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Radiotherapy Physics DepartmentNorfolk & Norwich University Hospital NHS Trust

UKhttp://www.rpunn.org.uk

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ABSTRACT:ABSTRACT:ABSTRACT:ABSTRACT:

A program of quality assurance was developed to ensure the accuracy of diagnostic CTscanners when used for radiotherapy treatment planning and virtual simulation. Theschedule was built up from existing recommendations for diagnostic CT scanners and forgeometric aspects of linear accelerators. Two major changes to the tests described in theseschedules were required which are not described elsewhere. These were CT number qualityassurance and alignment laser calibration.A monthly check was scheduled for the validity of the CT number to electron densitycalibration using materials of known electron density located within an anthropomorphicphantom. Tolerance levels were based on the dosimetric error for a 6MV beam at 10cmdeep in a material of constant electron density and a constant CT number error.A CT test tool was developed to enable optimal adjustment of the isocentre defining lasersto the image centre of the CT scan. This tool gives quantitative values for the offset of laserposition in all three planes and therefore allows adjustment to be performed using aminimum number of image acquisitions. An accuracy of 0.5mm could be achieved in thecoronal and sagittal planes and 0.7mm in the transverse plane. This accuracy was limitedby the pixel size of the image (1mm).

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INTRODUCTION:

CT SimulationThe role of CT in radiotherapy treatment planning is well established. The cross-sectionalnature of the images, their spatial accuracy, the clear indication of inhomogeneities and thesimple relationship between CT number and radiation absorption parameters give them aclear advantage of other imaging modalities. As CT scanning has increased in speed, it hasbecome feasible to acquire realistic 3-dimenisonal data which further enhances the value ofCT image sets. In recent years, the clarity of the 3-dimensional reconstructions from CTimages has lead to the development of CT-simulators incorporating image processing toolsdesigned to replace conventional radiotherapy imaging equipment.The CT simulation procedure fulfils several functions within the complete process ofradiotherapy. All data is acquired relative to a fixed patient position. This involves aligningmarks on the skin of the patient with orthogonal laser indicators and may also use physicalimmobilisation aids. This ensures that information which is obtained using the simulator isvalid when the patient is transferred to a linear accelerator for treatment. The simulator canthen be used to locate the tumour and any critical organs relative to that fixed referenceposition. The acquisition of patient surface information and density information are alsoimportant tasks for CT simulation.

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Current Procedures for CT Simulation Quality AssuranceWhere CT scanners are used as simulation tools adequate quality assurance must beperformed to ensure the validity of the data being produced. This need is not unique to theapplication of radiotherapy and there are existing techniques for the quality assurance ofCT scanners for radiological use [1]. Diagnostic quality assurance procedures are also vitalfor the quality assurance of CT simulators, as with conventional simulation.The simulator is, however, primarily a geometric tool and quality assurance cannot becarried out on the imaging performance alone. Geometric accuracy quality assurance is awell documented area of radiotherapy practice and it is possible to draw on the existingtechniques used for both conventional simulators and linear accelerators [2].By using existing techniques in this manner almost the entire quality assurance schedulecan be built up and tolerances taken from current references. In practice there are additionalprocedures which are not covered by existing schedules, or procedures which requirealteration before they are appropriate for use on CT simulation. In table 1 four QAprocedures are identified which cannot be taken from existing QA practice for either adiagnostic CT scanner or a linear accelerator.

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Table 1: Procedures and sources for CT Simulation QA

Image Quality Geometric Accuracy AdditionalIPEM Report 77 IPEM Report 54 Procedures

Image noise Couch movement accuracy CT number accuracyCT dose index Couch rigidity CT number uniformitySlice thickness Image distortion(irradiated and image) Isocentre alignmentSpatial resolution laser position

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Providing a complete CT simulation QA scheduleOne advantage of CT simulation is that the data can be used to provide electron densityinformation for inhomogeneity correction algorithms for patient dose calculations. This is alarge improvement over bulk density corrections. The effect of such an improvement canbe lost if the variations in CT number across an image or over time are not acceptable.Diagnostic QA standards do specify tolerances for the accuracy and uniformity of CTnumber. With the large difference in the way CT number information is used in CTplanning different tolerances may be required.Any errors in the geometric integrity of the CT images themselves would directly affect theprimary role of the simulator in locating critical structures relative to a known referenceposition. Similarly a critical inaccuracy would also be introduced into the treatment if thealignment lasers were improperly adjusted. Without accurate alignment lasers it isimpossible to know the position of a point relative to the reference position, andsubsequently for the measured data to have any validity on a treatment unit.On linear accelerators and conventional simulators the reference point indicated by thelasers is the mechanical and radiation isocentre of the unit. This point is simple to identifyby nature of its physical definition. With CT simulators the reference point is often taken asthe centre of the CT image co-ordinate system (x, y = 0) at a known point along the axis ofthe scanner. Such a point is defined by the internal mechanism and reconstruction softwareof the scanner and so can only be identified using images of purpose built phantoms.

