted below MLC replaces upper lower jaws jaws

* This leakage value is further reduced by backup collimators that track the maximal leaf opening. The table does not include miniature MLCs and is not intended as a comparison of relative merit. All quoted leaf widths are projected to isocenter.

data. If the mechanical tolerances in the leaf positions are too large, significant deviations from the expected, deliverable fluence distribution can be introduced. This will appear as a dosimetric error that is due to the MLC and beam delivery hardware. When discrepancies between calculated and measured doses are found, it is usually not obvious which of 3 factors, calculation inaccuracy, measurement inaccuracy, or MLC inaccuracy, is the cause. Another aspect of IMRT delivery affecting dosimetry is the choice of delivery technique. Techniques that have been employed in MLC-based IMRT include the static step-and-shoot technique3,4 and dynamic MLC techniques such as the sliding window technique5– 8 and multiple dynamic arcs.9,10 In the step-and-shoot technique, dose is delivered as a series of static, irregular fields. The beam is off while the MLC leaves are repositioned between fields. In dynamic techniques, the leaves continuously change position while the beam remains on. Generally speaking, dose verification for stepand-shoot techniques is less difficult than for dynamic techniques because both the calculations and the delivery process are simpler than for dynamic methods, and the distributions tend to be less modulated. Factors having to do with target localization such as immobilization techniques, isocenter shifts, and internal organ motion may also have a significant effect on the dose delivered to the patient.12–15 Gantry angle inaccuracy has little impact on multiple fixed-field IMRT plans.12 Although patient movement between fields can affect individual fields significantly, its overall effect on IMRT plans may be less because the effects of different fields tend to cancel each other.12 Repetitive motion of the target, such as occurs in the breathing cycle, may combine with the collimator motion in dynamic beam deliveries to create significant hot and cold spots in the target.14 Target localization issues such as these are certainly important for ensuring accurate treatment delivery to the patient. They do not, however, affect dosimetric comparisons between in-phantom measurements and the predictions of planning systems and will not be considered further.

MLC design and characteristics All MLCs provide the basic function of multiple, independently-driven vanes or leaves, which allow flexible, computer-controlled beam shaping. The agreement between calculated and measured dose depends upon adequate accounting of the various effects associated with MLC characteristics. Several important dosimetric characteristics related to leaf design of some commercial MLCs are given in Table 1. It should be borne in mind that Table 1 is not intended as a comparison of the relative merits of the MLCs of these manufacturers. Other important aspects of MLC design not listed here bear on the utility of a particular MLC, including such characteristics as maximum leaf velocity, maximum field size, leaf over-travel, and leaf positioning accuracy. As indicated in Table 1, MLCs from different manufacturers occupy different positions in the treatment head, where they may replace the upper or lower sets of collimator jaws or may be mounted below the lower jaws, providing tertiary collimation. The amount of head scatter radiation received at the isocenter plane depends, to some extent, on the location of the MLC.16,17 Characteristics such as leaf transmission and scatter and leaf end design are of special significance for dynamic IMRT methods such as the sliding window technique. In dynamic IMRT dose that is transmitted through or scattered from the leaves, which may be termed “indirect” (or leakage) radiation, may constitute a significant fraction of the dose received by the patient.6,7,11,18 –20 This fraction of the dose increases as the frequency and magnitude of fluctuations in the intensity distribution increases.20 The range of deliverable fluence maps can be significantly constrained by the degree of MLC leakage, because clearly the minimum fluence must exceed the background transmission, which is proportional to the beam on-time. The same consideration applies to radiation that is transmitted through rounded leaf ends. Thus, unavoidable indirect radiation often places a lower limit on the intensity of “valleys” in the dose pattern. Although the leakage of commercial MLC leaf banks is in the range of 1.0% to 2.0%, as indicated in Table 1, the

Dosimetric validation for MLC-based IMRT ● M.R. ARNFIELD et al.

