- Email: [email protected]
Dosimetry measurement tools for commissioning Enhanced Dynamic Wedge
Copyright
Medical Dosimetry, Vol. 22, No. 3. pp. 171-176. 1997 0 1997 American Association of Medical Dosimetnsts Printed in the USA. All rights reserved 0958-3947/97 $17.00 + .oO
PI1 so958-3947(97)ooo14-9
ELSEVIER
DOSIMETRY
MEASUREMENT TOOLS ENHANCED DYNAMIC DENNIS D. LEAVI?T, PH.D.’
’ Department
FOR COMMISSIONING WEDGE
ERIC KLEIN,
M.S.2
of RadiationOncology, University of Utah Health SciencesCenter,Salt Lake City, UT 84132:and ’ Mallinkrodt Institute of Radiology,RadiationOncologyCenter.St. Louis,MO 63110
Abstract-Measurement of Enhanced Dynamic Wedge parameters requires that the dose at a measurement point be integrated during the entire exposure as the moving jaw closes to form the dose distribution. This major difference from static fields, where dose rate measurements can be made using a single moving probe, requires major modification in measurement techniques to use devices which can measure multiple points within the field simultaneously. Each of these new tools introduce unique problems in dose measurement, which must be understood prior to their use. This paper discusses these tools. 0 1997 American Association of Medical Dosimetrists. Key Words:
Dosimetry,
Enhanced
Dynamic
Wedge,
Detectors.
making this a more efficient process than previously available. Each measurementdevice may have some advantage for one or more of the required measurements.
INTRODUCTION Dosimetry measurements’ are required to validate the key characteristics of Enhanced Dynamic Wedge. (‘m7’ Measurements should be evaluated vs. field size and wedge angle for the following: surface dose, depth dose, wedge factor, beam profiles, and peripheral dose. In some computerized treatment planning systems these parameters will be entered in tabular form, while in other systems these terms will be generated from the segmented treatment tables (STT) provided by Varian specific to each beam energy. Several dosimetry measurement tools are available, and the choice of tools may depend upon the data requirements of the treatment planning system. Several measurementdevices are available for dynamic wedge measurements.These include ionization chambers, diodes, radiographic verification film, and thermoluminescent dosimeters (TLD). Both parallelplate and cylindrical ionization chambersare commonly used in radiation therapy measurements.Energy compensateddiodes are similar in energy responseto ionization chambers, but offer a smaller measurementcrosssection and thinner depth, thereby making possibledose measurementsin the buildup region. Radiation therapy verification film is commonly used in radiation therapy departments, and is supported by several commercial film densitometry systems. TLDs are available in reusable rods, chips, and discs, and may be used in vivo and in anthropomorphic phantom studies. Additionally, TLD rods of 1 mm diameter have been used -in solidwater phantoms to measuredepth doses,buildup doses, and beam profiles. Newer TLD readerscan sequentially processup to 50 TLD rods without intervention, thereby Reprint
requests to: Dennis Leavitt, University of Utah Health 84 132.
ationOncology. City,
UT
Ph.D., Department Sciences Center,
DEPTH
DOSE MEASUREMENT
Ion chamber measurementsshould be used to measure the central axis depth doses.The small-volume ion chambers used in water phantom dosimetry systems work well for these measurements.Water phantom systemsallow measurementof depth doseswithout repeated entry into the treatment room to adjust probe depth position. Newer water phantom systems allow integration of dose at programmedpoints. The integrated reading of both the movable scanning ion chamber and the fixed-position reference chamber can be recorded for each dynamic wedge exposure. The ratios can then be calculated to determine the depth doses.Using the reference chamber for these measurementswill allow correction for any change in machine output with time. Since each depth dose measurementrequires integration of a complete dynamic wedge treatment, the number of depths at which dosesare measuredcan be abbreviated, and the intermediate values interpolated. Typically, measurementsat d,,,, 2.5 cm, 5 cm, and additional depthsin stepsof 5 cm to a depth of 30 or 40 cm are adequateto construct the depth dosecurve. The depth dose measurements should be repeated with the collimator rotated 180” from its initial position, and the two setsof readings averaged to account for any misalignment of the mechanical setup. The measurementsrequired for depth dosesor beam profiles may be very time consuming,relative to similar measurementsfor standardwedge fields or open fields; therefore, special care must be exercised to verify the water surface level in the water phantom. For measurements extending over several hours or days, the water
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level in the tank becomes subject to change due to evaporation. An independent indicator, such as a front pointer attached near the periphery of the tank, can be usedto confirm the water level. This should be checked frequently, and water added to the tank as needed. Failure to keep the water 1eveI constant will lead to error in the relative depth dose measurements. Water phantom alignment with the central axis of the radiation field (registration) is critically important, since minor angular misalignment can introduce larger errors in depth dose measurementsfor wedge fields, as the ion chamber may view a different effective wedge transmissionif it deviates from the central axis of the field with depth. Typically, misalignment errors can be minimized by measuring the depth doses twice, once with the collimator at 90” and a second time with the collimator at 270”, and averaging the measurements. This sametechnique is repeated when determining effective wedge transmissionfactors. The following alignment tips are suggested: 1. Place the water phantom on a leveling device, such asa 3-point systemwhich allows adjustmentof the plane on which the tank rests.This may be included in the measurementor transport cart provided with the water phantom, or may be a separatedevice which would attach to the bottom of the phantom and rest on the treatment couch or someother fixed device. 2. Place a circular bubble level in the bottom of the water phantom. Adjust the leveling device until the bubble is centered in the circle. 3. Check the angular alignment of the gantry by placing a spirit level on the flat face of the accessory mount. (There may be an offset error in the digital or analogreadout of gantry angle on the face of the linear accelerator.) 4. Center the detector in the cross-hairsof the light field. Move the detector acrossthe extended range of depths through which measurementswill be made. Verify that the cross-hairsremain centered on the detector. Any deviation of the cross-hair projection acrossthe range meansthat the verticality of the water phantom, the detector track, and the linear accelerator are not aligned. 5. Verify the motion of the detector again after the phantom has been filled with water. Misalignment can occur due to settling or flex after the tank has been filled. Tissue-equivalentor water-equivalent phantomsare an alternative to water phantoms.Solid water phantoms can be usedto measurerelative dosevs. isocenter depth. From thesemeasurements,depth doses,tissue-phantomor tissue-maximumratios can be constructed for use in treatment planning systems.This technique has the advantage that the probe is always positioned at isocenter, thereby removing the uncertainty of misalignment encountered if the probe is repositionedto greater depthsas
