Objectives. Tolerance Limits and Action Levels for IMRT QA. The Overall Process of IMRT Planning and Delivery. Chain of IMRT Process

Transcription

1 Tolerance Limits and Action Levels for IMRT QA Jatinder R Palta, Ph.D Siyong Kim, Ph.D. Department of Radiation Oncology University of Florida Gainesville, Florida Objectives Describe the uncertainties in IMRT Planning and Delivery Describe the impact of spatial and dosimetric uncertainties on IMRT dose distributions Describe the limitations of current methodologies of establishing tolerance limits and action levels for IMRT QA Describe a new method for evaluating quality if IMRT planning and delivery The Overall Process of IMRT Planning and Delivery Positioning and Immobilization File Transfer and Management Image Acquisition (Sim,CT,MR, ) Structure Segmentation IMRT Treatment Planning Plan Validation Position Verification IMRT Treatment Delivery Chain of IMRT Process Position Verification File Transfer and and Management Positioning and Immobilization Image Acquisition Structure Segmentation IMRT Treatment Planning and Evaluation ASTRO/AAPM Scope of IMRT Practice Report IMRT Treatment Delivery and Verification Plan Validation Adapted from an illustration presented by Webb,

2 Uncertainties in IMRT Delivery Systems 0.0 MLC Leaf Position Gap error Dose error MLC leaf position Gantry, MLC, and Table isocenter Beam stability (output, flatness, and symmetry) MLC controller % Dose error Range of gap width Gap error (mm) Nominal gap (cm) Data from MSKCC; LoSasso et. al. Beam stability for low MU Beam Flatness and Symmetry for low MU Flatness versus time (s) after beam on MU 0x MU integrated Data from Christie Hospital: Geoff Budgell Symmetry versus time (s) after beam on 1 1 MU@ 500 MU/min sec sec

3 cm 0.4 cm diff(%) Relative OFF-Axis Distance (cm) MLC Controller Issue Uncertainties in IMRT Planning 1 MU per strip Dose Rate: 600 MU/min Varian 100 C/D Dose Rate: 600 MU/min Elekta Synergy Film measurements of a 10-strip test pattern. The linacs were instructed to deliver 1 MU per strip with the stepand-shoot IMRT delivery mode for a total of 10 MU. The delivery sequence is from left to right. Can be attributed to: Dose calculation grid size MLC round leaf end none divergent MLC leaf-side/leaf end modeling Collimator/leaf transmission Penumbra modeling; collimator jaws/mlc Output factor for small field size PDD at off-axis points The effect of dose calculation grid size at field edge Grid size becomes critical when interpolating high dose gradient. 6MV Photon step size effect for 0x0 fields both data set use trilinear interpolation Can result in large errors seg 1 seg seg3 sum May not be accurately accounted for in the treatment planning system Leakage radiation can be 15% or higher Closed Leaf Position 10 Relative Output Dose Off-axis Distance (cm) Fine grid size is needed for interpolation 3

4 Tongue-and-groove effect MLC X-Jaw Leaf-side effect MLC upper jaw MLC Tertiary Collimator MLC Upper Jaw Tertiary Collimator FWHM Avg. Min cm 0 cm collimator 0 leaf side Relative Profile Leaf side tongue Collimator Relative output cm cm Off-Axis Distance (cm) Off-axis distance (cm) Relative Profile Beam Modeling (Cross beam profile with inappropriate modeling of extra-focal radiation) ADAC(4mm) wobkmlctr Diff Measured vs. Calculated Off-Axis (cm) Relative Difference Beam Modeling (Cross beam profile with appropriate modeling of extra focal radiation) diff(w/bk) diff(wobk) wbkmlctr wobkmlctr Off-Axis (cm) 4

