Commissioning, beam matching and small field dosimetry experience with Elekta Versa

Transcription

1 Commissioning, beam matching and small field dosimetry experience with Elekta Versa Raj Varadhan, PhD Director of Physics, Technology Minneapolis Radiation Oncology

2 Outline Elekta Versa/Infinity platform Brief commissioning workflow/overview Beam modeling, pmc, emc & CC Elekta FFF beams, 6DOF Couch Model Verification & QA (MGDR) End to End testing Beam matching Synergy with Versa Small Field Dosimetry & SRS Conclusion

3 Agility 160 leaf MLC 5mm leaf width Greater clearance Full Interdigitation Real time leaf position 6.5 cm/sec leaf speed Low leakage/transmission Rubicon based video optic Greater reliability

4 Elekta Agility 160 leaf / 5mm Leaf width: 5mm at isocenter for all leaves Interdigitation Fieldsize: Up to 40 x 40cm No splits fields needed (All dynamic movements) Mechanical speeds: Leaf speed \ DLG = 6.5cm \ sec Image courtesy of Vivian Cosgrove, Ph.D., (2012) Adapted from Huq et al. PMB 49 (2002) M Real time leaf position monitoring and display: Half the moving parts of an encoder based system = Greater reliability Low leakage Less than 0.5% across the entire field VP Cosgrove et al., ASTRO 51st annual meeting (2009) See PMB 49 (2002)

5 MLC comparisons: Elekta Agility: Varian Millennium 120:

6 MLC comparisons: Elekta Agility: Varian Millennium 120:

7 FFF Mode Up to 70% dose reduction Reduction in Dose Thyroid 22% Breast 31% Ovaries 62% Testes 67% Reduced dose outside of the treatment area

8 Rapid MLC leaf speeds Head and Neck Previous MLCi Generation Versa HD PTV prescription 54 Gy 54 Gy Rapid MLC leaf speed Improve treatment times with sophisticated modulation Number of fractions Dose per fraction 1.8 Gy 1.8 Gy Beam on time per fraction 4 mins 53 seconds 2 mins 49 seconds 42% faster

9 High dose rate delivery Lung Previous Generation Versa HD PTV prescription 60 Gy 60 Gy High Dose Rate mode Improve treatment times with escalated SRS/SRT dose regimens Number of fractions 5 5 Dose per fraction 12 Gy 12 Gy Beam on time per fraction 3 mins 50 seconds 2 mins 10 seconds 45% faster

10 5 Brain Mets 1 Arc 5 Gy/Fx 3 Min Delivery Excellent critical structure sparing and DVH results

11 Commissioning Data collection * Submission to Web Portal Compare data to 10 different institutions benchmarked data. Integrity of submitted data and 95% confidence intervals MC modeling done in Shanghai. CC Model New Delhi Elekta FFF Beams

12 Elekta FFF Elekta matches the PDD of FFF and flattened beam within 1%. Our D10 at FVSD was within 0.2% (~ 67.8% PDD) Existing literature largely relates to the Varian True Beam accelerators that implement FFF beams in an unmatched mode, whereby FFF beams are produced by removing the flattening filter without any subsequent change in beam energy or PDD matching. This is why the beam is often softer in Varian FFF beams. However Elekta supports energy tuning of the beam, by alteration of the gun current to increase the mean electron energy incident on the target and changing the bending magnet currents to adjust the electron energy selection window. There are 3 parameters in Elekta that are varied to achieve this. 1) Bending Coarse current 2) Bending fine current and 3) Gun current. Bending coarse adjusts the bending current in the slalom of the linac head acting as an energy selection window, bending fine controls the focusing of the electron beam once through the slalom onto the target. The removal of the flattening filter results in an increased dose per pulse for FFF beams compared with flattened beams.

