A 2-D diode array and analysis software for verification of intensity modulated radiation therapy delivery

A 2-D diode array and analysis software for verification of intensitymodulated radiation therapy delivery

Paul A. Jursinica)

Medical College of Wisconsin, Radiation Oncology Department, Milwaukee, Wisconsin 53792

Ben E. Nelmsb)

TomoTherapy, Inc., Madison, Wisconsin 53717

~Received 19 November 2002; revised 22 January 2003;accepted publication 24 February 2003; published 22 April 2003!

An analysis is made of a two-dimensional array of diodes that can be used for measuring dosegenerated in a plane by a radiation beam. This measuring device is the MapCHECK™ Model 1175~Sun Nuclear, Melbourne, FL!. This device has 445 N-type diodes in a 22322 cm2 2-D array withvariable spacing. The entire array of diodes is easily calibrated to allow for measurements inabsolute dose. For IMRT quality assurance, each beam is measured individually with the beamcentral axis oriented perpendicular to the plane of diodes. Software is available to do the analyticalcomparison of measurements versus dose distributions calculated by a treatment planning system.Comparison criteria of percent difference and distance-to-agreement are defined by the operator.Data are presented that show the diode array has linear response when beam fluence changes byover 300-fold, which is typical of the level of modulation in intensity modulated radiation therapy,IMRT, beams. A linear dependence is also shown for a 100-fold change in monitors units delivered.Methods for how this device can be used in the clinic for quality assurance of IMRT fields aredescribed. Measurements of typical IMRT beams that are modulated by compensators and MLCsare presented with comparisons to treatment planning system dose calculations. A time analysis isdone for typical IMRT quality assurance measurements. The setup, calibration, and analysis timefor the 2-D diode array are on the order of 20 min, depending on numbers of fields. This issignificantly less time than required to do similar analysis with radiographic film. The 2-D diodearray is ideal for per-plan quality assurance after an IMRT system is fully commissioned. ©2003American Association of Physicists in Medicine.@DOI: 10.1118/1.1567831#

Key words: 2-D diode array, IMRT quality assurance, radiation therapy quality assurance

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INTRODUCTION

The primary goal of radiation therapy is to deliver dosesionizing radiation to a target, while minimizing the dose this given to adjacent healthy tissue. How this is accompliswith external sources of radiation has evolved as significchanges in imaging, treatment planning and delivery techogy have occurred in the past 20 years.1

Standard radiation therapy involved the use of radiatfields with approximately constant x-ray fluence acrossbeam.2 Beam intensities could be modified with wedges3 andcompensator filters4,5 as needed for 2-D and 3-D planninConformal radiation therapy became possible with the deopment of computed tomography imaging and 3-D treatmplanning programs. Conformal therapy required the usemultiple, non-coplanar beams that were individually shapto the beams-eye-view of the target.6,7

During the past 10 years the use of external beams wmodulated fluence across the beam, intensity modulateddiation therapy, IMRT, has been introduced.8,9 IMRT is anextension of 3-D conformal therapy that uses intensmodulated beams. This technique can produce dose disttions that have improved conformity to the target with cosequent avoidance of critical structures.10–13

One of the challenges of radiation therapy is the establ

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ment of quality assurance tests, which demonstrate thatlivered treatments produce dose distributions as calculaby the treatment planning system. This problem is especidifficult for IMRT, which uses modulated beams that prduce high dose-gradients. The ideal test would be to doin vivo dosimetry and have detectors in the patient. Thisnot a practical solution from the patient point of view. Coventional practice, before IMRT, has relied on recommendmethods for quality assurance of linear accelerators14,15 andtreatment planning systems.15,16 Careful calibration17 andmaintenance of linear accelerators and detailed commissing and routine checks of treatment planning systemsused in combination as an assurance of correct treatmdelivery. Verification of the treatment planning systemsbased on agreement of measured and computed beamfiles, depth dose curves, field-size dependence, and dosmonitor unit, MU, for a variety of field sizes for open anwedged fields.

IMRT with its highly modulated beams has required spcialized dosimetric verification18–20 in addition to what hasbeen recommended for 2-D and 3-D conformal radiattherapy.14–16One method that is now commonly used18 is totake beams that have been optimized for a patient andthem to irradiate simple-geometry phantoms. The resul

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871 P. A. Jursinic and B. E. Nelms: Diode array for verification of IMRT delivery 871

dose distribution in the phantom is calculated by the trement planning system and compared to the measuredThe relative dose distributions have been measuredfilm18–21 and when combined with additional ion chambmeasurements can be used to check absolute dosimetcheck of the modulated beam fluence pattern19,22 has alsobeen carried out using a 2-D beam imaging system.

The present work characterizes a new device for meaing dose in a plane under an IMRT field. The physical adosimetric properties are measured. Methods for howdevice can be used in the clinic for quality assuranceIMRT fields and typical results are presented.