Volume 22, Number 3, 1997
in the water phantom. The disadvantageof this technique is that repeatedentry into the treatment room is required to change the amount of phantom material above the measurementprobe. Surface dose and build-up dosescan be measured with a variety of instruments.Most commonly used is a parallel-plate ionization chamber. Thesechambersallow the measurementof surface doseand build-up curves by the sequential addition of thin plastic sheetsabove the parallel-plate chamber,until the depth of maximum dose hasbeenreached.Corrections must be applied relative to the effective measurementposition within the parallel plate chamber. These measurementsare very time consuming, since the measurementsmust be repeated for both polarities of the electrometer, and the readings averaged.Differences in surface doseand build-up doses have been reported between different commercially available parallel-plate chambers.This difference is attributed to the design of the chambers. Alternatively, water-tight parallel plate chambersare available for measurementin a water phantom. These chambers, such as the NACP and Markus chambers,require an approximate 1 mm thick entrance window, so surface measurements must be extrapolated from the build-up measurements. Surface dose and build-up dosescan also be measured using diodes. In a fashion similar to that using parallel-plate ionization chambers, a diode can be mountedin a solid phantom with its face coincident with the face of the phantom facing the incident radiation beam. Additional sheets of buildup material can be added,and readingsaccumulateduntil the depth of maximum doseis achieved. Alternatively, diodesmounted in a water phantom can be positioned at incremental depths from the surfaceto the depth of maximum doseand dose readings accumulated. In both these techniques using diodes, a true surface dosecannot be measured,sincethe effective measurementpoint of the diode is beneath 0.5 mm epoxy resin buildup material. Thus, the effective surface dose must be extrapolated from the buildup curve. Relative build-up curves can also be estimatedusing film densitometry. As discussedbelow, extreme care must be exercised in the use of radiographic film for dynamic wedge measurements.These same concerns extend to estimation of dosesin the buildup region. WEDGE FACTOR
MEASUREMENT
Ionization chamber measurements are recommendedto verify the effective wedge factor. With the ion chamberpositionedat 10 cm depth in phantom on central axis, the reading for each dynamic wedge angle is recorded for a fixed number of monitor units. This is repeated for the open field for the same number of monitor units. The ratio of readings is the effective wedge factor. This process is repeated for all square fields from 4 to 40 cm wide. Since intermediate wedge anglescan be createdfrom 10 to 60”, the effective wedge
Dosimetry measurement tools 0 D. D. LEAVIIT
factor for these intermediate angles can also be measured. This serves as an independent verification of the ratio of tangents method of calculating the effective wedge factors for the intermediate wedge angles. BEAM
PROFILE
MEASUREMENT
Two techniques are available to measuredynamic wedge beam profiles. Film densitometry has long been used in radiation measurement.More recently, direct measurementof radiation dose distributions using a linear array of ionization chambersor energy-compensated diode detectors hasbeen introduced. The primary advantage of linear detector arrays is that the measurements made in a standard water phantom apply directly in profile determination with no additional corrections. The primary disadvantageis that a large number of sequential exposures4-10must be completed at each depth in order to define the beam profile at the number of depths typically required by treatment planning systems.Compared to measurementusing a single ionization chamber or diode, the measurementtime is reduced proportional to the number of detectors in the array. Measurement using film densitometr?, Film densitometry offers the advantage that a complete field can be defined in one exposure. Two-dimensional profiles can be measured by mounting a single film parallel to the central axis of the beam; 3D profile data can be accumulated by mounting individual films perpendicular to the central axis of the field at the depths in phantom required by the treatment planning data acquisition system. Proper indexing of the films is accomplished by marking the central axis using thin solder wire. This creates a narrow indentation in the beam profile at the location correspondingto the central axis of the field, and allows an independent verification of the field alignment. The primary disadvantagesof film are that depth and energy spectrum corrections must be determined and applied to the film density measurements, in addition to the normal density-to-dose conversion factors. in order to achieve reliable results. Additionally, care must be exercised in the film processor quality control and positioning techniques of film in phantom. Several commercially available film dosimetry cassettescan be applied to dynamic wedge dosimetry. Additionally, companiesand individuals now offer film densitometry services which include reading processed films and reporting the dosimetry data back in a form compatible with the treatment planning system requirements. This makesit possiblefor institutions without an on-site film densitometry system to still use this technique for dose measurement. Although commercial cassettes are available in which barefilm can be exposed,the useof thesecassettes requires that each exposure be preceded by loading the bare film in a darkroom, then unloading the film in the darkroom after exposure. This adds considerabletime to
AND
E. E. KLEIN
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the measurementprocess.Several institutions have made all exposuresusing the radiation therapy verification film in its paper envelope. The paper lip of the envelope above the film edgemust be folded over, SOthat the film edge can be madecoincident with the leading edgeof the phantom; and each film envelope should be pierced with a pin to evacuate any air in the envelope during compression into the phantom. This technique has been found much more efficient and still delivering good results. Extreme care is required in processingradiographic film for dynamic wedge measurements.The low-volume film processorscommonly used in radiation therapy departments are not designedto processlarge numbersof films rapidly. The processortemperature rises if a large number of films are fed through the processor, thereby changing the film density to dose response, and the processor rollers often introduce streak artifacts to the film. Becauseof thesedifficulties, it is recommendedthat a high-volume film processor which is subject to very tight quality control be used to process the dynamic wedge films. Such processorsare readily available in diagnostic radiology departments,and are typically used to process mammographic films where high detail is required. The softer rollers on theseprocessorsminimize any streak artifacts. If using film densitometry measurements,two techniques can be used to measurebeam profiles. Radiation therapy verification film (XV2) can be sandwichedbetween polystyrene or “solid water” sheetsand placed parallel to the central