actual portion of delivered indirect dose through dynamic MLC may be much greater than this. Thus, accurate dose calculation demands that the transmission, including dynamic MLC, be carefully assessed and used as an input to the dose calculation algorithm.6,18,20 The focal characteristics of the MLC and the associated leaf end design are also significant for dosimetry. In Table 1, “double focus” denotes that the leaves move along an arc centered on the x-ray target, while “single focus” denotes that the leaves move perpendicular to the beam central axis. In MLCs with a double focus and flat leaf-end design, the fluence transmission is unity on the open side of the leaf and falls immediately to the fullthickness transmission in the shadow of the leaf. Singlefocus MLCs have leaves that move in a plane and have rounded ends. In this case, the transmission of fluence through the leaf end falls from unity in the open field to the full-thickness transmission value over a finite distance, the distance depending on the leaf-end shape. The purpose of the rounded design is so that a relatively constant penumbra is maintained for different values of leaf displacement. The overall penumbra is larger for rounded than for flat-end designs. For rounded-end designs, the dose calculation algorithm must account for the fluence transmitted through the rounded end. This may be modeled as a transmission function or, less precisely, as an equivalent leaf-end displacement. The equivalent displacement is approximately 1.0 mm for the Varian MLC.11,19,21 Details related to the dosimetry of leaf ends can be found in the literature.11,19,21–23 A variety of additional factors contribute to the fidelity and reproducibility of dose delivery. Many of these could be categorized as quality assurance issues arising from the MLC mechanical characteristics and tolerances, rather than dosimetric issues per se. These include, for example, leaf position reproducibility and tolerance, leaf speed stability, and leaf acceleration effects.24 It is also important to realize that certain characteristics and limitations of MLC hardware place constraints on the range of fluence distributions that can be realized in practice. The effects of MLC leakage have already been discussed. Some other hardware restrictions include the minimum gap between opposing leaves, maximum leaf velocity, dose rate, and the leaf length.25–27 For example, the maximum leaf velocity together with the dose rate determines the minimum fluence gradient achievable by that leaf. As another example, very low dose rates may be necessary to produce certain fluence distributions. However, at low dose rates, output instability of the accelerator could result in inaccurate delivery, due to numerous beam interrupts of a dynamic treatment. Machine calibration or output uncertainty for deliveries of a few monitor units can also produce significant dosimetric errors.28 –31 This concern applies in particular to stepand-shoot techniques, which employ numerous low exposure segments. Another issue related to machine design is the

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tongue-and-groove effect. Illustrations of the tongueand-groove designs of several commercial MLCs can be found in Das et al.32 If leaves are not properly synchronized, underdosage of a part of the field will result from shadowing by the tongue-and-groove part of the leaf design.8 The necessary constraints on leaf motion to avoid tongue and groove underdosage effects are now well understood.8,33,34 Leaf sequencing algorithms to minimize the tongue-and-groove effect have been implemented in commercial IMRT planning systems. IMRT planning system Commissioning of a conventional treatment planning system requires both nondosimetric and dosimetric tests.35,36 Nondosimetric tests include such items as image registration and conversion, image and isodose display, and hardcopy devices. Dosimetric tests include comparisons between measurements and planning system predictions of, for example, depth doses, open and wedge beam profiles, and handling of inhomogeneities. Both measurements and dose calculations must be accurate, if the two are to match. Unlike conventional systems, IMRT planning systems must account for the additional complexity due to modulation of the fluence distribution. In general, the more complex the modulation the greater are the additional uncertainties that are introduced above those that already exist for conventional field calculations. We are primarily concerned with those aspects of IMRT planning systems that specifically relate to dose accuracy. The accuracy of the dose calculation algorithm itself is fundamental to the goal of achieving agreement with measurements. It is well established that modelbased algorithms such as superposition methods are accurate over a wider range of conditions than semiempirical dose calculation (conventional) models.37,38 However, because of the many dose calculation iterations needed for fluence optimization, most current IMRT systems use the less computationally demanding conventional models such as pencil beam algorithms39 or scatter integration-based methods.40,41 Although less accurate algorithms are used during the optimization process to compute the numerous iterated dose distributions, it is imperative that after optimization is completed, the most accurate algorithm available on the planning system be used to compute the final dose map of the optimal plan. Significant differences have been noted between head and neck IMRT distributions when calculated by model-based vs. conventional methods.40 Ideally, the algorithm used to calculate the final plan should incorporate accurately the limitations and properties of the delivery system, to make the plan distribution match as closely as possible with measurements. Even modelbased methods require some modifications or adaptations to handle intensity modulated beams. In the step-and-shoot technique, an IMRT field is segmented into a series of static, irregular fields. There is