MATERIALS AND METHODS

The x-ray beam used in this work was provided bySiemens MD2 linear accelerator. A beam with nominal eergy of 6 MV was used. Absolute dose calibration of tlinear accelerator was done according to TG-5123 and thisbeam had a percent-depth-dose of 66.9 for x rays only acm depth in water. Calibration conditions are 1 cGy/MU sat depth-of-maximum dose of 1.6 cm, source-to-surfacetance, SSD, of 98.4 cm, and a 10310 cm2 field defined at adistance of 100 cm. A cylindrical ion-chamber, modN30001~PTW Hicksville, NY!, calibrated at the Universityof Wisconsin Accredited Dosimetry Calibration Laboratowas used in these measurements. When measuring lineacelerator outputs in air the ion chamber was covered witcylindrical, acrylic, buildup cap with a 1.35 cm thick walThe ion chamber was operated with 300 V of bias, whprevented measurement errors from charge-recombinatio

The number of pulses produced by the linear accelerin an irradiation was determined by the pulse counter oProfiler, a diode linear-array detector~Sun Nuclear, Mel-bourne, FL!. The period of the pulses was determineddividing the irradiation time by the number of pulsecounted. The pulse width was determined by measuringtime duration of the target current with an oscilloscopModel 2247A~Tektronics, Beaverton, OR!.

Scans of beam profiles were made with a thrdimensional, scanning, water-phantom system~WellhoferDosimetrie, Schwrazenbruck, Germany!. Scans were madewith a cylindrical ion chamber, Model IC-15~Wellhofer Do-simetrie, Schwrazenbruck, Germany!, which had a0.125 cm3 volume and a diameter of the sensitive area ofmm.

The two-dimensional dose measuring device used inwork is the MapCHECK™ Model 1175~Sun Nuclear, Mel-bourne, FL!. The MapCHECK consists of 445 N-type diodethat are in a 22322 cm2 2-D array with variable spacingbetween diodes, which is shown in Fig. 1. Each detectoran active area of 0.830.8 mm2. These N-type diodes areproprietary design that makes them very resistant to damby radiation compared to N-type diodes commercially avable before 2001. Two acrylic plates that have conducsurfaces envelop the diodes, which are mounted on a mlayered circuit board. This provides shielding from radio frquency fields generated in a linear accelerator and radiol

Medical Physics, Vol. 30, No. 5, May 2003

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cal buildup of 2 g/cm2 to the detector junctions. Eacdetector is connected to the input of a low leakage, high gMOSFET operational amplifier, which integrates the signduring irradiation. Signal processing is done by a persocomputer connected through an amplifier interface circuitdiode-relative-sensitivity calibration procedure, performwith a built-in software application, determines the sensitity of each diode with respect to the central diode. The mtrix of diode sensitivities is recorded as a file in the compuand is applied to subsequent diode measurements.

The entire array of diodes was calibrated in dose byfollowing procedure immediately before its use for IMRbeam measurements. A linear accelerator that has beenbrated according to TG-5123 was used to irradiate theMapCHECK with a 10310 cm2 open field. The plane of thediodes was at 100 cm from the accelerator radiation souand the diodes were at a radiological depth of 5 g/cm2. Thiswas provided by 3 cm of solid water on top of thMapCHECK with its intrinsic 2 g/cm2, 1.35 cm physicalthickness, of buildup. The source-to-surface distance ofsetup is 95.65 cm. The dose on the central axis at the dplane is 92.7 cGy/100 MU for the calibrated linear acceletor. Based on the central diode reading and the matrixdiode sensitivities a built-in software application generadose calibration factors for every detector. These factorsrecorded as a file in the computer and are applied to suquent diode measurements to give results in terms of cbrated dose.

For quality assurance of IMRT beams, the MapCHECKused to measure every beam with the beam central axisented perpendicular to the plane of diodes. In order to domeasured versus calculated dose analysis, the treatmentning system must be able to use the IMRT beams~optimizedfor a patient! to calculate a separate dose distribution toflat, solid-water phantom. The FOCUS 3-D Treatment Plning System~Computerized Medical Systems, St. LouiMO! automatically calculates these quality assurance plaof dose for individual IMRT beams at a user-specified Sand depth in phantom. In this study, the dose is calculateda plane at 5 cm depth, which corresponds to the planediodes of the MapCHECK with 5 g/cm2 buildup, and SSD of95 cm. The dose calculation is carried out with a 3convolution/superposition algorithm. A software applicatiodesigned and prototyped by one of the authors, Ben Neand Sun Nuclear, is available to do the analytical compariof measured versus calculated dose. The dose distributiothe quality assurance plane is calculated at 2–3 mm restion and interpolated to 1 mm resolution. It is viewed in tsoftware as a grayscale ‘‘virtual film.’’ The diode array reresents up to 445 point dose measurements that are supposed on the virtual film for both percent difference adistance-to-agreement,24 DTA, analysis. The operator can define separately a percent difference and DTA criterion~e.g.,3% percent difference/3 mm DTA, 3%/2 mm, 2%/2 mmetc.!.

Each diode reading is converted to dose, based oncalibration factors, and then compared to its correspond

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FIG. 1. The diode array pattern of the MapCHECK. The black and gray squares are the positions of the diode detectors. The center 10310 cm2 region hasdetectors spacing of 7.07 mm. The outer ring, which is 6 cm wide, has detectors spacing of 14.14 mm. The horizontal and vertical lines are 22 cmdetector spacing of 10 mm. The diagonal lines are 25.4 cm long with detector spacing of 7.07 mm.