5 Segmental Multileaf Collimator (SMLC) Delivery System What should be the tolerance limits and action levels for delivery systems for IMRT? MLC* Leaf position accuracy Leaf position reproducibility Gap width reproducibility Gantry, MLC, and Table Isocenter Beam Stability Low MU Output (<MU) Low MU Symmetry (<MU) Tolerance Limit 1 mm 0. mm 0. mm 0.75 mm radius % % Action Level mm 0.5 mm 0.5 mm 1.00 mm radius 3% 3% * Measured at all four cardinal gantry angles Palta et.al. AAPM Summer School, 003 Dynamic Multileaf Collimator (DMLC) Delivery System MLC* Leaf position accuracy Leaf position reproducibility Gap width reproducibility Leaf speed Gantry, MLC, and Table Isocenter Beam Stability Low MU Output (<MU) Low MU Symmetry (<MU) Tolerance Limit 0.5 mm 0. mm 0. mm ±0.1 mm/s 0.75 mm radius 3% % Action Level 1 mm 0.5 mm 0.5 mm ±0. mm/s 1.00 mm radius 5% 3% What should be the tolerance limits and action levels for IMRT Planning? 5

6 Are TG53 recommendation acceptable for IMRT TPS? Probably not!!!! Absolute Normalization Point (%) 1.0 Central-Axis (%) Inner Beam (%) Outer Beam (%) Penumbra (mm) Buildup region (%) Outer Inner Build-up Normalization point Most subfields have less than 3 MU (Data from 100 Head and Neck IMRT patients treated at UF) TG 53 Recommendation for 3DRTPS Penumbra Need D/3D analyses tools. How to quantify the differences? Qualitative Methods (1) Measurement Calculation Overlaid isodose plot visual comparison Adequate for ensuring that no gross errors are present evaluation influenced by the selection of isodose lines; therefore it can be misleading. 6

7 Methods () Quantitative methods: Dose Difference Methods (3) Quantitative methods: Distance-to to-agreement (DTA) Possible to quickly see what areas are significantly hot and cold; however, large errors can exist in high gradient regions Distance between a measured dose point and the nearest point in the calculated distribution containing the same dose value More useful in high gradient regions; however, overly sensitive in low gradient regions Hogstrom et.al. IJROBP., 10, , 1984 Quantitative methods: Composite distribution Overlaid isodose distribution Composite distribution Methods (4) A binary distribution formed by the points that fail both the dose-difference and DTA criteria D > D tol B DD = 1 d > d tol B DTA = 1 B = B DD x B DTA Useful in both low- and high- gradient areas to see what areas are off However, No unique numerical index-that enables the analysis of the goodness of agreement Harms et.al. Med. Phys., 5, ,1998 Quantitative methods: Gamma index distribution P j P j P i P i distance, d dose-difference, D Methods (5) γ i = min d d tol D tol Combined ellipsoidal dosedifference and DTA test D + γ 1, calculation passes, and γ> 1, calculation fails D d = D tol = d tol Low et.al. Med. Phys., 5, ,1998 j 7

8 Methods (6) Quantitative methods: Normalized Agreement Test (NAT) D < D tol, NAT = 0 d < d tol, NAT = 0 %D < 75% & D meas < D cal, NAT = 0 Otherwise, Where, NAT = D scale x (δ-1) D scale i = larger[d cal, D meas ] i / max[d cal ] δ i = smaller[( D/ D tol ), ( d/ d tol )] i Average NAT value NAT index = x 100 Average of the D scale matrix Try to quantify how much off overall Childress et.al. IJROBP 56, ,003 Current Dose Verification Methods Dose difference Possible to quickly see what areas are significantly hot and cold; however, large errors at high gradient regions Distance-to-agreement (DTA) Distance between a measured dose point and the nearest point in the calculated distribution containing the same dose value More useful in high gradient regions; however, overly sensitive in low gradient regions Composite distribution 1 A binary distribution formed by the points that fail both the dose-difference and DTA criteria Gamma index distribution Gamma function criteria using the combined ellipsoidal dosedifference and DTA tests 1 Harms et al, Med. Phys., 6(10), 1998, pp Low et al, Med. Phys., 6(5), 1998, pp Tolerance limits based on statistical and topological analyses Region δ 1 (high dose, small dose gradient) δ 1 (high dose, large dose gradient) δ 1 (low dose, small dose gradient) δ 90-50% (dose fall off) Confidence Limit* (P=0.05) 3% 10% or mm DTA 4% mm DTA Action Level 5% 15% or 3 mm DTA 7% 3 mm DTA Are there any limitations of current methodologies of establishing tolerance limits for IMRT QA???? * Mean deviation used in the calculation of confidence limit for all regions is expressed As a percentage of the prescribed dose according to the formula, δ i = 100% X (D calc D meas. /D prescribed ) Palta et.al. AAPM Summer School, 003 8

