I M R Tintro


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I M R Tintro

  1. 1. An Introduction to Intensity Modulated Radiotherapy PHY778 Lecture April 1, 2009
  2. 2. Outline <ul><li>Introduction </li></ul><ul><li>Process </li></ul><ul><li>Physics </li></ul><ul><li>Technological Implementation </li></ul><ul><li>Sites </li></ul>
  3. 3. Introduction <ul><li>IMRT </li></ul><ul><ul><li>Ability to deliver many beamlets of varying radiation intensity within one treatment field </li></ul></ul><ul><li>Beamlet </li></ul><ul><ul><li>Smallest element to be modified </li></ul></ul>Beamlet Field Width Field Length
  4. 4. Introduction <ul><li>Inverse treatment planning process </li></ul><ul><ul><li>Clinical objectives (goals) specified first </li></ul></ul><ul><ul><li>Planning system optimizes the plan’s parameters to satisfy the defined clinical objectives </li></ul></ul><ul><ul><li>Field’s fluence is optimized for IMRT </li></ul></ul>
  5. 5. Introduction <ul><li>Optimization yields the set of beamlet weights (fluence map) </li></ul><ul><li>Fluence map is converted to deliverable sequences which depend on technique </li></ul>
  6. 6. Implementation
  7. 7. Process Patient Selection ↓ Simulation ↓ Target and Tissue Delineation ↓ Treatment Planning/Optimization ↓ Plan Evaluation ↓ Quality Assurance ↓ Treatment Delivery ↓ Followup
  8. 8. Patient Selection <ul><li>Nasopharynx T2N2 </li></ul><ul><ul><li>Improved salivary outcome </li></ul></ul><ul><ul><li>No difference in reported xerostomia score </li></ul></ul>
  9. 9. Patient Selection <ul><li>Prostate Cancer </li></ul><ul><ul><li>Dose escalation improves outcome </li></ul></ul><ul><ul><li>Rectal toxicity is dose dependent </li></ul></ul>Rx to 76 Gy  95%  50%
  10. 10. Patient Selection <ul><li>Early stage breast cancer </li></ul><ul><ul><li>Less heterogeneity </li></ul></ul><ul><ul><li>Less moist desquamation </li></ul></ul><ul><ul><li>Improved late cosmesis </li></ul></ul>
  11. 11. Patient Selection <ul><li>Lung </li></ul><ul><ul><li>Proximity to large critical structures </li></ul></ul><ul><ul><li>Pulmonary toxicity life-threatenning </li></ul></ul><ul><ul><li>Dose-volume relationship poorly understood </li></ul></ul><ul><ul><li>Target motion may be a problem </li></ul></ul>
  12. 12. Simulation <ul><li>Tumour localization is critical </li></ul><ul><ul><li>Conformal dose distribution </li></ul></ul><ul><ul><li>Minimize expansion of PTV into OAR </li></ul></ul>
  13. 13. Simulation <ul><li>Patient immobilization depends on site </li></ul><ul><ul><li>Masks </li></ul></ul><ul><ul><li>Chest board </li></ul></ul><ul><ul><li>Characterize device </li></ul></ul>
  14. 14. Simulation <ul><li>Inter-fraction motion </li></ul><ul><ul><li>Implanted gold seed fiducials </li></ul></ul><ul><ul><li>Surgical clips </li></ul></ul><ul><ul><li>Bony anatomy </li></ul></ul><ul><ul><li>Calcifications </li></ul></ul>
  15. 15. Simulation <ul><li>Intra-fraction motion </li></ul><ul><ul><li>Bladder/bowel preparation </li></ul></ul><ul><ul><li>Compression </li></ul></ul><ul><ul><li>Gated therapy </li></ul></ul>
  16. 16. Simulation <ul><li>Fuse images from various modalities </li></ul>
  17. 17. Contouring <ul><li>Evidence-based </li></ul><ul><li>Gross disease </li></ul><ul><li>Potential routes of local and regional spread of disease </li></ul><ul><li>Patterns of failure </li></ul><ul><li>Principles of ICRU 50/62 </li></ul><ul><li>Normal tissue tolerances/avoidance criteria </li></ul><ul><li>Radiobiology of dose-fraction-time </li></ul>
  18. 18. Target Delineation ICRU 50, 62 <ul><li>GTV+CTV </li></ul><ul><li>(A) + IM (organ motion) </li></ul><ul><li>(B) + SM (setup uncertainty) </li></ul><ul><li>SM + IM </li></ul><ul><li>PTV = non-linear combination of SM + IM </li></ul>