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calculated dose value from the quality assurance dose pSince the measured and calculated dose are both in absdose, the operator need only specify the normalizat~100%! dose value, and the absolute dose relationshipspreserved as the dose arrays are renormalized. Each dposition is analyzed for percent difference25 using

Percent Difference51003~Dose12Dose2!

Normalization Dose.

The DTA analysis is performed with a search of all calclated dose values in a radius around each diode position.any given diode, if there exists within the search radius eit~a! a calculated dose value equal to or~b! a calculated dose


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value greater than and a calculated dose value less thanmeasured dose, then the analysis at that point satisfiesDTA criterion.

Three different types of diode detectors were also usethis work. What is called an old N-type diode is a mod30-487-8, 6–12 MV, N-type silicon diode~Sun Nuclear,Melbourne, FL!, which was manufactured prior to 2001This diode has an intrinsic buildup cap made of brass thasuitable for 6 to 12 MV x rays. An N-type~MapCHECK!diode, which is an individual MapCHECK diode, thatpackaged in a cylindrical, brass buildup cap, which is idetical to that of the old N-type diode. A P-type diode, whicha model 1113000-0, is a P-type silicon diode~Sun Nuclear,Melbourne, FL!. This diode has an intrinsic buildup of les

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than 0.1 gm/cm2 and is designed for dose measurementsthe surface of a patient.

A model 80T-150U Universal Temperature Probe~FlukeCorp., Everett, WA! and Model 87 digital multimeter~FlukeCorp., Everett, WA! were used in combination for measurintemperature. The 80T-150U consists of a low mass, thmistor, temperature probe and a resistance-to-voltageverter.

To determine the temperature characteristics of the N-t~MapCHECK! diode it was forward biased and operated athermistor.26 To do this the N-type~MapCHECK! diode wasconnected to the 80T-150U resistance-to-voltage convertevoltage-temperature calibration was done as follows: theode and a laboratory thermometer were packaged in a wproof, latex sleeve, the combination was placed in a wabath, the temperature was varied, thermal equilibrium westablished in about 7 min as evidenced by a stable volreading, and the temperature and voltage were measuThe voltage measured with the 80T-150U resistancevoltage converter was then converted to the junctitemperature of the N-type~MapCHECK! diode.

To test the diode sensitivity at various temperatures it wirradiated and the charge output was measured with a laratory built charge integrator and amplifier. The diode wirradiated with a 10310 cm2, 6 MV x-ray beam. The diodewas secured to the side of a water bath that had a thin-plawall. The temperature of the water bath was changed, tmal equilibration was allow for 5 min, and the junction temperature of the diode was monitored by measuringforward-biased diode resistance.

RESULTS

The linear accelerator delivers dose by giving squpulses of the electron beam at a frequency of a few tentha kilohertz. Changing the frequency of the pulses, notamplitude or the duration of the pulses of the electron bevaries the dose rate of the accelerator. For 6 MV x raysthe Siemens MD2 linear, the pulse duration was measurebe 5.5ms. A dose of 1 Gy at source-to-axis distance of 1cm was delivered with 5723 pulses in 30 s. This is a dose


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pulse, an instantaneous dose rate, of 1.7531024 Gy/pulse or31.8 Gy/s during the pulse. The sensitivity of diodes hbeen shown26–31to change when the dose-per-pulse is of tmagnitude. This has been found to be a more significproblem for N-type diodes.26–28

The transmission of the multileaf collimator, MLC, wameasured with an ion chamber at a depth of 5 cm in sowater. The ratio of the signal when the ion chamber wunder the MLC to when it was in the open field was 0.3IMRT beams are modulated from 100% transmission to lels of 0.3%, which is the transmission of a MLC leaf. Thisa potential 330-fold change in the instantaneous dose acthe IMRT beam. Since the MapCHECK uses N-type diodits dependence on dose-per-pulse is a concern.

The response of the MapCHECK and various types

FIG. 2. Dose response of various diodes and the MapCHECK as a funcof dose-per-pulse of a 6 MV x-ray beam. The MapCHECK values are aaverage of the five diodes in the center of the array. Exposures were mwith a 10310 cm2 field measured at 100 cm from the radiation source olinear accelerator. The dose-per-pulse value was varied from 1.8 to 0.1changing the distance of the detector to the radiation source from 75 tocm. The dose-per-pulse value of 0.003 was obtained by irradiating thetector with it positioned under a collimator jaw. The total range of dose-ppulse change is 1.8/0.0035600-fold. The dose-per-pulse values were nomalized to the value at a detector-to-source distance of 100 cm.corresponding dose-per-pulse value was 1.7531024 Gy/pulse.

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FIG. 3. Response of the MapCHECK as a functionthe dose delivered. The MapCHECK values are anerage of the five diodes in the center of the array. Tdose was altered by changing the number of moniunits set on the accelerator.