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a practical limit of approximately 50 subfields that can be delivered in a single treatment session for some machines.42 A conventional approach is to calculate the dose as a summation of the dose from the constituent irregular fields.43 For dynamic IMRT, when the fluence modulation is low or moderate, the situation is closer to that for the step and shoot technique. In this case, dose has been calculated by approximating the effect of the dynamic field as a series of static fields, and including a correction to account for dynamic movement.44 A more accurate approach is to explicitly construct the deliverable fluence distribution, based on the actual leaf sequence file produced by the planning system.27 The improvement in accuracy by accounting for leaf trajectories in the calculation increases as the degree of modulation of the field increases. This method should take into account the fluence transmitted through the MLC leaves and, if applicable, rounded leaf ends.6,7,20,39 The amount of head scatter from the continuously changing leaf configuration must also be calculated. In the step-and-shoot method, the head scatter of each subfield is calculated separately, and then they are all added together at the end. Various authors have published approaches to calculating the head scatter for different manufacturer’s MLCs, i.e., Siemens,32 Varian,45 Philips/ Elekta,16,44 and General Electric.46,47 In dynamic IMRT, the head scatter will be overestimated if it is based on the maximum extent of the dynamically delivered field.10 Instead, the head scatter should be computed from an area integration of the average irregular opening due to the field shapes occurring during dynamic delivery. An extended 2-component source distribution based on, for example, exponential44 or Gaussian functions17,48 can be used to model the head scatter component.17,44,45,49,50 The head scatter correction is more significant when the MLC replaces the upper jaws than when it sits lower in the treatment head.16,17

MEASUREMENTS OF INTENSITY-MODULATED FIELDS The first question regarding IMRT validation measurements is which fields should be measured. As in any planning system, the recommended procedure is to work from the simple to the complex. The first step is to verify in phantom the output and depth doses of uniform fields vs. the predictions of the planning system. For dynamic IMRT, dose validation should be made for both conventional and dynamically-generated uniform fields.11,20,21,39 For the sweeping window method, the required 1D leaf motion files are quite simple and can be created with a text editor.21 It should be verified that the depth doses of such fields are the same as for conventional fields of the same field size. The verification of intensity-modulated fields is always of the actual, deliverable dose rather than the optimized (desired) dose. In MLC-based IMRT, even

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nominally uniform fields will have a “waviness” in the deliverable field profile, due to the greater transmission through the interleaf region vs. the intraleaf region.20 The effect of this variable transmission is not taken into account in current dose algorithms and can cause dosimetric errors where isodose lines coincide with the interleaf leakage region.39 In the Varian MLC design, the interleaf division between the 2 central leaves coincides with central axis. A “point” dose measurement on central axis will read higher than the average dose of the central region of the field. Dose must be averaged over a sufficient area to account for the nonuniformity, for example, by film measurement or a sufficiently large ion chamber to subtend several leaves.19 Measurements of uniform fields may be followed by measurements of specially designed 1D and 2D intensity patterns. Geometric shapes and other idealized distributions of varying complexity are useful to test certain aspects of the planning system, such as effects of leakage18,20,34 and head scatter,17 or as representative of clinical applications such as compensators and wedges.51 Some distributions that have been considered are cones, horns, inverted cones or pyramids, multipeak distributions, sinc function, rectangular patterns, etc.17,18,20,34,39,51,52 A centrally blocked (e.g., “donut”) field is a useful distribution for testing a planning system leaf sequencing algorithm for the presence of the tongueand-groove effect.53 An example of a comparison of calculations and measurements of a simple 1D doublepeak intensity distribution is given in Fig. 1. Figure 1a shows the delivered fluence profile through central axis, while Fig. 1b shows the corresponding calculated and measured dose profiles. Satisfactory performance of the planning system on specially contrived, geometric fields leads to the final commissioning phase, which is the testing of optimized clinical fields for hypothetical patients. The CT data used as the basis for the optimization may come from an anatomic phantom or from the CT scan of a patient or patients who have previously undergone conventional radiotherapy. However, the dose distribution used for comparison with measurements must be computed with CT data of the phantom that is used for measurements. The dose distribution to the phantom will be somewhat different than the original optimized plan on the patient. A technique for verification of single fields is described in the later section on phantoms and measurement geometry. In addition to single-field measurements, it is also necessary to test, in phantom, the total 3D dose distribution from all fields of an optimized plan or plans. This may be done with cylindrical,39,54 anatomic,28,41 or rectilinear phantoms.39,55 Multiple point measurements to test the predictions of the planning system can be made at selected locations representing the target and critical structures.