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The principle of the design of phantoms for the alignment of the laser markers isstraightforward. A physical point is defined which can be viewed both physically andradiographically. By aligning the point with the lasers and then scanning the registration ofthe laser reference point to the CT reference point can be determined. This method workswell for alignment of the sagittal and coronal lasers. In the transverse plane there is adifficulty identifying a single point on an image set. The finite slice width and non-discreteslice sensitivity profile may give several slices in which a radiographic marker may bevisible. A qualitative decision is then required to determine the slice which corresponds tothe position of the marker.Guidelines for performing image distortion test as well as general notes on the qualityassurance for CT simulators in radiotherapy are available elsewhere [3]. In order to be ableto provide a complete quality assurance schedule for CT simulation it was necessary to re-analyse the test which was carried out for CT number uniformity and reproducibility andprovide a means to perform accurate and qualitative measurements on the position of thetransverse alignment laser.

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CT NUMBER UNIFORMITY/ REPRODUCIBILITY:CT NUMBER UNIFORMITY/ REPRODUCIBILITY:CT NUMBER UNIFORMITY/ REPRODUCIBILITY:CT NUMBER UNIFORMITY/ REPRODUCIBILITY:

TheoryThe relationship between electron density and CT number for biological tissues can beexpressed as two linear regions applying at high and low electron densities [4]. It ispossible to acquire CT images of objects of known electron density and accuratelydetermine this relationship. The CT images can than be converted to provide a voxel mapof patient electron density for use in dosimetric calculations by the treatment planningcomputer. With CT numbers being used in this way any CT number quality assuranceessentially becomes a check on the validity of this calibration.The calibration measurements should consider the possibility of CT number non-uniformity. Uniformity as assessed in a water phantom may give good results, however thepresence of beam hardening artefact may give large non-uniformity’s in the presence ofhigh or low density regions. Systematic errors may occur if these are not considered. Inorder to avoid this the quality assurance procedure which was developed used ananthropomorphic phantom with both high density bone and lung regions†. The phantomwas supplied with a range of inserts of known composition which could be located in oneof three positions within the phantom.

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y = 2032.2x - 2168

y = 999.92x - 991.45

-1000

-500

0

500

1000

1500

0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800

Electron Density

CT

No

Fig 1. Example of electron density calibration data

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Fig 2. Anthropomorphic phantom for CT number QA

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ProcedureThe quality assurance procedure, which is carried out monthly, uses a high, medium andlow density insert. The positions of the inserts changes in a three month cycle so thatvariations in CT number across the field of view and due to beam hardening can be takeninto account. If a discrepancy is found then a wider range of measurements can beperformed. The CT numbers measured for the three inserts are compared to the CT numbervalue which corresponds to the electron density of the insert, derived from the calibration.The initial consideration in deriving a tolerance for this test was that the error introducedinto any dosimetric calculation from CT number inaccuracy should not exceed 0.5%. Thecalculation used a simple equivalent depth algorithm in which the dose at a depth d in amaterial of electron density ρ is equivalent to the dose at a depth D in water, where D = d ×ρ / ρwater and an additional inverse square law correction is applied. An error in electrondensity will give a proportional error in equivalent depth. The effects of electron densityerrors can be simply estimated from the tissue maximum ratio data for the radiation beam.

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Table 2: Dosimetric errors resulting from errors in CT number at 10cm deepfor a 10cm square 6MV field.

It can be seen in table 2 that the dosimetric error remains constant for a fixed error in CTnumber. For this reason it is most appropriate to use a tolerance based on a fixed number ofHounsfield units and not a percentage error in CT number or electron density. This type oftolerance becomes difficult to achieve at very high CT numbers. It is possible to relax thesetolerances for high CT numbers because large areas of high density bone are not clinicallyevident. Based on this data tolerances of 10HU for CT number <100 and 20HU for CTnumber > 100 were chosen.