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diodes at different dose-per-pulse values are shown in FigThe value of dose-per-pulse was varied by making measments at various distances from the source or with the detor positioned under a collimator jaw. The dose-per-puvalue itself was measured with an ion chamber. The datFig. 2 indicate that the MapCHECK N-type diodes are vesimilar to a P-type diode and have about a 2% changesensitivity for a dose-per-pulse change of 600-fold. TheN-type diode shows greater than 14% change in its sensity in this same range of dose rates.

Since IMRT treatments are given at a variety of dosesresponse of the MapCHECK to various doses is importaThe dose is what is normally referred to when makingirradiation. This is different than the dose-per-pulse dscribed earlier. In the present context, dose is the sum ofdose delivered by each pulse in an irradiation. Figureshows that the response of the MapCHECK is linear ove100-fold change in dose.

The short-term, over a period of hours, reproducibilityMapCHECK measurements was determined by repeat

TABLE I. Measurements of dose under an aluminum compensator shlike a wedge. Seven repeats of this measurement were made overperiod. The Cartesian coordinates,X andY, of each diode correspond to thsurface of the MapCHECK as shown in Fig. 1. These coordinates aredistance measured in the plane of the diodes.

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1 0 0 7838.4 2.4 0.032 5 0 7921.1 3.6 0.043 5 5 7266.9 3.0 0.044 0 5 7386.6 3.8 0.055 25 5 7232.5 2.1 0.036 25 0 7907.0 3.6 0.057 25 25 8388.5 4.5 0.058 0 25 8527.1 3.0 0.049 5 25 8407.6 2.6 0.03


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measuring the dose under an aluminum compensator-fiwhich was shaped like a wedge. The aluminum filter wirradiated by a 6 MV x-ray beam with a 15315 cm2 field.The MapCHECK diodes were at a source-to-axis distanc100 cm, a radiological depth of 2 g/cm2, and received 89cGy on the central axis. Table I shows the result of sevrepeat measurements made by nine diodes. The relativedard deviation of these measurements was<0.05%.

The long-term, over a period of months, reproducibilityMapCHECK measurements was quantified over a 261period of use. As explained in the Material and Methosection, a 10310 cm2 open field was measured prior to eameasurement session. These measurements provide a rof the relative sensitivity of each diode with respect to tcentral diode, presuming that the beam flatness and symtry are unchanged during this period of time. Figure 4 shothe measured relative sensitivity of four MapCHECK diodover a 9 month period of use. Over this time period appromately 150 IMRT fields were measured with thMapCHECK with an estimated total dose delivered of 50 GBased on the data in Fig. 4, there is no noticeable changthe sensitivity of these four diodes under these conditionsuse. The average sensitivity and standard deviation ofdata in Fig. 4 is as follows: diode 1, 1.00560.0017; diode 2,1.00460.0018; diode 3, 1.00560.0024; and diode 4, 1.00260.0016.

The sensitivity of diodes to radiation changes with teperature. To test the MapCHECK temperature sensitivitychanging its temperature is not practical due to its large sand weight. Instead, an individual diode, N-typ~MapCHECK! diode, was tested. Figure 5 shows the chanin the diode signal with diode temperature. The diode teperature coefficient,CT , at temperature,T, is defined as fol-lows:

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FIG. 4. The relative sensitivity of four MapCHECK di-odes plotted over a period of 261 days of use. For thmeasurements the MapCHECK is irradiated by an op10310 cm2 field as explained in the text. The measurments are for diodes with Cartesian coordinates,X andY, that correspond to the surface of the MapCHECKshown in Fig. 1. Measurements of diode 1 (X523,Y50), diode 2 (X53,Y50), diode 3 (X50,Y53), anddiode 4 (X50,Y523) are shown. The diode readingare normalized by dividing them by the reading otained by the central diode atX50, Y50.

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FIG. 5. The relative sensitivity of an individual N-type~MapCHECK! diode versus the temperature of the dode. The measured points are shown and the solidis a linear fit to the data: diode signal532.110.1953T, whereT is the temperature in °C.

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The temperature difference across the MapCHECK wdetermined by measuring the surface temperature at extrpositions. These data are shown in Table II. The temperadifference is 0.1 °C across a MapCHECK that is in tempeture equilibrated. Based on the measured temperature cocient of an N-type~MapCHECK! diode this corresponds to0.05% difference in diode sensitivity across the MapCHECdue to temperature difference.

In order to calibrate the MapCHECK the central diomust be irradiated to a known dose. To do this requiresthe scatter characteristics of the MapCHECK are known. Tscatter characteristics of the MapCHECK are determinedmeasuring the central diode response to irradiation by difent field sizes of a 6 MV x-ray beam. The same measuremeis made for an ion chamber in a water phantom. Theseare shown in Fig. 6. The MapCHECK central diode andion chamber signals increase almost identically with fisize. This demonstrates that the MapCHECK and waphantom have very similar scatter characteristics. In Figthe ion chamber shows slightly greater dose rate thanMapCHECK diode at field sizes greater than 18318 cm2.This is due to a smaller amount of scatter in the MapCHE

TABLE II. Measurements of the surface temperature of the MapCHECK wa low-mass thermistor. The MapCHECK was kept in a treatment room wtemperature regulated to62 °C for 8 h before the measurements were maIn this manner, the temperature difference across the plane ofMapCHECK diodes was minimized. The Cartesian coordinates,X andY, ofeach measurement point correspond to the surface of the MapCHECshown in Fig. 1.