13 Elekta FFF Effect on PDD and DMAX: The effect on PDD of removing a flattening filter is to increase the quantity of low energy photons, as compared to the equivalent flattened beam. This effect is especially prevalent on the central axis due to the removal of the attenuation caused by the flattening filter, which results in the unmatched FFF beams having a PDD resembling that of a ~ 5 MV flattened beam. Matching the FFF beam at 10 cm results in the PDD becoming more comparable to the associated flattened beam. The depth of maximum dose for Elekta FFF beam is more constant with field size for FFF beams due to the reduction in head scatter compared to flattened beams, which results in the depth of maximum dose being deeper for a matched FFF beam than that of a flattened beam for field sizes 10 cm 10 cm. This is why we see the Dmax as 1.8 cm for 6X FFF beam as compared to 1.5 cm for flattened conventional 6MV Elekta beam This implementation on Elekta is different from Varian which is why Varian s beam spectra is slightly softer

14 Commissioning steps on Versa/Infinity Point Doses with ion chamber Express QA Fields Planar dose using MC and AC (CC for 3D, wedges etc..) ArcCheck for VMAT. 22 different plans of varying complexity MGDR using 3DVH Mobius 3D End to End testing ( IROC H & N, Spine Lung, SRS Head )

15 Express QA methodology

16 MLC Geometry Editor

17 3 ABUT

18 Four L Evaluate MLC Offset, Leaf groove and MLC Tran

19 FourL Analyze A B first, and check MLC offset in picket fence region

20 Tune MLC Offset

21 MC vs Measured (6X)

22 MC Report

23 Post Model Adjustment Express QA Results 90SSD d=10 Gamma pass rates SD initial SD post adjust 10X 2%2mm 3%3mm 2%2mm 3%3mm 23ABUT 99.5% 100.0% 100.0% 100.0% 320x % 100.0% 100.0% 100.0% 410x % 100.0% 100.0% 100.0% 5DMLC1 92.6% 100.0% 100.0% 6HIMRT 99.4% 100.0% 99.7% 100.0% 7HDMLC 99.0% 100.0% 99.4% 100.0% 87SegA 91.0% 96.6% 95.6% 97.5% 9FOURL 82.6% 90.1% 87.6% 90.1% SD SD 6X 2%2mm 3%3mm 2%2mm 3%3mm 23ABUT 92.8% 97.3% 99.1% 100.0% 320x % 100.0% 100.0% 100.0% 410x % 100.0% 100.0% 100.0% 5DMLC1 94.6% 99.6% 99.8% 100.0% 6HIMRT 99.4% 100.0% 93.6% 99.5% 7HDMLC 99.4% 99.7% 99.4% 100.0% 87SegA 91.1% 98.1% 98.8% 100.0% 9FOURL 82.3% 89.4% 90.4% 91.3% SD SD 6FFF 2%2mm 3%3mm 2%2mm 3%3mm 23ABUT 85.0% 94.8% % 320x % 98.3% 98.10% 100% 410x % 100.0% 100% 100% 5DMLC1 65.9% 89.1% 99.80% 100% 6HIMRT 97.4% 99.4% 99% 100% 7HDMLC 98.7% 99.4% 99% 100% 87SegA 69.2% 87.9% 99.20% 100% 9FOURL 77.7% 87.7% 89.30% 90.70%

24 End to End tests: RPC HEAD & NECK PHANTOM

25 IROC H & N

26 IROC H &N

27 IROC

28 IROC Spine Lung

29 Clinical Patient QA Summary For VMAT, we use AC with PMMA plug inserted For small fields or highly modulated fields, AC used after doubling detector density. For hypofractionated SBRT/SRT cases using MLC based VMAT, we also analyze 3DVH using PDP methods.

