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IMRT: Treatment Planning and Dosimetry Nesrin Dogan, Ph.D Department of Radiation Oncology Virginia Commonwealth University Medical College of Virginia Hospitals Richmond, VA, USA
For small fields, minor uncertainties due to approximations in dose calculation models, methods for determining MLC leaf sequences and other factors may form a large fraction of dose delivered, and lead to inaccuracies in delivered dose.
10 cm 1 cm 1 cm 1 cm 1 cm 1 cm 1 cm 1 cm 1 cm 1 cm 1 cm
Need to be measured with microchamber, film or diode.
Subtle effects make a difference in IMRT.
Beam model based on penumbra measured with 6 mm diameter chamber Beam model based on penumbra measured with film Courtesy of G. Ezzel, Ph.D., Mayo Clinic
Need to determine energy dependence and angular response.
Small field detectors required for small field characterization.
Sensitive to position
Detector should be smaller than homogeneous region of dose to be measured
Assess electrometer response.
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Courtesy of Jean Moran, Ph.D, UofMichigan Small 1-D Detectors 0.0019 NA NA 0.3 0.015 0.009 Volume (cm 3 ) 0.45 0.4 0.73 NA 0.2 0.6 Diameter (cm) < resolution than diodes, dose rate dependence, expensive Diamond Non-linear dose response for <30 cGy MOSFET Stereotactic diode p-type Si diode Over-respond to low energy photons Martens et al. 2000 Pinpoint chamber Poorer resolution than diodes Micro-chamber Disadvantages Detector
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Pasma Med Phys 26: 2373-2378 (2376) 1999 Predicted EPID Ion Chamber + Discrepancies in the penumbra region (up to 10%) Overall: Good agreement 10 MV 25 MV EPID: DMLC measurements Courtesy of Jean Moran, UofMichigan
Current IMRT systems use simplified dose calculations during plan optimization: e.g., pencil beam -> uses very simple heterogeneity corrections, causing significant dose errors (10% or more non-IMRT cases)
Final dose calculation is performed using a separate independent dose calculation that incorporate the influence of the MLC: e.g, convolution / superposition; more accurate than Pencil beam; however, inaccuracies persist under certain circumstances
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Conventional dose algorithms can be inaccurate for
Small fields
Regions of dose gradients (radiation disequilibrium)
Heterogeneous conditions
IMRT is typically delivered through a sequence of small static fields or with a dynamically moving aperture with a small width. Dose gradients are common place in IMRT fields. For such fields, assumptions used in conventional algorithms regarding scatter equilibrium and output factor variation with field size typically break down.
Significant fraction of the dose within targets and organs at risk is due to scattered or leakage radiation
calculated dose distributions have the greatest uncertainties due to approximations inherent in conventional methods of transforming intensities into MLC leaf sequences
Experimental checks of IMRT fields routinely shows discrepancies between the planned ( desired ) and actual.
For IMRT
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Dose Calculation Algorithms Calculation Speed Calculation Accuracy Pencil Beam Monte Carlo Superposition/Convolution Courtesy: Jeff Siebers, VCU
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Comparison of SC and MC Comparison of a) Superposition-Convolution (SC) and b) MC dose calculations
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Monte Carlo Pencil Beam Pawlicki et al., Med Dosim, 26 157 (2001) Comparison of PB and MC Pencil Beam Monte Carlo
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Superposition Monte Carlo Slice 45 Monte Carlo Slice 55 Monte Carlo Slice 64 Comparison of SC and MC Superposition Superposition
For a given intensity distribution, dose predicted differs from that actually delivered to the patient/phantom
Can be avoided by performing final calculation with accurate algorithm
Optimization Convergence Error (OCE)
Consequence of systematic error during optimization
Optimization with an inaccurate algorithm results in different intensities than those predicted by an accurate algorithm
Actual dose is not optimal, a better solution exists
Can be avoided by optimization with an accurate algorithm
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DPE (same intensities) PB computed SC computed Make sure your final dose calculation is with an accurate algorithm 68 Gy 64 Gy 60 Gy 50 Gy 40 Gy 30 Gy
Optimized plans are converted to deliverable plans through leaf-sequencing process that takes into account the limitations and effects (leakage/scatter) of the MLC
The idealized optimal plan is replaced with “deliverable” plan
Optimized and deliverable IMRT plans differ
Different intensity distributions
More complex the intensity distribution, the greater the deviation
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Comparison of Isodoses a) An optimized intensity distribution b) A deliverable distribution using DMLC calculated using Convolution/Superposition algorithm
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Final dose is deliverable Deliverable IMRT Optimization Process combine optimization and delivery into one process Leaf Sequencing Initial Intensity (I I (x,y)) Evaluate Plan Objective Converged? Adjust I(x,y) Compute Dose (D O ) Optimized Intensity (I O (x,y)) and Dose D O = D D No Yes 1 3 4 5 2 Create Leaf Sequence 7 Create Deliverable Intensities (I D (x,y)) 8 6
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Deliverable Optimization Deliverable optimization can restore original optimized plan Head and Neck IMRT plan Original SC opt Deliverable Plan SC MC of Deliverable MC opt (deliverable)
More important for IMRT than conventional treatments.
Heterogeneities may effect some beamlets more than others -> causing different localized dose differences.
The reliability of clinical experience with DVH prescriptions and results may be significantly compromised if heterogeneity corrections are not used (e.g., Lung).