Error in CT number Error in dose /%CT number < 100 CT number > 100

10 HU 0.35 % 0.20 %20 HU 0.70 % 0.40 %30 HU 1.05 % 0.60 %

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LASER ALIGNMENT:LASER ALIGNMENT:LASER ALIGNMENT:LASER ALIGNMENT:

TheoryIn order to allow accurate adjustment of the alignment lasers relative to the scan imagecentre a tool was developed which combined both radiographic markers and visiblereference lines. The tool allowed quantitative assessment of the error in laser adjustment sothat correct adjustment could be achieved using only a single scan. The measurementscould be made independently of the angle of the couch although a reference position wasused to ensure correct alignment between the couch surface and the scanning plane.The problems in alignment of the transverse laser were overcome using a technique similarto that used in many stereotactic head-frame devices where it is important to know theposition of the head frame in any transverse CT slice. The technique uses radiographicmarker pairs at a known gradient to each other. In a transverse image the markers willappear as points with a separation which can be measured from the CT image. Thisseparation will correspond to the transverse position of the phantom.

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Design of the test toolThe radiographic markers used were channels in a perspex body. The position of themarker was taken as the centre of the channel; this method ensured the measured markerposition was independent of the CT image window used. To allow separate alignment ofthe left and right sets of transverse and coronal lasers sets of radiographic markers wereplaced on either side of the phantom with a hinge and screw system for independent heightadjustment. A single radiographic marker was positioned midway along the top of thephantom to allow for adjustment of the sagittal laser.The gradient between the radiographic marker pairs was chosen to give a simple 2:1 ratiobetween separation on the image and transverse distance. A reference position was markedon the outside of the phantom which corresponded to a known separation of theradiographic marker pair. The upper marker in each pair could be used for alignment of thecoronal laser.

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Fig 3: Transverse image of the laser alignment test tool showing the left andright pairs of radiographic markers and the sagittal adjustment marker above.

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ProcedureThe test tool is aligned with the lasers and moved into the scanning position. As there wasa certain amount of couch sag the couch is evenly loaded with 70 kg to simulate a patientweight. This was necessary because of the large (50cm) movement between the laseralignment position and the scanning position. A single transverse slice is acquired fromwhich any errors in laser marker position can be quantified. The adjustments are performedusing the test tool scales as a guide and a single slice is acquired to confirm the newpositions.

Fig 4: The test tool aligned with the lasers on the CT couch

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It is possible to detect errors in laser alignment of 0.5mm with this technique in the sagittaland coronal planes and 0.7mm in the transverse plane. The limiting factor is the pixel sizeof 1mm. With a single adjustment the procedure can be performed in 15 minutes.Originally the procedure was scheduled weekly but with experience this has been reducedto a fortnightly interval.

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CONCLUSION:CONCLUSION:CONCLUSION:CONCLUSION:

It is possible to derive the majority of a quality assurance schedule for radiotherapy CTsimulation from existing recommendations either for diagnostic CT scanners or for linearaccelerators. It is necessary to alter the form of some of these studies.CT number reproducibility and uniformity can be combined into a single procedure whichchecks the validity of the electron density conversion data. This should be performed withmaterials of known electron density in an anthropomorphic phantom to simulate theclinical situation. On the basis of an analysis of dosimetric error at a point 10cm deep intissue a tolerance for CT number of 10HU for materials of HU<100 and 20HU formaterials of HU>100 was chosen.A test tool was designed for the purpose of adjustment of the alignment lasers to the centreof the CT image data at a known transverse position. The tool uses a pair of radiographicmarker lines at a gradient to measure the transverse position of the tool to within 0.7mm.With these changes a complete quality assurance schedule for CT simulation can beproduced from existing recommendations [1-3].

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REFERENCES:REFERENCES:REFERENCES:REFERENCES:

1. Recommended Standards for the Routine Performance Testing of Diagnostic X-RayImaging Systems, IPEM Report No. 77, 1998

2. Commissioning and Quality Assurance of Linear Accelerators, IPSM Report No. 54,1988.

3. Physics Aspects of Quality Control in Radiotherapy, IPEM Report No. 81, 1998.

4. Parker R P, Hobday P A, Cassell K J, The direct use of CT numbers in radiotherapydosage calculations for inhomogeneous media, Phys Med Biol, 24(4), 1979, 802-809.

† St. Bartholomew’s Hospital, Clinical Physics Group.