30 Current Verification

31 What does conventional QA results mean?

32 Gamma QA a) Per beam, planar IMRT QA passing rates do not predict clinically relevant patient dose errors, B. Nelms et al Med Phys, 38 October 2011:

33 Moving from gamma passing rates to patient DVH based QA metrics in pretreatment dose QA, Zhen H. et al., Med. Phys Oct;38(10):

34 ArcCHECK Phantom QA Patient 2

35 3DVH QA Patient 2

36 3DVH analysis

37 ArcCHECK Phantom QA Patient 3

38 3DVH Patient 3

39 MGDR QA Summary: Traditional VMAT/RapidArc QA employs detector based geometry using the ubiquitous 3%/3mm dose and DTA threshold for gamma QA pass rates. The traditional QA provided no information on the size and spatial location of errors in patient geometry. MGDR correlates measurement to actual errors in patient geometry

40 MGDR At least for VMAT, phantom geometry based gamma QA metrics have a weak correlation with actual dose delivered to patient. Need to take a step back and think what is the goal of QA? Are you checking the machine/tps model or errors that have patient specific impact? If the answer is latter, then MGDR analysis is the likely solution

41 FVSD MGDR for SRS/SABR

42 Elekta 6DOF Hexapod Couch The HexaPOD RT System is guided by an infrared camera that enables patient positioning accuracy in six degrees of freedom. The HexaPOD is mounted on the base of the standard Elekta table. Movement of the HexaPOD is computercontrolled via the iguide software and executed by means of the robotic legs. Accurate positioning is ensured by means of an optical tracking system which consists of an infrared stereo tracking camera mounted on the ceiling and set of five passive sensors. The sensors are spherical reflectors positioned at nonsymmetrical distances on a base plate. This arrangement is rigidly attached to the HexaPOD by means of a C shaped bridge as shown below..

43 Varian Perfect Pitch 6 DOF Couch The TrueBeam system uses the machine isocenter as the origin of its reference coordinate system. The actual mechanical rotation point (origin of the mechanical coordinate system) of the 2DoF module is away from this isocenter. Therefore, substantial translations of the couch top and couch pedestal are required to generate a pure pitch or roll change at the isocenter. The 2DoF module sits on top of the movable stage of the couch pedestal. Consequently, the mechanical rotation point of the 2DoF module moves vertically, longitudinally, and laterally relative to the isocenter, necessitating calculations to convert changes in mechanical parameters of the couch pedestal and 2DoF module into changes of the couch position/orientation in the reference (isocentric) coordinate system. For example, a change in pitch of 3 degrees will result in approximately an 8 cm vertical change in the location of the couch top at the isocenter.

44 Varian 6DOF Perfect Pitch Couch The Varian 6DoF couch adds a two degrees of freedom (2DoF) module to the pedestal of the already existing 4DoF TrueBeam couch. The IGRT table top is fixed permanently on the top of the new 2DoF module, so that the 2DoF module moves together with the table top. It has the following specifications: the 2DoF module is 12 cm high; can handle a maximum load of 200 kg; does not reduce the vertical travel range of the existing 4DoF couch; can pitch and roll in the range of ±3.0 o ; has no mechanical assemblies extending into the rotation volume of the gantry head; and uses no external devices (e.g., optical cameras like Elekta) to control its position and is completely integrated within the TrueBeam control system.

45 Poster Number: PO BPC Foyer 33 To assess the accuracy of 6 Degrees of Freedom (6DOF) couch for patient positioning in both Varian and Elekta platforms using a custom developed phantom that can be used for daily quality assurance. Clinical Significance: An ideal daily QA phantom should be both precise and simple to use. We have developed and validated a QA phantom that with the aid of implanted markers can be used to verify the accuracy of six degrees of freedom (6 DOF) couch in both Varian and Elekta Linacs. The test can be performed by therapists as part of daily QA in less than 10 minutes.

