MVCT Image Guidance and QA Robert Staton, PhD DABR MD Anderson Cancer Center Orlando ACMP Annual Meeting 2011
Disclosures MDACCO has received grant funding from TomoTherapy, Inc.
Overview TomoTherapy MVCT review Recommended QA procedures Implications for Adaptive Radiotherapy
Megavoltage CT Imaging 6 MV Accelerator (tuned to 3.5 MV for MVCT) Primary Collimator 6mm slice width (1mm for 4.x software) 85cm Binary MLC (all open during MVCT) 85 cm Gantry Aperture 40 cm CT FOV Approximately 50cm
Spectral Energy Distribution
MVCT Imaging 3 choices of reconstructed slice thickness Fine (2mm) Normal (4mm) Coarse (6mm) Gantry Period = 10 sec Couch speed: Fine = 4 mm/rotation Normal = 8 mm/rotation Coarse = 12 mm/rotation Typical scan times 2-4 mins
Imaging Time: Examples Prostate: t Fine, 34 slices, 6.8 cm => 170 seconds Head and Neck: Normal, 42 slices, 16.8 cm => 210 seconds Thorax: Coarse, 44 slices, 26.4 cm => 220 seconds
Prostate Diagnostic KVCT TomoTherapy MVCT
Head and Neck Diagnostic KVCT TomoTherapy MVCT
Imaging Frequency At M. D. Anderson-Orlando, we MVCT for every tx What if. we had imaged only 1st day? we had imaged only first 5 days? we had imaged weekly?
Imaging Frequency Zeidan et al., IJROBP, 67(3), pp. 670 677, 2007 Retrospective analysis of 24 HN patients (802 tx) All 802 fx were IGRT Replay different protocols: Use the daily MVCT images Calculate positioning errors for different frequencies of IGRT
Imaging Frequency IG % - Protocol A: no imaging (system. corr.) - Protocol B: Image 1, 1-3 3, 1-5 5,1-7 day - Protocol C: Weekly imaging 0 % 3-21% 20% - Protocol D: first 5, then weekly 31% - Protocol E: every other day IG, running average 50%
Imaging Frequency Results systematic errors are reduced with minimal image workload (i.e. 9% IGRT) Random errors are not reduced for non-ig treatments
Imaging Frequency % of Residual Error vs. % IGRT 90 80 70 60 50 40 30 20 10 0 0 10 20 30 40 50 60 % Image guidanced > 3mm (all-tx) > 5mm (all-tx) > 3mm (non-ig) > 5mm (non-ig)
MVCT Imaging QA What do I need to QA? How often?
TG-148 Imaging QA Geometric Image Quality Imaging Dose
TG-148 Daily QA
Daily QA Example -Test imaging, registration, alignment chain 3) Align & test automatic couch setup 1) Scan 2)R Register & compare to known offsets Tolerance: Consistency within 1 mm
TG-148 Monthly
TG-148 Quarterly
Monthly QA Example One MVCT scan Geometric Noise Uniformity Spatial resolution MVCT HU Dose
Suggested Test & Tolerances Geometric Verify dimensions of phantom are consistent with known values Alignment of fiducials TG-148: 2mm/1mm (SRS/SBRT) Noise Select a consistent ROI in a homogeneous portion of the cheese phantom TG-148: Consistency with Baseline MDACCO Std Dev of Central ROI Reference value: ~40 HU
Suggested Test & Tolerances Uniformity Compare central and peripheral MVCT HU in uniform phantom TG-148 Compare to Baseline Within 25 HU if using MVCT for dose calculations
Suggested Test & Tolerances Noise Assess low contrast visibility using phantom with various density plugs TG-148: Compare to baseline For example, can you visualize the 1.02 density plug? Tomo spec: 3-cm object of 3% difference in electron density
Suggested Test & Tolerances Spatial Resolution TG-148 1.6mm high contrast object 3 rd row of contrast detail plug
Image Artifacts Button/Zipper Artifact Due to sharp dropoff of detector response in the center of the detector array Possible solutions Energy spectral recalibration Repositioning of detector
Suggested Test & Tolerances MVCT Dose Measure MSAD in phantom with ion chamber in a consistent location Typical values 1-3 cgy TG-148: compare to baseline MDACCO Scan of entire cheese phantom (18 cm) Normal 1.2 1.8 cgy
Suggested Test & Tolerances MVCT HU calibration Important if using dose calculations on MVCT TG-148: Near Water within 30 HU of baseline Lung/Bone within 50 HU of baseline
The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. TG-148 Annual QA Verification of imaging/treatment/laser coordinate system Design a phantom end-to-end test Film or diode array TG-148 tolerance: 2mm/1mm (SRS/SBRT)
Component Replacements Imaging QA is recommended after major component replacements Magnetron SSM Target Linac
Adaptive Radiotherapy Allows assessment of the dosimetric impact of anatomical changes during the course of treatment
Adaptive Protocol at MDACCO Daily MVCT scans are collected for H&N patients Images are run through a research version of TomoTherapy Planned Adaptive software Automatically ti deforms contours to MVCT images and recalculates dosimetry for that fraction Cumulative and daily DVHs can be used to tract changes in the dosimetry Replans are triggered by physician review
Planned Adaptive Research Version
Thorax Phantom DVH No Deformation
Previous MVCT Study Thorax Phantom DVH Previous Calculation K M Langen et al, The use of megavoltage CT (MVCT) images for dose recomputations, Phys Med Biol 50, 4259-76 (2005).