X~cm!

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0 0 21.620 0 21.6

220 0 21.70 20 21.60 220 21.7


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compared to the 64364340 cm3 water phantom that wasused. Most importantly, for a 10310 cm2 field theMapCHECK central diode and the ion chamber have virally identical scatter behavior. Under the geometry of tmeasurement, the dose rate measured by the calibratedchamber is found to be 0.927 cGy/MU. This value is usedcalibrate the MapCHECK central diode in terms of dose.

The ability of the MapCHECK to measure dose acrosfield was compared to the profile of dose measured withion chamber in a water scanning-system. Figure 7 shodata for dose at a radiological depth of 2 g/cm2 for a 20320 cm2 field of a 6 MV x-ray beam, whose central axisincident perpendicular to the surface of the phantom. T‘‘horns’’ of the beam are easily seen and the MapCHECmeasurements are in very close agreement to the ion chber scan data. Figure 8 shows data for a 10310 cm2 beam,whose central axis is incident at a 30° angle to the surfacthe phantom. The sloped dose distribution caused bybeam incident angle is easily seen and the MapCHECK msurements are in very close agreement to the ion chamscan data.

Typical MapCHECK measurements and comparisonsshown in Figs. 9 and 10. These are examples of IMRT fiemodulated by a MLC or a compensator. For these plotscomparison criteria of63% difference and a distance-toagreement<3 mm were used. Since the diodes are cabrated in dose these comparisons are in absolute dosejust relative dose. Figures 11 and 12 show comparisons wthe compensator was misaligned laterally or rotationally wrespect to the intended dose distribution that was calculaThe MapCHECK measurement and comparison is quite ssitive to misalignments.

The use of MapCHECK for routine quality assuranceIMRT fields has been found to be very efficient. Tablegives typical values for the time needed for various stepsthe measurements. The beam irradiation time is variabledepends on the type of modulation: compensator, step-ashoot MLC, dynamic MLC, the number of intensity leveused, number of MLC segments required, and the dose

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FIG. 6. The response of the MapCHECK central diodand an ion chamber versus the field size of a 6 MVx-ray beam used for irradiation. The MapCHECK diodwas at source-to-axis distance of 100 cm and at a radlogical depth of 5 g/cm2. The ion chamber was asource-to-axis distance of 100 cm and at 5 cm deptha water phantom. Dose rates were based on thesponse of the ion chamber, which was 0.927 cGy/Mfor the 10310 cm2 field. The MapCHECK central di-ode response was normalized to this value.

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DISCUSSION

Dose rate dependence has been reported to be a profor N-type silicon diodes26–28but not for preirradiated P-typedetectors.28,29 It has since been shown30,31 that P-type silicondiodes do show a dependence on dose-per-pulse, whichcurs at much higher accumulated irradiation dose thanN-type diodes. The improved N-type diode used in tMapCHECK shows dose-per-pulse dependence that is qsimilar to what is normally found for P-type diodes.

The radiation damage that underlies the diode dependon dose-per-pulse increases with accumulated dose, e


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cially from high energy photon beams that contain neutcontamination.26,29,30,32,33Since IMRT for head and necksites is predominantly done with 6 MV beams, which do nhave contamination neutrons, one expects a slow accumtion of radiation damage to the N-type diodes of tMapCHECK. However, if a clinic does a large numberIMRT treatments with 10–20 MV x rays, such as for protate, then neutron damage may be important. It has bshown33 that N- and P-type diodes have a drop in sensitivof 1.5% to 2.2% per 1000 Gy of absorbed dose. The averIMRT treatment delivers about 2 Gy per fraction. Thisapproximately the dose delivered to the MapCHECK for tquality assurance check of an IMRT patient. If one allows2% drop in MapCHECK sensitivity before a diode-relativsensitivity calibration is done, then the frequency of calibtion would be every 500 measured patients. For most clinan annual calibration of the MapCHECK would be adequa

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FIG. 7. A comparison of MapCHECK and ion chambemeasurements of a beam profile. Irradiation is dowith a 6 MV x-ray beam, 20320 cm2 field, whose cen-tral axis is incident perpendicular to the surface of tphantom. The ion chamber is 2 cm deep in a waphantom and measurements are shown as a solidThe MapCHECK diodes are at a radiological depth2 g/cm2. The MapCHECK diodes are along theX axisshown in Fig. 1 and measurements are shown as dThe data are normalized to the measured value atcentral axis position.

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FIG. 8. A comparison of MapCHECK and ion chambemeasurements of a beam profile. Irradiation is dowith a 6 MV x-ray beam, 10310 cm2 field, whose cen-tral axis is incident at a 30° angle to the surface of tphantom. The isocenter of the beam is at depth 10 cThe ion chamber is 10 cm deep in a water phantom ameasurements are shown as a solid line. TMapCHECK diodes are at a radiological depth10 g/cm2. The MapCHECK diodes are along theX axisshown in Fig. 1 and measurements are shown as dThe data are normalized to the measured value atcentral axis position.