  19. 19. PTV versus OAR <ul><li>Conflicts can arise </li></ul>
  20. 20. Contouring <ul><li>Nodal atlas (normal anatomy) </li></ul>
  21. 21. Contouring example
  22. 22. Planning <ul><li>Number and placement of beams </li></ul><ul><li>Objectives </li></ul><ul><li>Specify dose </li></ul><ul><li>Concurrent boost </li></ul>
  23. 23. Number and Placement of beams <ul><li>Standard beam arrangements usually used </li></ul><ul><li>Usually 5-9 fields sufficient </li></ul><ul><li>Some planning systems capable of Beam Angle Optimization prior to Fluence map optimization </li></ul>
  24. 24. Planning <ul><li>Number and placement of beams </li></ul>1 9 8 7 6 5 4 3 2 1 2 3 4 5 6 7
  25. 25. Beam number and placement <ul><li>Considerations: </li></ul><ul><ul><li>Complexity of the target shape </li></ul></ul><ul><ul><li>Proximity to critical organs </li></ul></ul><ul><ul><li>Previous RT? </li></ul></ul><ul><ul><li>Central or lateralized target volume </li></ul></ul><ul><ul><li>Equally spaced </li></ul></ul><ul><ul><li>Gantry angles that best cover the target and miss critical structures </li></ul></ul><ul><ul><li>Ideally no opposing beams (some exceptions) </li></ul></ul>
  26. 26. Parallel Opposed Fields <ul><li>During optimization, rays (beamlets) 2 & 2’ compete with each other </li></ul><ul><li>Not possible to optimize dose to one point without changing dose to the other points </li></ul><ul><li>Rays do not compete with each other </li></ul><ul><li>All rays are useable </li></ul><ul><li>Possible to optimize dose to all points A, B, and C individually </li></ul>
  27. 27. Beam number and placement <ul><li>Use of non-coplanar fields may help to significantly decrease the dose to critical structures </li></ul>
  28. 28. Collimator Rotation <ul><li>Minimize leakage </li></ul><ul><li>Maximize coverage </li></ul>
  29. 29. Objectives <ul><li>Allows definition of clinical goals </li></ul><ul><li>Each objective has a priority assigned </li></ul><ul><li>Objectives may be base on: </li></ul><ul><ul><li>dose </li></ul></ul><ul><ul><li>clinical knowledge </li></ul></ul><ul><ul><li>equivalent uniform dose (EUD) </li></ul></ul><ul><ul><li>TCP or NTCP (estimate local control or toxicity) </li></ul></ul>
  30. 30. Objectives <ul><li>Minimum dose objective </li></ul><ul><ul><li>Penalty if any point in a TARGET structure receives less than a specified dose </li></ul></ul><ul><li>Maximum dose objective </li></ul><ul><ul><li>Penalty if any point in an OAR structure receives more than a specified dose </li></ul></ul>
  31. 31. Objectives- Max and Min dose Maximum = 50 Gy Minimum = 70 Gy
  32. 32. Objectives – Uniform dose Maximum Dose Maximum = 70 Gy Minimum = 70 Gy
  33. 33. Objectives – Dose volume objectives Maximum Dose > 70 Gy to 90% Target < 60 Gy to 10% OAR
  34. 34. Objectives – Equivalent Uniform Dose 43 49 30 71 EUD 7.4 Cord 4.6 Brainstem 5.0 Parotid -8.0 Target a Region
  35. 35. Physics <ul><li>Objective Functions: </li></ul>
  36. 36. Physics <ul><li>Minimize objective function using gradient based methods: </li></ul><ul><li>I is a vector of beamlet weights </li></ul><ul><li>If M old = 1, steepest descent algorithm </li></ul><ul><li>If M old = H old , Newton’s method where H old is the inverse of the Hessian matrix </li></ul>
  37. 37. Physics – optimization algorithms <ul><li>Simulated annealing </li></ul><ul><ul><li>Attempts to find global minimum </li></ul></ul><ul><ul><li>The probability for accepting a trial configuration is controlled by the temperature and is given by: </li></ul></ul><ul><ul><li>where Δ F is the increase of the objective function and T is the system temperature. </li></ul></ul><ul><ul><li>The temperature is gradually lowered according to an empirically chosen cooling schedule. </li></ul></ul>