9 Comparison of Measured and Calculated Cross Plot RPC IMRT Phantom Results Posterior Anterior Posterior Profile Anterior 8 6 Data analysis 10 mm DTA difference Measured with a diode-array (Map Check; Sun Nuclear Corp.) RPC criteria o f acceptability: 7% for PTV 4 mm DTA for OAR Dose (Gy) Organ at Risk Distance (cm) RPC Film Primary PTV Institution Values Fréquence Radiotherapy Oncology Group for Head &Neck 0 0,87 0,89 0,91 1 Centres- 118 patients 0,93 0,95 0,97 0,99 1,01 1,03 Number of meas. = 679 Mean = 0,995 SD = 0,05 1,05 1,07 1,09 1,11 1,13Dm/Dc 7 Points outside 4σ In 1600 observations % C.L. ~Same odds as wining Power ball Multi-state Lotto 4 times IMRT QA Outliers Surely there is something systematic at work in some cases.. Recommandations for a Head and Neck IMRT Quality Assurance Protocol: 8 (004), Cancer/Radiothérapie , M. Tomsej et al. Dong et al. IJROBP, Vol. 56, No. 3, pp , 003 9

10 What could be the reason??? Evidence That Something Could Be Amiss It could be delivery error Mechanical Errors? MLC Leaf Positioning Fluence and Timing? Orchestration of MLC and Fluence It could be dosimetry artifacts Some measurement Problem? It could be algorithmic errors Source Model, Penumbra, MLC Modeling More than likely a conspiracy of effects, each with it own uncertainty. An uncertainty model Space-oriented uncertainty (SOU) Dose uncertainty is proportional to the gradient of dose A new method for evaluating the quality of IMRT Planned dose profile Based on space and dose-specific uncertainty information Hosang Jin MS; Pre Doctoral Candidate University of Florida with 1 mm with mm 10

11 An uncertainty model Non-space-oriented uncertainty (NOU) Dose uncertainty is inversely proportional to the dose level It follows from the fact that the signal-tonoise ratio (SNR) is proportional to the magnitude of the measured dose Dose Dose Uncertainty An uncertainty model U 1 3 ( r) = w SOU + w NOU + w SOU NOU + ε Assuming that the SOU and NOU are uncorrelated and that there is no offset, meaning that w 1 =w =1, w 3 =0 and ε=0, Space-oriented SOU ( σ ) G( r ) σ sou r Uncertainty sources Non-space-oriented σ σ sou + σ nou 1 NOU ( σ nou ) D Jin et al, Medical Physic, 3(6), pp , 005 Space-oriented sou An uncertainty model σ ( r) = D( r) σ σ r = σ r r I i _ i 1-D Simulation 3-Segmented IMRT Field Non-space-oriented for SOU sources i through I, I is the total number of sources of SOU, σ Dr_i is a SD of the spatial uncertainty of the source i σ ( r ) = σ D( r) D σ ro = σ ro nou r o o J j _ j K ( r ) total = σ sou ( r ) σ nou( r ) σ ( r) overall = σ total _ k ( r) k For a combination of multiple fields σ + for NOU sources j through J, J is the total number of sources of NOU, σ ro_j is a SD of the spatial uncertainty of the source j Beam set 1 Different beam width Same beam fluence Beam set Same beam width Different beam fluence 11