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to the MapCHECK show no discernable change in diosensitivity. This is in agreement with earlier results33 thatwould predict 0.075% to 0.11% decrease in diode sensitifor 50 Gy of irradiation, which one could not observe aboexperimental uncertainty.

The temperature coefficient of the N-type~MapCHECK!diode was found to be 0.54%/°C, which is higher than the0.05 to 0.40%/°C values reported in the literature26,29,30,34

for N-type and P-type diodes. It is recommended thatMapCHECK be stored at a temperature close to that oftreatment room where it is going to be used. In that wtemperature differences across the MapCHECK will be smas shown in Table II. Any changes in temperature inmeasurement environment from day-to-day will not be iportant since the MapCHECK is calibrated in absolute dimmediately before a measurement session.

The comparisons done with MapCHECK measuremesuch as shown in Figs. 9–12, were done with criteria63% difference and a distance-to-agreement<3 mm. Thequantitative measurement and comparison tool t

FIG. 9. A plot of the calculated dose distribution in gray scale andcomparison of the measured and calculated data for a MLC-modulIMRT field. The points, shown as squares, in this plot indicate the locaof a measured point outside the comparison criteria of63% difference anda distance-to-agreement<3 mm. In this plot 363 points were measured wi95.6% of them meeting the comparison criteria.


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MapCHECK has allows one to ask a new question. Wcriteria and what percent of points passing the criteria wmake a significant difference in final IMRT dose distribtions and clinical outcomes? Now that this tool is availabthe medical physics community can attempt to answerquestion.

There are two obvious advantages of using this 2-D dioarray as an IMRT quality assurance tool. First is the abito perform absolute dose comparisons for hundredsmeasurement positions using only a single beam delivas compared to the many multitudes of delivery repetitionecessary to perform absolute point measurements wimicro ionization-chamber. Second is the efficiency in timand effort.

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FIG. 10. A plot of the calculated dose distribution in gray scale andcomparison of the measured and calculated data for a compensmodulated IMRT field. The points, shown as squares, in this plot indicthe location of a measured point outside the comparison criteria of63%difference and a distance-to-agreement<3 mm. In this plot 291 points weremeasured with 95.5% of them meeting the comparison criteria.

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The use of MapCHECK for routine quality assuranceIMRT fields is very efficient as shown in Table III. Thsetup, calibration, and analysis time are on the order ofmin, depending on numbers of fields. To do this typequality assurance, in absolute dose, with film requires mhours of work and extreme care in processing of film, geration of the dose-response curve, and measuremendose at a point with a calibrated ion chamber. Also, firequires an expensive 2-D film-density scanner and expsive software for converting optical density to dose and coparing the 2-D dose distribution to the dose distribution pvided by the treatment-planning system.

FIG. 11. A plot of the calculated dose distribution in gray scale andcomparison of the measured and calculated data for a compensmodulated IMRT field. This plot is identical to Fig. 10 except that tcompensator was misaligned by 4 mm laterally. In this plot 291 points wmeasured with 69.8% of them meeting the comparison criteria.

FIG. 12. A plot of the calculated dose distribution in gray scale andcomparison of the measured and calculated data for a compensmodulated IMRT field. This plot is identical to Fig. 10 except that tcompensator was misaligned by a 5° rotation around the central axis. Inplot 291 points were measured with 76.3% of them meeting the comparcriteria.


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One of the disadvantages of the MapCHECK is the sparesolution of the diode array, which is shown in Fig. 1. Fthe 22322 cm2 measurement plane of the MapCHECK theare 445 measurement points. For film with assay resoluof 1 mm spacing there are 48 400 measurement points22322 cm2 field of view. This high resolution allows film tomore accurately measure regions of high dose-gradiwhich occur in IMRT fields. However, as shown in Figs.and 10, for routine quality assurance MapCHECK can shthat measured and calculated data agree within reasoncriteria for greater than 95% of hundreds of points in tfield.

The resolution of film dosimetry is unrivaled. Film is neessary in commissioning an IMRT program. However, t2-D diode array is ideal for per-plan quality assurance aan IMRT system is fully commissioned. It would be possibto increase the resolution of the diode array by designingaccurate stepper platform to be placed under the fixed aMultiple beam deliveries with the diode array in differepositions would, in effect, provide more points of measument that could be superimposed to give a higher resolumeasurement array.

Some would argue that a major deficiency of the 2diode array is the inability to measure true composite plawith the beams in their intended treatment angles incidena measurement phantom. It is true that the diode arrayquires beam irradiation normal to the plane of measuremand thus is applicable for beam-by-beam analysis rathermulti-beam composite analysis. However, beam-by-beanalysis is generally more stringent than composite analyas small errors in any single field are more easily detecrather than being potentially lost in a larger sea of compodose. While composite IMRT film analysis has its merits,is primarily popular because it reduces the time requiredquality assurance by limiting the number of films irradiateprocessed, calibrated, and analyzed. The 2-D diode analeven on a beam-by-beam basis, is much more time efficthan a single composite film analysis.