axis of the radiation field. The dose delivered to the film can be adjusted so that the maximum dose near the tip of the wedge remains within the films’ doserange. (Typically, 75 cGy delivered at 10 cm depth satisfies this requirement.) A standard 2-dimensional isodosedistribution can be derived from this film. Smooth apposition of film and phantom can be achieved by inserting sheetsof “superfiab” between the film and the solid phantom sheets.The “superflab” expands laterally under compression. thus avoiding pressureartifacts and air cavity artifacts. One film is required for each field size and wedge angle. Radiation therapy verification film (XV2) can be sandwiched between polystyrene or “solid water” sheetsand placed perpendicular to the central axis of the radiation field. This “beam’s eye view” arrangement of the film requires that multiple films be used for each field size and wedge. but allows the evaluation of 3D dose distributions for each wedge field. For typical fan-line/grid-line tabular data treatment planning systems, five or six films would have to be used for each field size and wedge, with the films placed at d,,,, and at equal depth increments beyond d,,,. Beam profiles have been measuredand compared
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for the two film measurement techniques. These comparisons show that the measured profiles are in good agreement at all depths, as long as reasonable care is taken in the film processing. TWO specific problems must be addressed with the use of film densitometry: film alignment (registration) and film density-to-dose conversion. Film alignment can be verified by placing a thin solder wire across the film edge along the cross-hair shadow defining the perpendicular bisectors of the light field on the phantom surface. This creates a narrow line along the entire length of the film corresponding to the central axis of the field. The center of the scannedprofile can then be aligned using this line. Alternatively, an alignmentjig can be built into the film cassette.This can consist of pins embeddedin one half of the cassette, correspondingto the position of central axis just below the entrance surface of the phantom and just above the exit edge of the film. A mark on the entrance surface of the cassette,correspondingto the position of the pinprick in the film, is then used to center the field cross-hairson the phantom. The pinprick at the bottom of the film then allows alignment of the central axis of the field relative to the film, and accounts for any skewing of the film during placement in the cassette.These pinpricks show up on the film as localized dark spotson the film and can be aligned with the film densitometertemplate. Altematively, the pinpricks can be placed at the edgesof the film and matched with positions on the film densitometer template to assureproper alignment of the film for scanning. Film density-to-dose conversion (H&D curve) must be carefully addressed.Use of a single density-to-dose conversion curve will not account for change in film response due to energy change with depth. Similarly, film responsein the penumbraregion may changedue to the changein energy spectrumof the scatteredradiation. These effects can be studied using techniques such as those describedby Williamson et al.* The film H&D curve shouldbe measuredusing film placed in the phantom in the same orientation used to expose the dynamic wedge fields. That is, if the wedge films are exposed using the film parallel to the central axis of the field, then the H&D curve should be determined using a series of films exposed parallel to the central axis of the field. The H&D curve can be determined using a fixed open field size of 10 cm X 10 cm for a seriesof exposuresup to 200 cGy. The density-to-dose relationship can then be determined by searchingfor the maximum on each film and relating that to the delivered dose. Most newer film densitometry systems have an option to automatically perform this step. In order to achieve maximum differentiation of dose from the film densities, the maximum dose delivered should be adjusted downward so that the maximum film density is still within the steeply ascending portion of the H&D curve. Film sensitivity vs. depth is dependent on photon
Volume
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3, 1997
The film densitometer output signal was measured vs. dose at dmaxand at 30 cm depth on central axis. Similar measurementswere made at intermediate depths in 5 cm increments. These curves suggest an increased film sensitivity of approximately 4.2% at 25 cm for 6 MV photons, while showing no increase in film sensitivity for 18 MV photons. This effect can easily be folded into film densitometry scanning by including a simple dose modifier which corrects the densitometer reading by the inverse of the sensitivity factor vs. depth. This can be represented as: energy.
D(d,R) = R*(H/D)“(l/(l + b(d - d,,,)))
(1)
where D(d,R)= correcteddose R = film densitometer reading H/D = film densityto doseconversion b =film sensitivity coefficient determined by least squaresfit to measured sensitivitydata. Evaluation of data for 6 MV suggest a value for b = 0.0018 cm-‘. If the smallcorrection due to energy changesoff-axis is ignored, this sensitivity effect is linear with depth; therefore, the entire film density profile at any given depthcan be modified by this singlecorrection factor. The addition (and magnitude)of a small correction factor for energy changes off-axis is reportedby Clewlow et al2 As an alternative to the standardXV2 film paper envelope, a newly introduced vacuum packaging provides a water-tight film packet that can be positioned in a water phantom to enable measurementsin an environment closer to that commonly done with ion chambers and diodes. Radiochromic film is also available, but has not been applied to enhanceddynamic wedge measurementsdue to the higher doserequired and the higher cost per film. Three types of film densitometershave beenapplied to radiotherapy dosimetry measurement:a video camera mounted above a light box and acquiring a gray scale image via a frame grabber, a scanning light source and photodiode which continuously records transmittance and can be sampled via an Analog-to-Digital Circuit (ADC) or can be plotted in analog form, and a linear film scanner which passesthe film across a line source and readstransmittance to a linear array of 4000 CCDs. Measurement using linear detector arrays Linear detectorarrays areavailablefrom severalcommercialvendors.Linear diodearraysare composedof 11 or 25 energy-compensated p-type diodesspaced2.5 cm apart (11 diodes)or 2 cm apart (25 diodes)with an option to foId the array inward to decreasethe diodespacingto 1 cm. This allows narrow fields to be integrated in fewer steps, or wider fields to be integrated in a larger number of steps. Ionization chamberarrays consist of 41 ionization chambersspaced1 cm apart.Recentdosimetryintercomparisons show nearly identical responseof diode and ionization
Dosimetry measurement tools 0 D. D. LEAVITT AND E. E. KLEIN