46 Phantom Description: QA Phantom An 80mm acrylic cube (shown below) with external markings and various titanium and aluminum fiducial markers imbedded at accurately known positions was used in this study. The fiducials are used in the study to determine positional and angular accuracy. The cube was mounted on a novel platform that was precisely milled using a computerized numerical control (CNC) milling machine so that the cube is angled 2.5 degrees in each of the rotational axis ( pitch, roll and yaw). Platform

47 QA Phantom Fiducial Markers 4 spherical 2.38 mm Diameter Aluminum fiducials located at the axial CAX and positioned peripherally ANT, POST and lateral to center of the cube (fig 1). 8 Aluminum Wires 10mm long and 2mm in diameter located at the 8 vertices of the cube (fig 2) 3 spherical 2mm dimeter Titanium fiducials, the offset fiducials are used determine the linear position (fig 3). Titanium Marker Locations in Cube: One (1) at Isocenter: X; Y; Z=0 (fig 1) One (1) located from Isocenter: X: 2 cm; Y: 2 cm; Z: +2 cm (fig 3) One (1) located from Isocenter: X: +2 cm; Y: +1 cm; Z: 1 cm

48 CT images of fiducials Fig 2: Wires located at the superior 4 vertices Fig 1: CA Axial cut shows 4 peripheral fiducials and the fiducial at CA Fig 3: Fiducial located at ( 2, 2,2)

49 Methods: To provide a reference image dataset, the phantom was leveled and aligned to 0 in all three axes and scanned in a GE CT scanner. Images were acquired with 1 mm slice thickness and reconstructed at 1 mm slice spacing. The images were transferred to the Monaco(Elekta) and Pinnacle 3 (Philips) treatment planning system respectively and the isocenter was defined at the center of the phantom image with the guidance of the titanium markers (fig1). The image and isocenter position was exported to the XVI(Elekta)/ARIA(Varian) application and used as the reference dataset for this study.

50 Elekta XVI: The XVI software enables the CBCT images to be reconstructed at high (0.5 mm), medium (1 mm), and low (2 mm) resolutions. We reconstructed the images at 1mm resolution ( same as kvct) for this study. Three rigid body registration algorithms (namely, gray value, bone, and seed matching) are available for matching the reference and verification image datasets. The gray value automatic registration technique uses a gray level correlation ratio algorithm 1. The bone registration technique uses a chamfer matching algorithm 2. The seeds matching algorithm uses the same chamfer matching algorithm as the bone algorithm, except the algorithm has been optimized to match small objects of high density that are nonlinearly separated. 1.. Roche, G. Malandain, X. Pennec, and N. Ayache, The correlation ratio as a new similarity measure for multimodal image registration, in Proceedings of the Medical Image Computing and Computer Assisted Intervention MICCAI 98 (Springer Verlag, Cambridge MA, 1998),pp Brogefors, Hierarchical chamfer matching: A parametric edge matching algorithm, IEEE Trans. Pattern Anal. Mach. Intell. 10, (1988).

51 Methods: The baseline accuracy of each 6DOF couch was measured using a dual axis digital protractor (Model DXL360S) as shown below

52 Methods: CBCT images were acquired in both Varian True Beam and Elekta Versa linacs and analyzed using the vendor supplied software. The 6DOF couch was adjusted based on the alignment of markers and aluminum wires embedded in the acrylic cube. We used the Head and Neck CBCT protocol on the XVI platform in the study. A scan angle of 200 degrees using 100 kv and 36.6 mas was used Varian OBI used the Head, full fan, half trajectory CBCT protocol. Total scan angle 200 degrees, 100 kv and 150 mas.

53 Results: In both the Elekta and Varian platforms the vendor supplied software correctly calculated the rotational off set introduced in all 3 rotational axis within ± 0.2 o. The X and Y rotation off set ( 2.5 degrees)can be verified using dual axis protractor as shown below