The 2-D array can also be used for routine quality assance for open beams. The array is positioned so that dioon the diagonals that have 7 mm separation, see Fig. 1parallel to the in-plane and cross-plane directions ofbeam. The MapCHECK control software generates an AStext file of the diode measurements. A spreadsheet progra35

written for Microsoft Excel by one of the authors, PAJ, reathis file. Beam flatness, area symmetry, and light-field ver

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TABLE III. Analysis of time required in order to carry out quality assuranmeasurements with the MapCHECK.

Activity Time ~min!

Set up: connecting, computer and softwarestartup, leveling, centering, and SSDadjustment

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Irradiation for dose calibration 0.5Beam irradiation per field 1 to 5Analysis of data per field 1 to 2

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radiation-field coincidence are determined. This is a veryficient quality assurance method since a single irradiagives both in-plane and cross-plane data.

Also useful in the future would be a quality assuranphantom constructed to hold a 3-D array of diodes, not juplane. Careful engineering would be required to ensurethe diodes respond equally from any direction, and thwould need to be negligible perturbation of dose due to srounding electronic circuitry. Such a volumetric phantowould provide a very efficient tool for analysis of composIMRT plans, and would lend itself to more advanced delivesystems such as Tomotherapy or other dynamic rotatiomethods.

ACKNOWLEDGMENTS

The authors would like to thank the Sun Nuclear Corpration for the prototype MapCHECK used in this work athe fabrication of the individual N-type~MapCHECK! diode.Also we would like and to thank Bill Simon and Jie Shi fomany useful discussions.

a!Electronic mail: [email protected]!Part of this work was completed while at Computerized Medical Syste

St. Louis, Missouri 63132.1S. Webb, ‘‘Three-dimensional radiation-therapy treatment planning,’The Physics of Three-dimensional Radiation Therapy~Institute of Phys-ics Publishing, Bristol, 1993!, pp. 1–64.

2S. H. Levitt and F. M. Khan, ‘‘The rush to judgement: does the evidesupport the enthusiasm over three-dimensional conformal radiatherapy and dose escalation in the treatment of prostate cancer?’’ IRadiat. Oncol., Biol., Phys.51, 871–879~2001!.

3F. M. Khan, ‘‘Treatment planning I,’’ inThe Physics of RadiationTherapy~Williams and Wilkins, Baltimore, 1994!, pp. 226–259.

4F. Ellis, E. J. Hall, and R. Oliver, ‘‘A compensator for variations in tissthickness for high energy beams,’’ Br. J. Radiol.32, 421–422~1959!.

5P. A. Jursinic, M. B. Podgorsak, and B. R. Paliwal, ‘‘Implementation othree-dimensional compensation system based on computed tomoggenerated surface contours and tissue inhomogeneities,’’ Med. Phys21,357–365~1994!.

6D. L. McShan, B. A. Fraass, and A. S. Lichter, ‘‘Full integration of thbeam’s eye view concept into computerized treatment planning,’’ InRadiat. Oncol., Biol., Phys.18, 1485–1494~1990!.

7G. W. Sherouse, D. Bourland, K. Reynolds, H. L. McMurry, T. P. Mitcell, and E. L. Chaney, ‘‘Virtual simulation in the clinical setting; Sompractical considerations,’’ Int. J. Radiat. Oncol., Biol., Phys.19, 1059–1065 ~1990!.

8S. Webb, Intensity-Modulated Radiation Therapy~Institute of PhysicsPublishing, Bristol, 2001!.

9J. A. Purdy~Chairman of working group!, ‘‘Intensity-modulated radiationtherapy: current status and issues of interest,’’ Int. J. Radiat. Oncol., BPhys.51, 880–914~2001!.

10R. Mohan, G. S. Mageras, B. Baldwin, L. Brewster, G. Kutcher, S. Lbel, C. M. Burnman, C. C. Ling, and Z. Fuks, ‘‘Clinically relevant optmization of 3-D radiation treatments,’’ Med. Phys.19, 933–944~1992!.

11M. P. Carol, ‘‘Conformal radiosurgery: Stereotactic surgery and radiosgery,’’ in 3D Radiation Treatment Planning and Conformal Therapy, ed-ited by J. A. Purdy and B. Emami~Medical Physics, Madison, WI, 1993!,pp. 249–266.

12C. Burman, C. Chui, G. Kutcher, S. Leibel, M. Zelefsky, T. LoSasso,Spirou, Q. Wu, J. Yang, J. Stein, R. Mohan, Z. Fuks, and C. C. Li‘‘Planning, delivery, and quality assurance of intensity-modulated radtherapy using dynamic multileaf collimator: a strategy for large-scimplementation for the treatment of carcinoma of the prostate,’’ IntRadiat. Oncol., Biol., Phys.39, 863–873~1997!.

13M. A. Hunt, M. J. Zelefsky, S. Wolder, C. Chui, T. LoSasso, K. Rosenweig, L. Chong, S. Spirou, L. Fromme, M. Lumley, H. A. Amols, C. C


f-n

aater-

yal

-

s,

enJ.

phy

.

l.,

-

r-

.,-

e.

-

Ling, and S. A. Leibel, ‘‘Treatment planning and delivery of intensitmodulated radiation therapy for primary nasopharynx cancer,’’ Int. J.diat. Oncol., Biol., Phys.49, 623–632~2001!.