chamberin measurement of openfield and dynamic wedge beam profiles.6 In addition to the alignment checksdiscussedabove, additional checksmust be evaluatedfor the linear detector arrays. Specifically, the depth of each detector relative to the others must be verified. Although each detector may have a fixed position relative to the rest of the array, the entire array, when mounted in the water tank, may be skewedrelative to the water surface.Typically, shimsare provided to adjust the entire array in order to achieve the desiredcoincidencewith the water surface. The relative sensitivity from detector to detector must be carefully monitored during dose measurement. The sensitivity of individual diodes change with dose absorbedby the diode. These sensitivities are relatively slowly varying, but must be accounted for in the measurements.In the measurementof dynamic wedge fields, the individual diodes will be exposed to different dose levels, varying by orders of magnitude, depending upon the location of the diode, For example, the diodes near the tip of the wedge will receive roughly twice the dose received by diodes on central axis, while diodesnear the heel of the wedge will receive only half the central-axis dose. Similarly, diodes outside the geometric limits of the radiation field will receive only a few percent of the central axis dose. Calibration techniques are recommended by the system manufacturers for careful crosscomparison of the individual detector sensitivity. Thus, in the smaller arrays which can be moved across the water phantom, each diode will be sequentially positioned at the central axis of the radiation field and a fixed dose delivered in that configuration. This process is repeatedfor each diode in the array, This allows an exact intercomparisonof every diode. Alternatively, the largest possiblefield is set to irradiate the water tank, such that all detectors are simultaneouslyin the radiation field. For the sameexposure, the reading for each detector is recorded; the detector array is then shifted a distanceequal to the spacing between detectors, and the exposure is repeated. The relative sensitivity of each detector in the array can then be determined by evaluating the ratios of readingsbetween the neighboring detectors. As an added check on detector sensitivity, one or more overlap points is included in the positioning of the detector array. Thus the last point measuredby one detector will correspond to the first point measuredby the neighboring detector, allowing a direct intercomparison of readings. The linear detector arrays can measure dynamic wedge dose profiles at any depth in the water phantom. Care must be exercised, however, in evaluating measurements in the buildup region. The ionization chamber array mounts the individual detectors such that the long axis of the detector is parallel to the central axis of the beam. This compromisesthe ability of the detectors to measuredose in the buildup region where the dose gradient is changing rapidly acrossthe length of the detector. The linear diode array has an active thickness for each detector of less than 1 mm. This allows measure-
175
ment of dose in the buildup region; however, the mechanical placement of each diode must be carefully verified. Otherwise, large differences in reported surface doseand superficial doseswill be noted from detector to detector. This will introduce discontinuities in the dose profiles in the buildup region. Measurement using thermoluminescentdosimeters Re-usableLiF rods can be usedfor dynamic wedge measurements.These rods are of a diameter 1 mm, approximately 6 mm long. The rods have a higher content of LiF than did previous rods, and therefore are more consistent in response.These rods, in combination with readerswhich can processup to 50 TLDs without intervention, may be useful in determination of the buildup doses,depth doses,and beam profiles at various depths. Careful handling and processing of the TLD rods is required to ensureconsistentperformance. For example, the following recommendationsare made by Bicron for managementof their TLD-100 rods: “Handling: All looseTLD materialshouldbe handled with vacuum tweezers,not mechanicaltweezersor fingers. Small scratches,lossof mass,or foreign depositswill all affect the light emissioncharacteristicsof the TL material. Cleaning: Between normal uses,the dosimetersshouldbe rinsed, not soaked, with an analytical grade anhydrous methyl alcohol and dried by evaporation for at least one hour. They shouldthen be annealedaccordingto the correct procedurefor the given materialone time before actualuse. The annealwill aid in removing any residual alcohol deposits.Annealing: For annealingup to 400°C the containers shouldbe wade from high temperaturestainlesssteelor oxidized aluminum.Never use non-oxidized aluminum! It is recommendedthat the metal be thin to allow rapid cooling following annealing. A dedicated annealing oven shouldbe usedto help prevent contaminationfrom foreign substances. To avoid inconsistentheat gradients,the annealing containersshould not be stackedwithin the oven nor allowedto touch the oven walls. Theseshouldbe placedon open oven racks with air space all around.” (Harshaw RadiationMeasurementProductsSpecificationSheet:Handling andThermal Treatmentsof Bare Thermoluminescent Dosimeters).The heat treatmentfor TLD- 100rodsrequires pre-irradiation at 400” for 1 h followed immediately by 100°C for 2 h, then cooled on a cold surface to room temperature.Post-irradiationat 100°Cfor 10 min is recommendedto minimize the TLD fading. The consistencyof the TLD responsecan be improved by putting the rods through the read cycle 5-10 times prior to first irradiation. This hasbeenreferred to as“training the TLDs.” (personal communication,Harshaw). SUMMARY Dosimetry measurementtools are readily available for the measurementsnecessaryto commissionenhanced dynamic wedge. The choice of tools to apply will depend upon the specific data requirements of the individual
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Dosimetry
treatment planning system. Consistency in production quality control of the linear accelerators strongly suggests that, once the extensive commissioning measurements have been made and evaluated for a specific energy and machine type, subsequent installations can selectively verify those measurements specific to their own linear accelerator, and thus minimize the volume of measurements first made to evaluate and define the enhanced dynamic wedge characteristics.
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REFERENCES 1. Bidmead, A.M.; Garton. A.J.: Childs, P.J. Beam data measurements for dynamic wedges on Varian 600C (6 MV) and 2100C (6 and 10 MV) linear accelerators. Phys. Med. Biol. 40~393-411; 1995. 2. Clewlow, J.P.; Waggener, R.; Feldmeier. J.J.; Bite, W.S. Film
7. 8.
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dosimetry of a Varian dynamic wedge. (Poster Session S-7, AAPM Annual Meeting, Calgary, Canada, 1992). Elder. P.J.; Coveney, F.M.; Welsh, A.D. An investigation into the comparison between different dosimetric methods of measuring profiles and depth doses for dynamic wedges on a Varian 6OOC linear accelerator. Phys. Med. Biol. 40:683-689; 199.5. Klein, E.E.; Low, D.A.; Meigooni, A.S.; Purdy, J.A. Dosimetry and clinical implementation of dynamic wedge. Inf. J. Radiat. Oncol. Biol. Phys. 31:583-594; 1995. Leavitt, D.D.; Martin. M.; Moeller, J.H.; Lee, W.L. Dynamic wedge field techniques through computer controlled collimator motion and dose delivery. Med. Phys. 17:87-91; 1990. Leavitt. D.D.; Larson, L.G. Evaluation of a diode array for measurement of dynamic wedge dose distributions. Med. Phys. 20:381382; 1993. Lydon, J.M.; Rykers, K.L. Beam profiles in the nonwedged direction for dynamic wedges. Phys. Med. Biol. 41:1217-1225; 1996. Williamson, J.F.: Khan, F.M.; Sharma, S.C. Film dosimetry of megavoltage photon beam: a practical method of isodose-to-isodensity curve conversion. Med. Phys. 8:94; 1981.