12 Results (1-D: 3 IMRT Beam Fields) Confidence weighted Dose Distributions (CWDD) + Uncertainty distribution Isodose lines [Upper bound] Plan 1 Calculated dose distribution Isodose lines CWDD Beam set 1 Different beam widths Same beam fluences Beam set Same beam widths Different beam fluences - Uncertainty distribution Isodose lines [Lower bound] Plan Clinical Application of the Uncertainty Model Application 1: Dose Uncertainty Volume Histogram (DUVH) IMRT Plan 1 3D dataset of NOU 3D dataset of SOU Uncertainty Prediction model 3D uncertainty distribution X Binary map of organ of interest 3D uncertainty distribution of organ of interest Head and Neck IMRT Plan Plan I (5 beams) Plan II (6 beams) Plan III (9 beams) Plan 3 is selected due to minimal uncertainty in PTV and OARs Plan selection Applying other evaluation methods (e.g. DVH, isodose lines) Plan Plan 1 Plan 3 Plan evaluation Making DUVHs with different IMRT plans for the same target Volume Plan 1 Uncertainty Dose Uncertainty Volume Histogram (DUVH) Colorbar unit: cgy (1 SD)) In all plans, 95% of PTV was covered by the prescribed dose (180 cgy/fraction). 1

13 Dose uncertainty distribution Confidence-weighted dose distributions 95% confidence interval Plan I (5 beams) Plan II (6 beams) Plan III (9 beams) Colorbar unit: cgy (1 SD) SOU: 1 mm of a SD for spatial displacement NOU: 1 % of a SD for a relative dose uncertainty at 180 cgy Plan I (5 beams) Plan II (6 beams) Plan III (9 beams) 180 cgy (Red) 160 cgy (purple) 10 cgy (skyblue) 80 cgy (green) 40 cgy (yellow) PTV (green) Spinal cord (brown) Right parotid (yellow) DUVH (Dose uncertainty volume histogram) Head and Neck IMRT Plan PTV (absolute DUVH) Brainstem (relative DUVH) Plan I (5 beams) Plan II (7 beams) Plan III (9 beams) Spinal cord (relative DUVH) Colorbar unit: cgy In all plans, 95% of PTV was covered by the prescribed dose (180 cgy/fraction Smaller area under the DUVH curve is preferable (arrows). Both absolute and relative (point uncertainty/point dose) uncertainties are employed to make the DUVH. 13

14 Dose uncertainty distribution Confidence-weighted dose distributions 95% confidence interval As far as PTV coverage is concerned, Plan II is preferred. Plan I (5 beams) Plan II (7 beams) Plan III (9 beams) Colorbar unit: cgy (1 SD) SOU: 1 mm of a SD for spatial displacement NOU: 1 % of a SD for a relative dose uncertainty at 180 cgy Plan I (5 beams) Plan II (7 beams) Plan III (9 beams) 180 cgy (Red) 140 cgy (purple) 10 cgy (skyblue) 80 cgy (green) 40 cgy (yellow) PTV (green) Spinal cord (brown) DUVH (Dose uncertainty volume histogram) MapCHECK Analysis PTV (absolute DUVH) Brainstem (relative DUVH) Spinal cord (relative DUVH) A binary test using dose difference and distance-to-agreement (DTA ) Criteria Dose difference: 3% DTA: 3 mm Points less than 10% of reference dose were excluded Smaller area under the DUVH curve is preferable (arrows). Both absolute and relative (point uncertainty/point dose) uncertainties are employed to make the DUVH. 14

15 Planar Dose Distribution for an IMRT Field Planar Dose Distribution for an IMRT Field Dose Distribution Uncertainty Distribution Colorbar unit: cgy Blue: calculation is higher, green: measurement is higher Uncertainty distribution: 1 SD Dose Distribution Uncertainty Distribution Colorbar unit: cgy Blue: calculation is higher, green: measurement is higher Uncertainty distribution: 1 SD Planar Dose Distribution for an IMRT Field Dose Distribution Uncertainty Distribution Colorbar unit: cgy Blue: calculation is higher, green: measurement is higher Uncertainty distribution: 1 SD Summary The tolerance limits and action levels proposed in this presentation for the IMRT delivery system QA have justifiable scientific rationale The tolerance limits and action levels proposed in this presentation for the IMRT planning and patient specific QA also have justifiable scientific rationale. However, all commonly used metrics ( D, binary difference, gamma index etc.) for dose plan verification have limitations in that they do not account for spacespecific uncertainty information The proposed plan evaluation metrics will incorporate both spatial and non-spatial dose deviations and will have high predictive value for QA outliers 15