Use AAPM Report No:85 Tissue Inhomogeneity Corrections for Megavoltage Photon Beams.
4% - 10% error in relative e- density result in ~2% error in dose.
Important when target regions (PTV) extend into the buildup region.
Calculated doses are often inaccurate and lower than delivered doses.
Likely to cause hot spots in the target and elsewhere as a result of inverse planning engine fighting with the buildup effect – may cause excessive skin reactions and compromise the plan quality.
Bolus needs to be added if the target is in the buildup region: needs to be included during the scanning of patient.
Create nonPTVOARs for organs overlapping with PTVs:
NonPTVSmallBowell, NonPTVRectum,
NonPTVBladder, etc.
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Guidelines for Target Expansions Prostate CTV: Expand prostate by 0.5cm in all directions except posteriorly then + seminal vesicles (no expansion for seminal vesicles) Prostate PTV: Expand Prostate CTV by 0.5cm in all directions (3D expansion) Lymph Nodes CTV : Expand lymph nodes by 1.0 cm in anterior, posterior, right and left (2D expansion) with small bowel, bladder, rectum, bones, muscle, skin1cm and prostate PTV tissues being the limiting organs Lymph Nodes PTV : Expand Lymph Nodes CTV 0.5 cm in all directions (3D expansion) with only skin1cm and Prostate PTV as the limiting structures
Shift isocenter to provide best separation between target and tissues.
Courtesy of G. Ezzel, Ph.D., Mayo Clinic 7 rows to cover target One row hits target and structure 6 rows to cover target Split between target and structure
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Dose-Volume Based vs. EUD-Based Optimization – H&N Example EUD + tumor as “virtual normal tissue” 30 45 50 (c) 70 60 EUD unconstrained (b) 30 45 50 80 60 70 45 Dose-Volume (a) 30 50 60 70 45
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Minimize Number of Segments Segments MU 50 550 75 582 100 604 150 619 200 631 50 Segments 75 Segments 100 Segments 150 Segments 200 Segments
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Impact of Degree of Fluctuations (“Complexity”) in Intensity Patterns on MUs for IMRT 100 Total MUs= 100 100 Total MUs = 300 100 100 10 cm 10 cm 100 100 Total MUs = 200 10 cm Total MUs = 150 50 50 50 10 cm
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Target Volumes Critical Structure TLDs in Target Volumes Radiochromic film through multiple plans Delivery is required by RTOG for participation in IMRT trials Removable Dry Insert Water Water Anthropomorphic: RPC Head Phantom Courtesy of Jean Moran, UofM
ScV field needs to be included in the IMRT optimization
Feathering
Watch out for overlaps if the IMRT plan wants to open the jaws into the ScV area -> may need to adjust the jaw
ScV field IMRT field
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This row might be used by the IMRT plan if the target is drawn too close to the isocenter plane Courtesy of G. Ezzel, Ph.D., Mayo Clinic Human planners sometimes have to trim IMRT beams…..
Machine QA- Acceptance and routine QA of the MLC for IMRT delivery - dosimetric and geometric characteristics
Algorithm QA for IMRT - QA of planning system and data consistency with machine
Patient Specific QA – prove plan works
1D and 2D dosimetry of treatment components such IM beams and segments
3D dosimetry of entire treatment delivery
Post Treatment QA
Log-file analysis
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Phantom Dose Verification Beams on Patient Beams on Phantom
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Sample Film Dosimetry Results Other Analysis Distance to Agreement Gamma … Isodose Comparison Profiles Dose Differences
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Compare isodoses (film) and absolute dose (chamber) Current Practice Courtesy of G. Ezzel, Ph.D., Mayo Clinic
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Gamma Analysis Measured Film Adaptive Convolution Monte Carlo
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Calculation to Measurement Comparison (b) Measured Calculated 54% of points have a dose difference <2% or a DTA <2 mm
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MC to Measurement Comparison (b) (c) Measured Calculated Measurement and MC w transport through MLC 97% within 2%,2 mm Measurement and MC using Tx planning systems Intensity Matrix
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=10% Superposition Monte Carlo MC Verification
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The percentage of points, averaged over all of the plan’s treatment fields for each patient with ≥ 1 with 2% tolerance and 2 mm DTA. MC (8.1% ± 3.7% points failed; range = 4.9% – 18.4%) SC (16.7% ± 5.6% points failed; range = 11.3% – 30.7%) MC Verification of Prostate IMRT Plans
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Courtesy of G. Ezzel, Ph.D., Mayo Clinic Check standard patterns for constancy
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DMLC field 14x14 cm 2 at SSD =100 cm, 2 cm separated strips
Need to characterize the MLC system for IMRT with special emphasis on penumbra, leaf leakage and transmission.
Need to know the limits of the mechanical systems and interactions with controller and accelerator software for delivery.
Continued need for improvements to software for delivery system, measurement devices, phantoms, and dose analysis tools.
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Acknowledgements Jeffrey Siebers – VCU Gary Ezzel – Mayo Clinic Mark Oldham – Duke University Jean Moran – U of Michigan Ivaylo Mihalov - UofArkansas
IndiaCancerHospital.com is a facilitation service for cancer patients around the world who are looking for high quality treatment in India at a low cost Write to us for a Free No Obligation Opinion and Cost Estimate for Cancer from Top Doctors in India Please scan and email your medical reports to us at hospitalindia@gmail.com Call Us At +91-9899993637 or visit www.IndiaCancerHospital.com
A lot of thanks!