Medical Dosimetry, Vol. 22, No. 3. pp. 171-176. 1997 0 1997 American Association of Medical Dosimetnsts Printed in the USA. All rights reserved 0958-3947/97 $17.00 + .oO
PI1 so958-3947(97)ooo14-9
ELSEVIER
DOSIMETRY
MEASUREMENT TOOLS ENHANCED DYNAMIC DENNIS D. LEAVI?T, PH.D.’
’ Department
FOR COMMISSIONING WEDGE
ERIC KLEIN,
M.S.2
of RadiationOncology, University of Utah Health SciencesCenter,Salt Lake City, UT 84132:and ’ Mallinkrodt Institute of Radiology,RadiationOncologyCenter.St. Louis,MO 63110
Abstract-Measurement of Enhanced Dynamic Wedge parameters requires that the dose at a measurement point be integrated during the entire exposure as the moving jaw closes to form the dose distribution. This major difference from static fields, where dose rate measurements can be made using a single moving probe, requires major modification in measurement techniques to use devices which can measure multiple points within the field simultaneously. Each of these new tools introduce unique problems in dose measurement, which must be understood prior to their use. This paper discusses these tools. 0 1997 American Association of Medical Dosimetrists. Key Words:
Dosimetry,
Enhanced
Dynamic
Wedge,
Detectors.
making this a more efficient process than previously available. Each measurementdevice may have some advantage for one or more of the required measurements.
INTRODUCTION Dosimetry measurements’ are required to validate the key characteristics of Enhanced Dynamic Wedge. (‘m7’ Measurements should be evaluated vs. field size and wedge angle for the following: surface dose, depth dose, wedge factor, beam profiles, and peripheral dose. In some computerized treatment planning systems these parameters will be entered in tabular form, while in other systems these terms will be generated from the segmented treatment tables (STT) provided by Varian specific to each beam energy. Several dosimetry measurement tools are available, and the choice of tools may depend upon the data requirements of the treatment planning system. Several measurementdevices are available for dynamic wedge measurements.These include ionization chambers, diodes, radiographic verification film, and thermoluminescent dosimeters (TLD). Both parallelplate and cylindrical ionization chambersare commonly used in radiation therapy measurements.Energy compensateddiodes are similar in energy responseto ionization chambers, but offer a smaller measurementcrosssection and thinner depth, thereby making possibledose measurementsin the buildup region. Radiation therapy verification film is commonly used in radiation therapy departments, and is supported by several commercial film densitometry systems. TLDs are available in reusable rods, chips, and discs, and may be used in vivo and in anthropomorphic phantom studies. Additionally, TLD rods of 1 mm diameter have been used -in solidwater phantoms to measuredepth doses,buildup doses, and beam profiles. Newer TLD readerscan sequentially processup to 50 TLD rods without intervention, thereby Reprint
requests to: Dennis Leavitt, University of Utah Health 84 132.
ationOncology. City,
UT
Ph.D., Department Sciences Center,
DEPTH
DOSE MEASUREMENT
Ion chamber measurementsshould be used to measure the central axis depth doses.The small-volume ion chambers used in water phantom dosimetry systems work well for these measurements.Water phantom systemsallow measurementof depth doseswithout repeated entry into the treatment room to adjust probe depth position. Newer water phantom systems allow integration of dose at programmedpoints. The integrated reading of both the movable scanning ion chamber and the fixed-position reference chamber can be recorded for each dynamic wedge exposure. The ratios can then be calculated to determine the depth doses.Using the reference chamber for these measurementswill allow correction for any change in machine output with time. Since each depth dose measurementrequires integration of a complete dynamic wedge treatment, the number of depths at which dosesare measuredcan be abbreviated, and the intermediate values interpolated. Typically, measurementsat d,,,, 2.5 cm, 5 cm, and additional depthsin stepsof 5 cm to a depth of 30 or 40 cm are adequateto construct the depth dosecurve. The depth dose measurements should be repeated with the collimator rotated 180” from its initial position, and the two setsof readings averaged to account for any misalignment of the mechanical setup. The measurementsrequired for depth dosesor beam profiles may be very time consuming,relative to similar measurementsfor standardwedge fields or open fields; therefore, special care must be exercised to verify the water surface level in the water phantom. For measurements extending over several hours or days, the water
of RadiSalt Lake
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level in the tank becomes subject to change due to evaporation. An independent indicator, such as a front pointer attached near the periphery of the tank, can be usedto confirm the water level. This should be checked frequently, and water added to the tank as needed. Failure to keep the water 1eveI constant will lead to error in the relative depth dose measurements. Water phantom alignment with the central axis of the radiation field (registration) is critically important, since minor angular misalignment can introduce larger errors in depth dose measurementsfor wedge fields, as the ion chamber may view a different effective wedge transmissionif it deviates from the central axis of the field with depth. Typically, misalignment errors can be minimized by measuring the depth doses twice, once with the collimator at 90” and a second time with the collimator at 270”, and averaging the measurements. This sametechnique is repeated when determining effective wedge transmissionfactors. The following alignment tips are suggested: 1. Place the water phantom on a leveling device, such asa 3-point systemwhich allows adjustmentof the plane on which the tank rests.This may be included in the measurementor transport cart provided with the water phantom, or may be a separatedevice which would attach to the bottom of the phantom and rest on the treatment couch or someother fixed device. 2. Place a circular bubble level in the bottom of the water phantom. Adjust the leveling device until the bubble is centered in the circle. 3. Check the angular alignment of the gantry by placing a spirit level on the flat face of the accessory mount. (There may be an offset error in the digital or analogreadout of gantry angle on the face of the linear accelerator.) 4. Center the detector in the cross-hairsof the light field. Move the detector acrossthe extended range of depths through which measurementswill be made. Verify that the cross-hairsremain centered on the detector. Any deviation of the cross-hair projection acrossthe range meansthat the verticality of the water phantom, the detector track, and the linear accelerator are not aligned. 5. Verify the motion of the detector again after the phantom has been filled with water. Misalignment can occur due to settling or flex after the tank has been filled. Tissue-equivalentor water-equivalent phantomsare an alternative to water phantoms.Solid water phantoms can be usedto measurerelative dosevs. isocenter depth. From thesemeasurements,depth doses,tissue-phantomor tissue-maximumratios can be constructed for use in treatment planning systems.This technique has the advantage that the probe is always positioned at isocenter, thereby removing the uncertainty of misalignment encountered if the probe is repositionedto greater depthsas