Dosimetric validation for MLC-based IMRT ● M.R. ARNFIELD et al.

Fig. 1. (a) An example of a simple 1D double-peak test pattern for dosimetric validation of an IMRT planning system utilizing the sweeping window technique. Shown is the “deliverable” fluence profile, which takes into account the detailed leaf trajectory data that is sent to the MLC sequencing control mechanism. (b) A comparison of the computed dose for the fluence profile of (a) with the dose measured by film dosimetry, at 5-cm depth in a water-equivalent phantom.

Dosimeters Several types of detectors are necessary for the varied needs of IMRT dosimetry. Table 2 gives a summary of dosimeters that are commonly used at this time in IMRT applications. The choice of dosimeter for a given measurement depends on the geometry, the accuracy needed, and how extensive the measurements will be. Ion chambers are essential for calibration purposes and are ideal for measuring absolute dose at a single

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point in a low gradient region. However, they cannot be used to verify 2D fluence patterns. The other limitation of ion chambers is their relatively large size. Because dose gradients are always present in modulated fields, there is generally insufficient dose uniformity for accurate ion chamber measurements. The use of miniature ion chambers can significantly reduce errors due to volume averaging.56 Because of their small size, thermoluminescent dosimeters (TLD) are often more suited to point measurements than ion chambers. TLDs also give the option of measuring multiple points simultaneously. With careful handling and attention to standardized annealing and readout protocols, TLD dosimetry can routinely achieve an accuracy of at least ⫾ 3%.57,58 Our experience at the Medical College of Virginia is that excellent spatial resolution and 2% accuracy can be obtained with cubic LiF TLDs of the dimensions 1 ⫻ 1 ⫻ 1 mm (Bicron, Solon, OH). Because of the variation of fluence in 2D and in 3D of IMRT plans, film plays an important role in any IMRT program. Film has been shown to be highly useful for commissioning and quality assurance measurements of single IMRT fields.11,39 When properly calibrated, film can measure isodose charts of photon fields with an accuracy of about ⫾ 3% within the field edge, when placed perpendicular to the beam direction. Film measurements are not accurate outside the edge of the beam due to over-response to low-energy scattered photons.59 At least part of the discrepancy between calculations and measurements outside the field edge shown in Fig. 1b is due to the film over-response. Others have noted that for low isodose lines, doses measured by film tend to exceed calculated doses.11 This is not a significant drawback for IMRT dose verification, because, in general, it is the high isodose lines that are of most interest. Film has also been used in an orientation parallel to the plane of gantry rotation, to measure composite distributions of several IMRT fields.19,28,39 In general, this procedure is relatively inaccurate compared to the single-field technique, because film requires the application of depth-dependent corrections to ameliorate large dosimetric errors when used in this orientation.60 This type of correction is impossible to apply when multiple beams from different directions are employed. However, the inaccuracy can be confined to the regions of the phantom at shallower depths if the calibration films are irradiated at the same depth as isocenter.19 Used in this manner, film can at least provide a qualitative check on the total dose distribution. One of the most promising directions in IMRT dosimetry is the use of electronic portal imaging devices (EPID). Because of hardware limitations and calibration uncertainties, EPIDs have mainly been studied as QA tools for IMRT field verification.61– 63 However, recent studies suggest their potential as accurate 2D dosimeters, especially with the advent of amorphous silicon technology.64 Because EPIDs are accessories of the accelerator,

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Table 2. Detectors commonly used in IMRT dosimetry Dosimeter

Geometry

Ion chamber Point TLD Film EPID Diode

Type of measurement Single field or composite fields

Advantages

Limitations

Accurate

Point measurements only; large detector size (except stereotactic chambers) Multiple points Single field or composite fields Small detector size (1 ⫻ 1 ⫻ 1 mm Inconvenient because not real-time cubes available) Planar Single field or composite fields Inexpensive; high resolution 2D Time consuming measurement Planar Single field verification or in vivo Rapid, convenient, easy to automate Dosimetry techniques still under developexit dose measurement ment Point In vivo QA verification Real-time Limited accuracy, placement sensitive, low-dose gradient only