54 Results in Varian & Elekta: VARIAN Elekta

55 Beam Matching on Synergy after Agility head upgrade FS Infinity Synergy %Diff 3.00 cm 61.00% 61.60% cm 61.50% 62.30% cm 62.60% 63.00% cm 64.20% 64.70% cm 65.90% 66.40% cm 67.70% 68.00% cm 68.80% 69.00% cm 70.30% 70.40% cm 71.50% 71.20% FS Synergy Infinity D100 D100 %diff 3.00 cm 68.10% 67.30% cm 68.70% 68.20% cm 69.30% 68.90% cm 70.20% 70.00% cm 71.40% 71.30% cm 72.40% 72.00% cm 73.00% 72.50% cm 73.60% 73.50% cm 74.50% 74.30%

56 3 X 3 Field Size G T

57 Beam Match Sc & Scp within ~ 1% 6X Comparison Sc Factor SYN Sc Factor INF %diff 3x x x x x x x x x X Comparison Sc Factor SYN Sc Factor INF %diff 3x x x x x x x x x

58 Electrons

59 Electron Montecarlo

60 EMC Our results indicate that the Monaco emc algorithm can accurately predict depth doses, isodose distributions, and monitor units in homogeneous water phantom for field sizes as small as 3.0 cm diameter for energies in the 6 to 18 MeV range at 100 cm SSD. Consequently, the old rule of thumb to approximate limiting cutout size for an electron field determined by the lateral scatter equilibrium (E (MeV)/2.5 in centimeters of water) does not apply to Monaco emc algorithm.

61 Beam Matching Summary 20 VMAT plans 3D Gamma pass rates between Versa and Synergy upgraded with Agility Head within <2% of each other 3D Plans within 1% of each other Photons and electrons are beam matched and patients can be treated on either Linac w/o replanning However Therapists should consult physics before moving VMAT patients between Linacs

62 Small Field Dosimetry How small is small? Lateral charged particle disequilibrium Dependent on the range of secondary electron & photon energy. Collimator setting that obstructs the source size Detector size relative to field size

63 Crux of the matter Reference conditions (ref)cannot be achieved for most SRS machines including CK, Tomo, cones etc. Machine specific reference (msr) needs to be linked to ref. D1 / D2 M1/M2 D1 / D2 = M1/ M2 *

64 SRS CONES Cone Interlocked Each cone is assigned a unique interlock code (Elekta beam block tray table 101 through 113) Collimator Mounting Attaches directly to the existing Elekta collimator mounting system without modifying LINAC Precision Calibration System easily can be focused to both radiation target and isocenter.

65 SRS CONES Clearance There is 33cm between bottom of cone system and isocenter

66 SRS Cones Small field dosimetry EBT 3, Edge Detector, PTWdiode60017, Diamond Detector, Exradin A16 ( cc) EDGE A16 %diff 1X X X X X

67 Diodes PTW PTW60017 Design: Measured quantities: Nominal sensitive volume: Reference point: p-type silicon diode, waterproof, diskshaped sensitive volume perpendicular to detector axis absorbed dose to water 0.03 mm³, radius 0.6 mm, unshielded on detector axis, 1.33 mm from detector tip

68 Diodes SNC Edge Detector Cone OF PTW EDGETG51 %diff 7mm mm mm mm mm mm

69 SRS Cone Detectors EBT3 Film EBT 3 FILM

70 Correction factors

71 Choice of Detector (CK)

72 EBT3 vs Diodes Cone OF PTW EDGE EBT3 Film %diff PTW %diff EDGE 7mm % 1.8% 9mm % 2.6% 11mm % 1.8% 13mm % 1.6% 15mm % 3.6% 17mm % 0.1%

73 Detector Summary Until the correction factors are properly understood and verified for diodes, use EBT3 film or W1 Scintillator detector Do NOT use ion chamber

74 SRS COMMISIONING SINGLE CONE OUTPUT & SRS HEAD VALIDATION

75 End to end test:iroc SRS HEAD

76 IROC SRS HEAD

77 IROC SRS HEAD

78 Conclusions Commissioning & Beam matching Elekta Versa Linacs has been a productive clinical experience For any multi Linac department, the concept of beam matching is the model to follow for maximal patient care and efficiency Thank you for your attention