Volume 22, Number 3, 1997
in the water phantom. The disadvantageof this technique is that repeatedentry into the treatment room is required to change the amount of phantom material above the measurementprobe. Surface dose and build-up dosescan be measured with a variety of instruments.Most commonly used is a parallel-plate ionization chamber. Thesechambersallow the measurementof surface doseand build-up curves by the sequential addition of thin plastic sheetsabove the parallel-plate chamber,until the depth of maximum dose hasbeenreached.Corrections must be applied relative to the effective measurementposition within the parallel plate chamber. These measurementsare very time consuming, since the measurementsmust be repeated for both polarities of the electrometer, and the readings averaged.Differences in surface doseand build-up doses have been reported between different commercially available parallel-plate chambers.This difference is attributed to the design of the chambers. Alternatively, water-tight parallel plate chambersare available for measurementin a water phantom. These chambers, such as the NACP and Markus chambers,require an approximate 1 mm thick entrance window, so surface measurements must be extrapolated from the build-up measurements. Surface dose and build-up dosescan also be measured using diodes. In a fashion similar to that using parallel-plate ionization chambers, a diode can be mountedin a solid phantom with its face coincident with the face of the phantom facing the incident radiation beam. Additional sheets of buildup material can be added,and readingsaccumulateduntil the depth of maximum doseis achieved. Alternatively, diodesmounted in a water phantom can be positioned at incremental depths from the surfaceto the depth of maximum doseand dose readings accumulated. In both these techniques using diodes, a true surface dosecannot be measured,sincethe effective measurementpoint of the diode is beneath 0.5 mm epoxy resin buildup material. Thus, the effective surface dose must be extrapolated from the buildup curve. Relative build-up curves can also be estimatedusing film densitometry. As discussedbelow, extreme care must be exercised in the use of radiographic film for dynamic wedge measurements.These same concerns extend to estimation of dosesin the buildup region. WEDGE FACTOR
MEASUREMENT
Ionization chamber measurements are recommendedto verify the effective wedge factor. With the ion chamberpositionedat 10 cm depth in phantom on central axis, the reading for each dynamic wedge angle is recorded for a fixed number of monitor units. This is repeated for the open field for the same number of monitor units. The ratio of readings is the effective wedge factor. This process is repeated for all square fields from 4 to 40 cm wide. Since intermediate wedge anglescan be createdfrom 10 to 60”, the effective wedge
Dosimetry measurement tools 0 D. D. LEAVIIT
factor for these intermediate angles can also be measured. This serves as an independent verification of the ratio of tangents method of calculating the effective wedge factors for the intermediate wedge angles. BEAM
PROFILE
MEASUREMENT
Two techniques are available to measuredynamic wedge beam profiles. Film densitometry has long been used in radiation measurement.More recently, direct measurementof radiation dose distributions using a linear array of ionization chambersor energy-compensated diode detectors hasbeen introduced. The primary advantage of linear detector arrays is that the measurements made in a standard water phantom apply directly in profile determination with no additional corrections. The primary disadvantageis that a large number of sequential exposures4-10must be completed at each depth in order to define the beam profile at the number of depths typically required by treatment planning systems.Compared to measurementusing a single ionization chamber or diode, the measurementtime is reduced proportional to the number of detectors in the array. Measurement using film densitometr?, Film densitometry offers the advantage that a complete field can be defined in one exposure. Two-dimensional profiles can be measured by mounting a single film parallel to the central axis of the beam; 3D profile data can be accumulated by mounting individual films perpendicular to the central axis of the field at the depths in phantom required by the treatment planning data acquisition system. Proper indexing of the films is accomplished by marking the central axis using thin solder wire. This creates a narrow indentation in the beam profile at the location correspondingto the central axis of the field, and allows an independent verification of the field alignment. The primary disadvantagesof film are that depth and energy spectrum corrections must be determined and applied to the film density measurements, in addition to the normal density-to-dose conversion factors. in order to achieve reliable results. Additionally, care must be exercised in the film processor quality control and positioning techniques of film in phantom. Several commercially available film dosimetry cassettescan be applied to dynamic wedge dosimetry. Additionally, companiesand individuals now offer film densitometry services which include reading processed films and reporting the dosimetry data back in a form compatible with the treatment planning system requirements. This makesit possiblefor institutions without an on-site film densitometry system to still use this technique for dose measurement. Although commercial cassettes are available in which barefilm can be exposed,the useof thesecassettes requires that each exposure be preceded by loading the bare film in a darkroom, then unloading the film in the darkroom after exposure. This adds considerabletime to
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the measurementprocess.Several institutions have made all exposuresusing the radiation therapy verification film in its paper envelope. The paper lip of the envelope above the film edgemust be folded over, SOthat the film edge can be madecoincident with the leading edgeof the phantom; and each film envelope should be pierced with a pin to evacuate any air in the envelope during compression into the phantom. This technique has been found much more efficient and still delivering good results. Extreme care is required in processingradiographic film for dynamic wedge measurements.The low-volume film processorscommonly used in radiation therapy departments are not designedto processlarge numbersof films rapidly. The processortemperature rises if a large number of films are fed through the processor, thereby changing the film density to dose response, and the processor rollers often introduce streak artifacts to the film. Becauseof thesedifficulties, it is recommendedthat a high-volume film processor which is subject to very tight quality control be used to process the dynamic wedge films. Such processorsare readily available in diagnostic radiology departments,and are typically used to process mammographic films where high detail is required. The softer rollers on theseprocessorsminimize any streak artifacts. If using film densitometry measurements,two techniques can be used to measurebeam profiles. Radiation therapy verification film (XV2) can be sandwichedbetween polystyrene or “solid water” sheetsand placed parallel to the central axis of the radiation field. The dose delivered to the film