Thorax Phantom Density Comparison: Original IVDT
MVCT Uniformity Cup of water used to calibrate IVDT 27 HU On one machine, the HU value increased near the edge of the FOV 5 HU This is illustrated by the bottle of water shown at the right 15 HU 37 HU
Virtual Water (Cheese) CT Calibration Phantom Water plug shown at upper-right Cheese phantom Cheese phantom configuration shown at lower-right
Thorax Phantom Density Comparison: New IVDT
New Thorax Phantom DVH Less than 1% error at D 50
Tomo1 Temporal MVCT Number Variability 120 Tomo1 Solid Water Variation 100 103 HU MVCT Number 80 60 40 20 0 5 HU -20
Tomo2 Temporal MVCT Number Variability 120 100 Tomo2 Solid Water Variation 100 HU MVC CT Number 80 60 40 20 31 HU 0
Energy variation HI*ART II S/N 002, ENERGY CHECK 0.4350 0.4300 Target changes 0.4250 0.4200 0.4150 0.4100 DEPTH DOSE AT 15 cm 0.4050 0.4000 10-Dec-02 28-Jun-03 14-Jan-04 1-Aug-04 17-Feb-05 5-Sep-05 24-Mar-06 10-Oct-06 28-Apr-07 14-Nov-07 1J 08 DATE
Effect of Component Changes 110 Tomo1 Solid Water Variation for 2009 100 90 Ion Chamber Magnetron/Circulator Target MVC CT Number 80 70 60 Magnetron/Target SSM/Gun board Magnetron Magnetron/Gun board 50 40 30 20
IVDT Curve Comparison IVDT curve appears to maintain shape over time 1000 The slope of the curve appears to 0 increase over time (with target wear) 200 MVCT Numb ber 800 600 400 200 IVDT Temporal Variation Maximum Minimum 0 0.5 1 1.5 2 There is greater variation in MVCT number for higher density materials Therefore, dosimetric errors will likely be greater in regions containing higher proportions of these materials 400 600 800 1000 1200 Density (g/cc)
Dosimetric Error Due to Temporal MVCT Number Variability Solid Water MVCT Number 103 86 80 69 60 49 41 29 20 5 PTV1 D 50 (Gy) 2.02 2.03 2.03 2.04 2.05 2.05 2.06 2.07 2.07 2.08 Error 1.7% 1.2% 1.0% 0.6% 0.3% 0.0% 0.2% 0.5% 0.8% 1.2% Rt Parotid D 50 (Gy) 0.84 0.84 0.84 0.84 0.84 0.84 0.85 0.85 0.85 0.85 Error 0.7% 07% 0.5% 05% 0.5% 05% 0.2% 02% 0.1% 01% 00% 0.0% 02% 0.2% 04% 0.4% 05% 0.5% 08% 0.8% Lt Parotid D 50 (Gy) 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 Error 0.8% 0.6% 0.5% 0.5% 0.1% 0.0% 0.1% 0.2% 0.2% 0.4% All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient Total D 50 variation in the primary target was ~3% Parotid D 50 variation was ~1.5% This is consistent with estimates based on simple physics models If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal HU variations to 3% If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value, dosimetric errors should be within ±1% These errors should increase as depth increases
Dosimetric Error Due to Temporal MVCT Number Variability Solid Water MVCT Number 103 86 80 69 60 49 41 29 20 5 PTV1 D 50 (Gy) 2.02 2.03 2.03 2.04 2.05 2.05 2.06 2.07 2.07 2.08 Error 1.7% 1.2% 1.0% 0.6% 0.3% 0.0% 0.2% 0.5% 0.8% 1.2% Rt Parotid D 50 (Gy) 0.84 0.84 0.84 0.84 0.84 0.84 0.85 0.85 0.85 0.85 Error 0.7% 07% 0.5% 05% 0.5% 05% 0.2% 02% 0.1% 01% 00% 0.0% 02% 0.2% 04% 0.4% 05% 0.5% 08% 0.8% Lt Parotid D 50 (Gy) 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 Error 0.8% 0.6% 0.5% 0.5% 0.1% 0.0% 0.1% 0.2% 0.2% 0.4% All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient Total D 50 variation in the primary target was ~3% Parotid D 50 variation was ~1.5% This is consistent with estimates based on simple physics models If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal HU variations to 3% If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value, dosimetric errors should be within ±1% These errors should increase as depth increases
Dosimetric Error Due to Temporal MVCT Number Variability Solid Water MVCT Number 103 86 80 69 60 49 41 29 20 5 PTV1 D 50 (Gy) 2.02 2.03 2.03 2.04 2.05 2.05 2.06 2.07 2.07 2.08 Error 1.7% 1.2% 1.0% 0.6% 0.3% 0.0% 0.2% 0.5% 0.8% 1.2% Rt Parotid D 50 (Gy) 0.84 0.84 0.84 0.84 0.84 0.84 0.85 0.85 0.85 0.85 Error 0.7% 07% 0.5% 05% 0.5% 05% 0.2% 02% 0.1% 01% 00% 0.0% 02% 0.2% 04% 0.4% 05% 0.5% 08% 0.8% Lt Parotid D 50 (Gy) 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 Error 0.8% 0.6% 0.5% 0.5% 0.1% 0.0% 0.1% 0.2% 0.2% 0.4% All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient Total D 50 variation in the primary target was ~3% Parotid D 50 variation was ~1.5% This is consistent with estimates based on simple physics models If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal HU variations to 3% If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value, dosimetric errors should be within ±1% These errors should increase as depth increases