  38. 38. Physics – optimization algorithms <ul><li>Iterative algorithms most common </li></ul><ul><li>Simulated annealing and genetic algorithms too computationally intensive </li></ul><ul><li>Filtered back-projection and direct Fourier transformation methods have difficulty with handling negative fluence and non-invariant kernels. </li></ul>
  39. 39. Physics – Optimization in Eclipse <ul><li>All contours are sampled by point clouds </li></ul>
  40. 40. Physics - optimization in Eclipse <ul><li>Dose contribution from beamlet to each cloud point is established </li></ul><ul><li>In the process of optimization, “weight” of each beamlet is changed </li></ul>
  41. 41. Physics - optimization in Eclipse <ul><li>Multi-Resolution Dose Calculation (MRDC) Algorithm used for a fast dose estimation during an optimization phase </li></ul><ul><li>Objective (penalty) function combines all objectives into a single function </li></ul>
  42. 42. Physics - optimization in Eclipse <ul><li>Goal is to minimize a value of objective function </li></ul><ul><ul><li>Total penalty (black curve) always goes down </li></ul></ul><ul><li>Objectives may be changed on the fly </li></ul><ul><ul><li>Creates a discontinuity in the objective function </li></ul></ul>
  43. 43. Physics - optimization in Eclipse <ul><li>Dose volume optimizer (DVO) handles optimization of fluence </li></ul><ul><li>Optimized fluence passed on to the leaf motion calculator (LMC) </li></ul><ul><li>Delivery method chosen: </li></ul><ul><ul><li>Multiple static fields </li></ul></ul><ul><ul><li>Sliding window </li></ul></ul><ul><li>AAA algorithm used for final dose calculation </li></ul><ul><li>LMC determines MUs </li></ul>
  44. 44. Physics - optimization in Pinnacle <ul><li>First generation </li></ul>
  45. 45. Physics - optimization in Pinnacle <ul><li>Direct Machine Parameter Optimization (DMPO) </li></ul>
  46. 46. IMRT - optimization in practice <ul><li>Overlapping structures with competing dose objectives </li></ul><ul><ul><li>Assign overlap region to new structure </li></ul></ul><ul><ul><li>Assign distinct dose objective to each structure </li></ul></ul>
  47. 47. IMRT - optimization in practice <ul><li>PTV ring </li></ul><ul><ul><li>Represents margin around CTV </li></ul></ul><ul><ul><li>95% < PTV ring dose < 100% </li></ul></ul><ul><li>Outer ring </li></ul><ul><ul><li>Represents normal tissue surrounding PTV </li></ul></ul><ul><ul><li>Outer ring dose < 95% </li></ul></ul><ul><ul><li>Improves conformity of distribution </li></ul></ul>
  48. 48. IMRT - optimization in practice <ul><li>Treat aggressively/Spare lightly </li></ul><ul><ul><li>If optimization problem is constructed so that it is physically possible to meet objectives, then a solution will be found. </li></ul></ul><ul><ul><li>PTV coverage will be achieved and solution will not be sensitive to objective weights. </li></ul></ul><ul><ul><li>If it is not physically possible to meet objectives, solution will be very sensitive to choice of objective weights and typically clinically unacceptable </li></ul></ul><ul><li>IMRT plan not optimized but designed </li></ul><ul><ul><li>IMRT planning often a trial-and-error process. </li></ul></ul>
  49. 49. IMRT - optimization in practice <ul><li>Must set minimum segment area </li></ul><ul><ul><li>Dosimetry of small irregularly shaped segments is uncertain, particularly in the presence of heterogeneities. </li></ul></ul><ul><li>Must limit minimum segment MUs </li></ul><ul><ul><li>Some LINACS have limited dose linearity down to small MUs (not Varian due to gridded gun). </li></ul></ul><ul><li>Must limit maximum number of segments </li></ul><ul><ul><li>Plan must deliver efficiently. </li></ul></ul>