can be adjusted so that the maximum dose near the tip of the wedge remains within the films’ doserange. (Typically, 75 cGy delivered at 10 cm depth satisfies this requirement.) A standard 2-dimensional isodosedistribution can be derived from this film. Smooth apposition of film and phantom can be achieved by inserting sheetsof “superfiab” between the film and the solid phantom sheets.The “superflab” expands laterally under compression. thus avoiding pressureartifacts and air cavity artifacts. One film is required for each field size and wedge angle. Radiation therapy verification film (XV2) can be sandwiched between polystyrene or “solid water” sheetsand placed perpendicular to the central axis of the radiation field. This “beam’s eye view” arrangement of the film requires that multiple films be used for each field size and wedge. but allows the evaluation of 3D dose distributions for each wedge field. For typical fan-line/grid-line tabular data treatment planning systems, five or six films would have to be used for each field size and wedge, with the films placed at d,,,, and at equal depth increments beyond d,,,. Beam profiles have been measuredand compared
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for the two film measurement techniques. These comparisons show that the measured profiles are in good agreement at all depths, as long as reasonable care is taken in the film processing. TWO specific problems must be addressed with the use of film densitometry: film alignment (registration) and film density-to-dose conversion. Film alignment can be verified by placing a thin solder wire across the film edge along the cross-hair shadow defining the perpendicular bisectors of the light field on the phantom surface. This creates a narrow line along the entire length of the film corresponding to the central axis of the field. The center of the scannedprofile can then be aligned using this line. Alternatively, an alignmentjig can be built into the film cassette.This can consist of pins embeddedin one half of the cassette, correspondingto the position of central axis just below the entrance surface of the phantom and just above the exit edge of the film. A mark on the entrance surface of the cassette,correspondingto the position of the pinprick in the film, is then used to center the field cross-hairson the phantom. The pinprick at the bottom of the film then allows alignment of the central axis of the field relative to the film, and accounts for any skewing of the film during placement in the cassette.These pinpricks show up on the film as localized dark spotson the film and can be aligned with the film densitometertemplate. Altematively, the pinpricks can be placed at the edgesof the film and matched with positions on the film densitometer template to assureproper alignment of the film for scanning. Film density-to-dose conversion (H&D curve) must be carefully addressed.Use of a single density-to-dose conversion curve will not account for change in film response due to energy change with depth. Similarly, film responsein the penumbraregion may changedue to the changein energy spectrumof the scatteredradiation. These effects can be studied using techniques such as those describedby Williamson et al.* The film H&D curve shouldbe measuredusing film placed in the phantom in the same orientation used to expose the dynamic wedge fields. That is, if the wedge films are exposed using the film parallel to the central axis of the field, then the H&D curve should be determined using a series of films exposed parallel to the central axis of the field. The H&D curve can be determined using a fixed open field size of 10 cm X 10 cm for a seriesof exposuresup to 200 cGy. The density-to-dose relationship can then be determined by searchingfor the maximum on each film and relating that to the delivered dose. Most newer film densitometry systems have an option to automatically perform this step. In order to achieve maximum differentiation of dose from the film densities, the maximum dose delivered should be adjusted downward so that the maximum film density is still within the steeply ascending portion of the H&D curve. Film sensitivity vs. depth is dependent on photon
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The film densitometer output signal was measured vs. dose at dmaxand at 30 cm depth on central axis. Similar measurementswere made at intermediate depths in 5 cm increments. These curves suggest an increased film sensitivity of approximately 4.2% at 25 cm for 6 MV photons, while showing no increase in film sensitivity for 18 MV photons. This effect can easily be folded into film densitometry scanning by including a simple dose modifier which corrects the densitometer reading by the inverse of the sensitivity factor vs. depth. This can be represented as: energy.
D(d,R) = R*(H/D)“(l/(l + b(d - d,,,)))
(1)
where D(d,R)= correcteddose R = film densitometer reading H/D = film densityto doseconversion b =film sensitivity coefficient determined by least squaresfit to measured sensitivitydata. Evaluation of data for 6 MV suggest a value for b = 0.0018 cm-‘. If the smallcorrection due to energy changesoff-axis is ignored, this sensitivity effect is linear with depth; therefore, the entire film density profile at any given depthcan be modified by this singlecorrection factor. The addition (and magnitude)of a small correction factor for energy changes off-axis is reportedby Clewlow et al2 As an alternative to the standardXV2 film paper envelope, a newly introduced vacuum packaging provides a water-tight film packet that can be positioned in a water phantom to enable measurementsin an environment closer to that commonly done with ion chambers and diodes. Radiochromic film is also available, but has not been applied to enhanceddynamic wedge measurementsdue to the higher doserequired and the higher cost per film. Three types of film densitometershave beenapplied to radiotherapy dosimetry measurement:a video camera mounted above a light box and acquiring a gray scale image via a frame grabber, a scanning light source and photodiode which continuously records transmittance and can be sampled via an Analog-to-Digital Circuit (ADC) or can be plotted in analog form, and a linear film scanner which passesthe film across a line source and readstransmittance to a linear array of 4000 CCDs. Measurement using linear detector arrays Linear detectorarrays areavailablefrom severalcommercialvendors.Linear diodearraysare composedof 11 or 25 energy-compensated p-type diodesspaced2.5 cm apart (11 diodes)or 2 cm apart (25 diodes)with an option to foId the array inward to decreasethe diodespacingto 1 cm. This allows narrow fields to be integrated in fewer steps, or wider fields to be integrated in a larger number of steps. Ionization chamberarrays consist of 41 ionization chambersspaced1 cm apart.Recentdosimetryintercomparisons show nearly identical responseof diode and ionization
Dosimetry measurement tools 0 D. D. LEAVITT AND E. E. KLEIN