they present the possibility of 2D dosimetric verification of treatment fields as they are delivered to the patient (“exit dosimetry”).64 – 66 Alternatively, an EPID may be used in the same way as film, for the verification of individual IMRT fields in a flat phantom prior to patient treatment.61 Other devices that have been proposed for IMRT dosimetry include radiation sensitive polymer gels41,54,67 and silicon diodes. Polymer gels have the significant advantage of providing a true 3D map of dose distributions. Drawbacks of this method include the need for access to a NMR scanner and the considerable effort required achieving acceptable accuracy. Although diode detectors are not suitable for validation measurements, they might be of use for real-time verification of patient treatments, in the same manner as they are used for quality assurance in conventional radiotherapy.68 Because IMRT fields are nonuniform, imprecise placement of the device on the patient surface can result in large discrepancies between the expected and measured diode signals. Also, the correction factors needed for accurate interpretation of the diode readings of IMRT treatments have not yet been established. Phantoms and measurement geometry The most straightforward way to check individual IMRT fields dosimetrically is with calibrated therapy film (e.g., Kodak XV), placed normal to beam axis in a rectilinear, water-equivalent phantom.11 Film should be placed at a clinically relevant depth (e.g., 5 cm for head and neck cancer and 10 cm for prostate cancer). Film densities from each IMRT field are converted to dose via the film calibration data and then compared to predictions of the planning system, for the same geometry. Matching the measured and calculated distributions can be done ad hoc, by shifting the distributions relative to one another until the low isodose lines match.11 A better approach is to place marks on the film at the time of measurement, corresponding to the locations of the field cross hairs. These marks are made outside the borders of the IMRT field. This procedure allows an unambiguous registration of the calculated and measured dose distributions when the film is scanned or digitized. Fig. 2a shows a comparison of calculated and measured isodose

distributions for an IMRT field. A visual inspection of this example reveals only minor discrepancies in the isodose lines. For more detailed evaluation, linear dose profiles representative of selected areas of the dose map may be examined. For example, the profile in Fig. 2b corresponds to the dashed line in Fig. 2a. Film dosimetry of IMRT distributions is quite time consuming, the actual time depending on the available tools, i.e., the type of film scanner (mechanical or optical) and the available software for data manipulation and comparison. The routine use of EPID dosimetry can considerably speed up IMRT quality assurance.61 Dosimetric verification in anatomic or cylindrical phantoms has the advantage of most closely simulating actual patient treatments, both in geometry and in considering the combined effect of all beams. However, as in the case of verifying single fields, it is time consuming, regardless of the dosimetry technique that is chosen, because each field must be separately set up and delivered. As mentioned in the previous section, the difficulties in using film as a photon dosimeter parallel to the beam axis are well known. For this reason, absolute dosimetry of composite distributions of complete IMRT plans generally requires the use of ion chambers or TLD. Verification is made at selected points, e.g., at the approximate locations of the tumor and critical structures. A cylindrical water phantom with a movable ion chamber has been used for this purpose.39 A specially constructed rectilinear phantom designed for TLD and film dosimetry has also been used for IMRT verification.55,69 Other phantoms specifically designed for IMRT verification are now available commercially. When using an ion chamber, care must be taken to ensure the chamber is located in a region where the gradient does not change significantly over the active volume of the chamber. Chambers with a small active volume, such as ion chambers designed for stereotactic radiosurgery, are useful in this regard. As is the case for single fields, the composite distribution must be calculated for the same geometry as the measurement phantom. Cylindrical phantoms for verification of head and neck treatments should be 16 to 20 cm in diameter, for thorax and pelvis, 30 to 40 cm in diameter.

Dosimetric validation for MLC-based IMRT ● M.R. ARNFIELD et al.

Fig. 2. (a) An example of a comparison of calculated and film-measured isodose plots for a single field from a 9-field head and neck IMRT plan. The dashed vertical line indicates the location of the dose profile depicted in (b). The final dose calculation of the optimized plan was performed with a modelbased (superposition) algorithm. (b) Plots of the calculated and measured profiles corresponding to the vertical dashed line in (a), for detailed evaluation of the accuracy of dose calculation.