  50. 50. IMRT – radiation protection <ul><li>Increased MUs needed for IMRT </li></ul><ul><ul><li>Typical values for 200 cGy fraction, : </li></ul></ul><ul><ul><ul><li>700 MU for step-and-shoot </li></ul></ul></ul><ul><ul><ul><li>1200 MU for sliding window </li></ul></ul></ul><ul><li>Higher beam energy lowers peripheral dose but >10 MV neutron generation important </li></ul><ul><li>Secondary cancer risk is potentially greater </li></ul>
  51. 51. IMRT – MLC technical details <ul><li>Carriage (bank) </li></ul><ul><ul><li>Part of MLC which carries leaves </li></ul></ul><ul><li>Leaf </li></ul><ul><ul><li>Part of MLC used as final beam limiting device </li></ul></ul>
  52. 52. IMRT – MLC technical details <ul><li>MLC leaf design </li></ul>
  53. 53. IMRT – MLC technical details <ul><li>Tongue and groove effect </li></ul>
  54. 54. IMRT – MLC technical details <ul><li>Inter-digitation </li></ul>
  55. 55. IMRT – MLC transmission <ul><li>Amount of radiation transmitted through the leaves fully blocking the beam </li></ul><ul><li>Consists of : </li></ul><ul><ul><li>Inter-leaf transmission </li></ul></ul><ul><ul><li>Intra-leaf transmission </li></ul></ul>
  56. 56. IMRT – MLC dosimetric leaf gap <ul><li>Accounts for extra transmission through the rounded leaf edge </li></ul><ul><ul><li>Modeled as an apparent gap between two closed straight edge leaves </li></ul></ul>
  57. 57. IMRT – MLC minimum dose dynamic leaf gap <ul><li>Minimal tip to tip distance which needs to be maintained for any moving leaf pair in the dMLC mode </li></ul>
  58. 58. IMRT – MLC leaf speed <ul><li>Speed of the leaf at the level of isocenter. </li></ul><ul><ul><li>Maximum limit for Varian MLC is 3cm/ s. </li></ul></ul><ul><ul><li>Model 2.5 cm/s in LMC. </li></ul></ul><ul><ul><li>Allows for adjustment of any leaf during treatment. </li></ul></ul>
  59. 59. IMRT – Dose/Arc dynamic leaf tolerance <ul><li>Maximum allowed difference between the planned and actual leaf positions </li></ul>
  60. 60. IMRT – Jaw over travel <ul><li>The maximum distance the collimator jaw can extend over the central axis. </li></ul>
  61. 61. IMRT – Leaf over travel <ul><li>The maximum distance an MLC leaf can extend over the central axis </li></ul>
  62. 62. IMRT – Leaf Positions <ul><li>Fully retracted leaf is in the basic position within the carriage. The leaf is fully inside the carriage, at the “end” position. </li></ul><ul><li>Extended leaf is extended from the carriage </li></ul>
  63. 63. IMRT – Leaf Span <ul><li>Maximum distance from a tip of the most retracted leaf to the tip of the most extended leaf. </li></ul><ul><li>Limits a maximum field size, which can be delivered without repositioning of a carriage. </li></ul>
  64. 64. IMRT – Multiple carriage delivery <ul><li>Used for large modulated fields </li></ul><ul><ul><li>Carriages cannot move while the beam is on </li></ul></ul><ul><ul><li>Treatment field split into subfields with a width smaller than the leaf span </li></ul></ul>
  65. 65. Electronic compensation <ul><li>Replacement of physical compensator by means of dynamic MLC delivery. Forward planned IMRT. </li></ul><ul><ul><li>Electronic Compensator (planar) </li></ul></ul><ul><ul><li>Irregular Surface Compensator (curved surface) </li></ul></ul><ul><ul><li>Field in Field Compensator (segmented fields) </li></ul></ul>
  66. 66. Complete Irradiated Area Outline (CIAO) <ul><li>For portal verification of intensity modulated field </li></ul><ul><li>Based on threshold fluence </li></ul>
  67. 67. IMRT QA – Ion chamber array <ul><li>Reliable 2%/2mm digital  evaluation </li></ul><ul><li>Real-time analysis </li></ul><ul><li>4 patients in 30 minutes </li></ul>
  68. 68. IMRT QA Ion-chamber array
  69. 69. IMRT QA – Day 0 Pinnacle Fluence Map EPID image
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