chamberin measurement of openfield and dynamic wedge beam profiles.6 In addition to the alignment checksdiscussedabove, additional checksmust be evaluatedfor the linear detector arrays. Specifically, the depth of each detector relative to the others must be verified. Although each detector may have a fixed position relative to the rest of the array, the entire array, when mounted in the water tank, may be skewedrelative to the water surface.Typically, shimsare provided to adjust the entire array in order to achieve the desiredcoincidencewith the water surface. The relative sensitivity from detector to detector must be carefully monitored during dose measurement. The sensitivity of individual diodes change with dose absorbedby the diode. These sensitivities are relatively slowly varying, but must be accounted for in the measurements.In the measurementof dynamic wedge fields, the individual diodes will be exposed to different dose levels, varying by orders of magnitude, depending upon the location of the diode, For example, the diodes near the tip of the wedge will receive roughly twice the dose received by diodes on central axis, while diodesnear the heel of the wedge will receive only half the central-axis dose. Similarly, diodes outside the geometric limits of the radiation field will receive only a few percent of the central axis dose. Calibration techniques are recommended by the system manufacturers for careful crosscomparison of the individual detector sensitivity. Thus, in the smaller arrays which can be moved across the water phantom, each diode will be sequentially positioned at the central axis of the radiation field and a fixed dose delivered in that configuration. This process is repeatedfor each diode in the array, This allows an exact intercomparisonof every diode. Alternatively, the largest possiblefield is set to irradiate the water tank, such that all detectors are simultaneouslyin the radiation field. For the sameexposure, the reading for each detector is recorded; the detector array is then shifted a distanceequal to the spacing between detectors, and the exposure is repeated. The relative sensitivity of each detector in the array can then be determined by evaluating the ratios of readingsbetween the neighboring detectors. As an added check on detector sensitivity, one or more overlap points is included in the positioning of the detector array. Thus the last point measuredby one detector will correspond to the first point measuredby the neighboring detector, allowing a direct intercomparison of readings. The linear detector arrays can measure dynamic wedge dose profiles at any depth in the water phantom. Care must be exercised, however, in evaluating measurements in the buildup region. The ionization chamber array mounts the individual detectors such that the long axis of the detector is parallel to the central axis of the beam. This compromisesthe ability of the detectors to measuredose in the buildup region where the dose gradient is changing rapidly acrossthe length of the detector. The linear diode array has an active thickness for each detector of less than 1 mm. This allows measure-
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ment of dose in the buildup region; however, the mechanical placement of each diode must be carefully verified. Otherwise, large differences in reported surface doseand superficial doseswill be noted from detector to detector. This will introduce discontinuities in the dose profiles in the buildup region. Measurement using thermoluminescentdosimeters Re-usableLiF rods can be usedfor dynamic wedge measurements.These rods are of a diameter 1 mm, approximately 6 mm long. The rods have a higher content of LiF than did previous rods, and therefore are more consistent in response.These rods, in combination with readerswhich can processup to 50 TLDs without intervention, may be useful in determination of the buildup doses,depth doses,and beam profiles at various depths. Careful handling and processing of the TLD rods is required to ensureconsistentperformance. For example, the following recommendationsare made by Bicron for managementof their TLD-100 rods: “Handling: All looseTLD materialshouldbe handled with vacuum tweezers,not mechanicaltweezersor fingers. Small scratches,lossof mass,or foreign depositswill all affect the light emissioncharacteristicsof the TL material. Cleaning: Between normal uses,the dosimetersshouldbe rinsed, not soaked, with an analytical grade anhydrous methyl alcohol and dried by evaporation for at least one hour. They shouldthen be annealedaccordingto the correct procedurefor the given materialone time before actualuse. The annealwill aid in removing any residual alcohol deposits.Annealing: For annealingup to 400°C the containers shouldbe wade from high temperaturestainlesssteelor oxidized aluminum.Never use non-oxidized aluminum! It is recommendedthat the metal be thin to allow rapid cooling following annealing. A dedicated annealing oven shouldbe usedto help prevent contaminationfrom foreign substances. To avoid inconsistentheat gradients,the annealing containersshould not be stackedwithin the oven nor allowedto touch the oven walls. Theseshouldbe placedon open oven racks with air space all around.” (Harshaw RadiationMeasurementProductsSpecificationSheet:Handling andThermal Treatmentsof Bare Thermoluminescent Dosimeters).The heat treatmentfor TLD- 100rodsrequires pre-irradiation at 400” for 1 h followed immediately by 100°C for 2 h, then cooled on a cold surface to room temperature.Post-irradiationat 100°Cfor 10 min is recommendedto minimize the TLD fading. The consistencyof the TLD responsecan be improved by putting the rods through the read cycle 5-10 times prior to first irradiation. This hasbeenreferred to as“training the TLDs.” (personal communication,Harshaw). SUMMARY Dosimetry measurementtools are readily available for the measurementsnecessaryto commissionenhanced dynamic wedge. The choice of tools to apply will depend upon the specific data requirements of the individual
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treatment planning system. Consistency in production quality control of the linear accelerators strongly suggests that, once the extensive commissioning measurements have been made and evaluated for a specific energy and machine type, subsequent installations can selectively verify those measurements specific to their own linear accelerator, and thus minimize the volume of measurements first made to evaluate and define the enhanced dynamic wedge characteristics.
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REFERENCES 1. Bidmead, A.M.; Garton. A.J.: Childs, P.J. Beam data measurements for dynamic wedges on Varian 600C (6 MV) and 2100C (6 and 10 MV) linear accelerators. Phys. Med. Biol. 40~393-411; 1995. 2. Clewlow, J.P.; Waggener, R.; Feldmeier. J.J.; Bite, W.S. Film
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dosimetry of a Varian dynamic wedge. (Poster Session S-7, AAPM Annual Meeting, Calgary, Canada, 1992). Elder. P.J.; Coveney, F.M.; Welsh, A.D. An investigation into the comparison between different dosimetric methods of measuring profiles and depth doses for dynamic wedges on a Varian 6OOC linear accelerator. Phys. Med. Biol. 40:683-689; 199.5. Klein, E.E.; Low, D.A.; Meigooni, A.S.; Purdy, J.A. Dosimetry and clinical implementation of dynamic wedge. Inf. J. Radiat. Oncol. Biol. Phys. 31:583-594; 1995. Leavitt, D.D.; Martin. M.; Moeller, J.H.; Lee, W.L. Dynamic wedge field techniques through computer controlled collimator motion and dose delivery. Med. Phys. 17:87-91; 1990. Leavitt. D.D.; Larson, L.G. Evaluation of a diode array for measurement of dynamic wedge dose distributions. Med. Phys. 20:381382; 1993. Lydon, J.M.; Rykers, K.L. Beam profiles in the nonwedged direction for dynamic wedges. Phys. Med. Biol. 41:1217-1225; 1996. Williamson, J.F.: Khan, F.M.; Sharma, S.C. Film dosimetry of megavoltage photon beam: a practical method of isodose-to-isodensity curve conversion. Med. Phys. 8:94; 1981.