Achievable accuracy In 1994, the AAPM TG40 report on comprehensive QA for radiation oncology recommended a tolerance of 2% for dose calculation accuracy of single fields, or 2 mm in regions of high dose gradient.36 As of yet, no equivalent standard has been published for IMRT. In fact it is not clear exactly how to characterize the dosimetric error of IMRT plans. Significant dose gradients often

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exist throughout the irradiated area, even including the target. Errors may be small in one part of a 3D dose distribution, but large elsewhere. In this situation, it is not clear how to report discrepancies between measurements and the planning system. Correlation methods that take into account the entire 2D dose map are one alternative approach to dosimetric comparisons.61 Another important issue for which quality assurance standards have not been established is the question of how much patient-specific dosimetry must be performed on a routine basis after an IMRT program is established. For example, full dosimetric verification of all fields might be performed on the first 10 patients when a new treatment site is introduced (e.g., prostate, head and neck, breast). Then, after confidence is gained in the accuracy of beam delivery, the dosimetry can be reduced to a subset of randomly selected fields, rather than all fields of a given plan. Ideally, the accuracy of IMRT dose delivery should approach that of conventional fields. An interinstitutional study demonstrated that in the absence of inhomogeneities, calculation accuracy of 3% or 3 mm was achievable in general for conventional fields.70 Based on this, an acceptability criteria has been suggested for IMRT-distributions of 3% in dose or 3 mm displacement of isodose lines, whichever is lower.11 In a study of prostate patients treated via the sliding window technique,6 calculated doses in 20 of 21 separate fields were found to agree with single TLD measurements within 5%.71 In a series of 400 prostate IMRT patients, in most cases, calculations of the combined dose of 5 fields agreed within 2% with ion chamber measurements at a point and isodose lines generally agreed within 2 mm.72 In an implementation of the dynamic arc technique, it has been reported that ion chamber-measured doses at isocenter agree with calculations within 2%.9,10 It has been noted that dosimetric uncertainties in single fields may be greater than in the total distribution of all fields, presumably due to cancellation of errors in the multiple-field case.12,69 The achievable level of dosimetric accuracy partly depends on the degree of modulation and the delivery technique. As was mentioned in a prior section, the more highly modulated a field, i.e., with high amplitude and closely spaced peaks and valleys, the more monitor units are needed to deliver it. This leads to a greater portion of leakage radiation that must be corrected for, and corrections may only be approximate.20 In general, increasing the monitor units (lowering the efficiency) degrades the dosimetric accuracy.73 The narrow MLC openings required to deliver highly modulated fields can produce substantial dosimetric error. For example, the dosimetric accuracy of head and neck treatments, which generally require greater fluctuations in the intensity map, may be more difficult to achieve than for prostate, which requires less modulated intensities. Discrepancies of 15% were found between measured and computed output fac-

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tors for narrow beamlets used in head and neck IMRT plans.41 It was noted in the section on dosimeters that dosimetric discrepancies in some studies were larger for lower isodose lines, and that this is attributable to the increased sensitivity of film to low-energy photons. However, part of this effect may, at least in some cases, be caused by the lack of accounting for in-phantom lateral scatter in the dose calculation algorithm. Discrepancies of up to 3.8 mm between planned and measured isodose lines have been observed for predicted doses from a common IMRT planning system compared to measurements of rectangular fields in a water phantom.39

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5.

6.

7.

8.

9.

10.

CONCLUSIONS At present, the dosimetric needs for commissioning and implementing IMRT are considerable. When film dosimetry is used the irradiation, scanning and analysis of all fields of an IMRT plan is laborious and time consuming. Portal imaging dosimetry is still under active development, and eventually it will probably replace film as the standard tool for ongoing verification of IMRT fields, after the commissioning phase. The maturation of EPID methods will significantly reduce the time demands of the physics component of IMRT programs. Organizations such as AAPM and ICRU have not yet issued guidelines or standards for commissioning and quality assurance of IMRT systems. Despite several reports of good accuracy in clinical series, some problems in IMRT dosimetry have not been completely resolved. Remaining difficulties are mainly for highly modulated beams, which in practice, require dynamic techniques such as the sliding window method or tomotherapy to deliver. Some specific issues are the dosimetry of small fields, accelerator instability at low monitor units, and the correct implementation of leaf effect and head scatter corrections. The dose accuracy of treatments involving inhomogeneities will improve as model-based algorithms replace semi-empirical algorithms in IMRT planning systems.

Acknowledgment—This work was supported in part by National Cancer Institute Grant CA 74043.

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