Dosimetric Error Due to Temporal MVCT Number Variability Solid Water MVCT Number 103 86 80 69 60 49 41 29 20 5 PTV1 D 50 (Gy) 2.02 2.03 2.03 2.04 2.05 2.05 2.06 2.07 2.07 2.08 Error 1.7% 1.2% 1.0% 0.6% 0.3% 0.0% 0.2% 0.5% 0.8% 1.2% Rt Parotid D 50 (Gy) 0.84 0.84 0.84 0.84 0.84 0.84 0.85 0.85 0.85 0.85 Error 0.7% 07% 0.5% 05% 0.5% 05% 0.2% 02% 0.1% 01% 00% 0.0% 02% 0.2% 04% 0.4% 05% 0.5% 08% 0.8% Lt Parotid D 50 (Gy) 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 Error 0.8% 0.6% 0.5% 0.5% 0.1% 0.0% 0.1% 0.2% 0.2% 0.4% All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient Total D 50 variation in the primary target was ~3% Parotid D 50 variation was ~1.5% This is consistent with estimates based on simple physics models If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal HU variations to 3% If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value, dosimetric errors should be within ±1% These errors should increase as depth increases
Dosimetric Error Due to Temporal MVCT Number Variability Solid Water MVCT Number 103 86 80 69 60 49 41 29 20 5 PTV1 D 50 (Gy) 2.02 2.03 2.03 2.04 2.05 2.05 2.06 2.07 2.07 2.08 Error 1.7% 1.2% 1.0% 0.6% 0.3% 0.0% 0.2% 0.5% 0.8% 1.2% Rt Parotid D 50 (Gy) 0.84 0.84 0.84 0.84 0.84 0.84 0.85 0.85 0.85 0.85 Error 0.7% 07% 0.5% 05% 0.5% 05% 0.2% 02% 0.1% 01% 00% 0.0% 02% 0.2% 04% 0.4% 05% 0.5% 08% 0.8% Lt Parotid D 50 (Gy) 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 0.53 Error 0.8% 0.6% 0.5% 0.5% 0.1% 0.0% 0.1% 0.2% 0.2% 0.4% All values shown are given for dose recalculations performed on a single MVCT of a head & neck patient Total D 50 variation in the primary target was ~3% Parotid D 50 variation was ~1.5% This is consistent with estimates based on simple physics models If the IVDT or MVCT images were never recalibrated, one could expect to limit the dosimetric error due to temporal HU variations to 3% If recalibrations were performed to maintain the solid water MVCT number within 30 HU of the commissioned IVDT value, dosimetric errors should be within ±1% These errors should increase as depth increases
Patient Example: Linac Change Linac Change Solid water: 36 HU to 8 HU 1.2% increase in dose to PTV1
Baseline MVCT Recalculation Error Water Head Thorax MVCT kvct Error MVCT kvct Error MVCT kvct Error D 95 (Gy) 1.99 2.00 0.7% 1.97 2.00 1.3% 1.99 2.00 0.4% D 50 (Gy) 2.01 2.01 0.4% 2.01 2.04 1.3% 2.05 2.03 0.7% D 05 (Gy) 2.03 2.04 0.4% 2.04 2.07 1.1% 2.09 2.08 0.4% MVCT images were obtained for three phantoms The cheese phantom was scanned at the same time to obtain an accurate IVDT and eliminate any temporal effects The greatest baseline error occurred for the head phantom at -1.3%
Recommendations MVCT recalibration frequency depends on the dosimetric error that one is willing to accept Based on this data, if the MVCT were never recalibrated, a maximum dosimetric error of ~4% would be possible Including baseline error and temporal variations We recommend recalibrating the MVCT to maintain the MVCT number of water within ±30 HU of the commissioned value This would result in an overall dosimetric error of not greater than ±2.5% for the typical head & neck patient This is consistent with TG-148 recommendations We recommend checking the IVDT after major component changes affecting the beamline There appears to be a consistent reduction in MVCT number after linac or target changes Magnetron changes are less consistent, but some large variations were observed Apart from major component changes, the IVDT should be checked on a monthly basis
Conclusions Imaging QA is important for maintaining an IGRT program More frequent QA is needed when using the images for dose calculations TG-148 is a good source of information for TomoTherapyTh users
Acknowledgements Katja Langen, PhD MDACCO, chair of TG-148 Jason Pukala, MS UF graduate student