14R. Nath, P. J. Biggs, F. J. Bova, C. C. Ling, J. A. Purdy, J. van de Geand M. S. Weinhous, ‘‘AAPM code of practice for radiotherapy acceletors: Report of AAPM radiation therapy task group no. 45,’’ Med. Phy21, 1093–1121~1994!.

15G. J. Kutcher, L. Coia, M. Gillin, W. F. Hanson, S. Leibel, R. J. MortoJ. R. Palta, J. A. Purdy, L. E. Reinstein, G. K. Svensson, M. Weller,L. Wingfield, ‘‘Comprehensive QA for radiation oncology: Report oAAPM radiation therapy committee task group 40,’’ Med. Phys.21, 581–618 ~1994!.

16B. Fraass, K. Doppke, M. Hunt, G. Kutcher, G. Starkschell, R. Stern,J. Van Dyke, ‘‘American Association of Physicists in Medicine radiatiotherapy committee task group 53: Quality assurance for clinical radtherapy treatment planning,’’ Med. Phys.25, 1773–1829~1998!.

17P. R. Almond, P. J. Biggs, B. M. Coursey, W. F. Hanson, M. S. Huq,Nath, and D. W. O. Rogers, ‘‘AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams,’’ Med. P26, 1847–1870~1999!.

18J. S. Tsai, D. E. Wazer, M. N. Ling, J. K. Wu, M. Fagundes, T. DiPetrilB. Kramer, M. Koistinen, and M. J. Engler, ‘‘Dosimetric verification othe dynamic intensity-modulated radiation therapy of 92 patients,’’ IntRadiat. Oncol., Biol., Phys.40, 1213–1230~1998!.

19L. Xing, B. Curran, R. Hill, T. Holmes, L. Ma, K. Forster, and A. LBoyer, ‘‘Dosimetric verification of a commercial inverse treatment planing system,’’ Phys. Med. Biol.44, 463–478~1999!.

20X. R. Zhu, P. A. Jursinic, D. F. Grimm, F. Lopez, J. J. Rownd, and M.Gillin, ‘‘Evaluation of Kodak EDR2 film for dose verification of intensitymodulated radiation therapy delivered by a static multileaf collimatoMed. Phys.29, 1687–1692~2002!.

21J. Y. Ting and L. W. Davis, ‘‘Dose verification for patient undergoinIMRT,’’ Med. Dosim 26, 205–213~2001!.

22L. Ma, P. B. Geis, and L. Boyer, ‘‘Quality assurance for dynamic muleaf collimator modulated fields using a fast beam imaging systeMed. Phys.24, 1213–1220~1997!.

23P. R. Almond, P. J. Biggs, B. M. Coursey, W. F. Hanson, M. S. Huq,Nath, and D. W. O. Rogers, ‘‘AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams,’’ Med. P26, 1847–1870~1999!.

24D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, ‘‘A technique for thquantitative evaluation of dose distributions,’’ Med. Phys.25, 656–661~1998!.

25J. Van Dyke, R. B. Barnett, J. E. Cygler, and P. C. Shragge, ‘‘Commsioning and quality assurance of treatment planning computers,’’ InRadiat. Oncol., Biol., Phys.26, 261–273~1993!.

26P. A. Jursinic, ‘‘Implementation of anin vivo diode dosimetry programand changes in diode characteristics over a 4-year clinical history,’’ MPhys.28, 1718–1726~2001!.

27E. Grusell and G. Rikner, ‘‘Radiation damage induced dose rate nlinearity in an N-type silicon detector,’’Acta Radiol.: Oncol.23, 465–469~1984!.

28G. Rikner and E. Grusell, ‘‘General specifications for silicon semicoductors for use in radiation dosimetry,’’ Phys. Med. Biol.32, 1109–1117~1987!.

29E. Grusell and G. Rikner, ‘‘Linearity with dose rate of low resistivitp-type silicon semiconductor detectors,’’ Phys. Med. Biol.38, 785–792~1993!.

30J. Van Dam, G. Leunens, and A. Dutreix, ‘‘Correlation between tempeture and dose rate dependence of semiconductor response; influenaccumulated dose,’’ Radiother. Oncol.19, 345–351~1990!.

31D. Wilkins, X. A. Li, J. Cygler, and L. Gerig, ‘‘The effect of dose ratdependence ofp-type silicon detectors on linac relative dosimetry,’’ MedPhys.24, 879–881~1997!.

32J. Shi, ‘‘Characteristics of the Si diode as a radiation detector forapplication of in-vivo dosimetry,’’ Master’s thesis, Florida Institute oTechnology, May 1985.

33A. S. Saini, T. C. Zhu, J. R. Palta, and J. Shi, ‘‘A comparison of comercially available N- and P-type Si diode detectors,’’ Med. Phys.23,1071 ~1996!.

34A. S. Saini and T. C. Zhu, ‘‘Temperature dependence of commerciavailable diode detectors,’’ Med. Phys.29, 622–630~2002!.

35Copies of this program are available from the author.

Documents

A 2-D diode array and analysis software for verification of intensity modulated radiation therapy delivery