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Radiation Protection in Radiotherapy

Radiation Protection in Radiotherapy

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Radiation Protection in Radiotherapy

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  1. Radiation Protection inRadiotherapy IAEA Training Material on Radiation Protection in Radiotherapy Part 10 Good Practice including Radiation Protection in EBT Lecture 3: Radiotherapy Treatment Planning

  2. In BSS Treatment Planning is part of Clinical Dosimetry • BSS appendix II.20. “Registrants and licensees shall ensure that the following items be determined and documented: ... (b) for each patient treated with external beam radiotherapy equipment, the maximum and minimum absorbed doses to the planning target volume together with the absorbed dose to a relevant point such as the centre of the planning target volume, plus the dose to other relevant points selected by the medical practitioner prescribing the treatment; …” Part 10, lecture 3: Radiotherapy treatment planning

  3. …and BSS appendix II.21 • In radiotherapeutic treatments, registrants and licensees shall ensure, within the ranges achievable by good clinical practice and optimized functioning of equipment, that: (a) the prescribed absorbed dose at the prescribed beam quality be delivered to the planning target volume; and (b) doses to other tissues and organs be minimized. Part 10, lecture 3: Radiotherapy treatment planning

  4. Treatment planning is the task to make sure a prescription is put into practice in an optimized way Prescription Planning Treatment Part 10, lecture 3: Radiotherapy treatment planning

  5. Objectives • Understand the general principles of radiotherapy treatment planning • Appreciate different dose calculation algorithms • Understand the need for testing the treatment plan against a set of measurements • Be able to apply the concepts of optimization of medical exposure throughout the treatment planning process • Appreciate the need for quality assurance in radiotherapy treatment planning Part 10, lecture 3: Radiotherapy treatment planning

  6. Contents of the lecture A. Radiotherapy treatment planning concepts B. Computerized treatment planning C. Treatment Planning commissioning and QA Part 10, lecture 3: Radiotherapy treatment planning

  7. The need to understand treatment planning • IAEA Safety Report Series 17 “Lessons learned from accidental exposures in radiotherapy “ (Vienna 2000): • About 1/3 of problems directly related to treatment planning! • May affect individual patient or cohort of patients Part 10, lecture 3: Radiotherapy treatment planning

  8. A. Basic Radiotherapy Treatment Planning Concepts i. Planning process overview ii. Patient data required for planning iii. Machine data required for planning iv. Basic dose calculation Part 10, lecture 3: Radiotherapy treatment planning

  9. i. Planning process overview • Combine machine parameters and individual patient data to customize and optimize treatment • Requires machine data, input of patient data, calculation algorithm • Produces output of data in a form which can be used for treatment (the ‘treatment plan’) Patient information Treatment unit data Planning Treatment plan Part 10, lecture 3: Radiotherapy treatment planning

  10. ii. Patient information required • Radiotherapy is a localized treatment of cancer - one needs to know not only the dose but also the accurate volume where it has been delivered to. • This applies to tumour as well as normal structures - the irradiation of the latter can cause intolerable complications. Again, both volume and dose are important. Part 10, lecture 3: Radiotherapy treatment planning

  11. One needs to know • Target location • Target volume and shape • Secondary targets - potential tumour spread • Location of critical structures • Volume and shape of critical structures • Radiobiology of structures Part 10, lecture 3: Radiotherapy treatment planning

  12. It all comes down to the correct dose to the correct volume Dose Volume Histograms are a way to summarize this information

  13. Dose Volume Histograms Comparison of three different treatment techniques (red, blue and green) in terms of dose to the target and a critical structure Critical organ Target dose Part 10, lecture 3: Radiotherapy treatment planning

  14. Tumour: High dose to all Homogenous dose Critical organ Low dose to most of the structure The ideal DVH volume volume 100% 100% dose dose Part 10, lecture 3: Radiotherapy treatment planning

  15. Need to keep in mind • Always a 3D problem • Different organs may respond differently to different dose patterns. • Question: Is a bit of dose to all the organ better than a high dose to a small part of the organ? Part 10, lecture 3: Radiotherapy treatment planning

  16. Serial organs - e.g. spinal cord Parallel organ - e.g. lung Organ types High dose region High dose region Parallel organ What difference in response would you expect? Serial organ Part 10, lecture 3: Radiotherapy treatment planning

  17. In practice not always that clear cut • ICRU report 62 • Need to understand anatomy and physiology • A clinical decision Part 10, lecture 3: Radiotherapy treatment planning

  18. In many organs, dose and volume effects are linked -e.g. Boersma*et al., classified the following (Dose,Volume) regions to be regions of high risk for developing rectal bleeding: *Int. J. Radiat. Oncol. Biol. Phys., 1998; 41:84-92. Part 10, lecture 3: Radiotherapy treatment planning

  19. In EBT practice • Need to know • where to direct beam to, and • how large the beam must be and how it should be shaped Part 10, lecture 3: Radiotherapy treatment planning

  20. Target design and reference images • In radiotherapy practice the target is localized using diagnostic tools: • Diagnostic procedures - palpation, X Ray, ultrasound • Diagnostic procedures - MRI, PET, SPECT • Diagnostic procedures - CT scan, simulator radiograph Part 10, lecture 3: Radiotherapy treatment planning

  21. BSS appendix II.18. • Therapeutic exposure: “Registrants and licensees shall ensure that: (a) exposure of normal tissue during radiotherapy be kept as low as reasonably achievable consistent with delivering the required dose to the planning target volume, and organ shielding be used when feasible and appropriate” ... Part 10, lecture 3: Radiotherapy treatment planning

  22. Optimization of protection • One part of the optimization of radiotherapy • Strategies: • Employ shielding where possible • Use best available radiation quality • Ensure that plan is actually followed in practice = verification Part 10, lecture 3: Radiotherapy treatment planning

  23. Selection of treatment approach • Requires training and experience • May differ from patient to patient • Requires good diagnostic tools • Requires accurate spatial information • May require information obtained from different modalities Part 10, lecture 3: Radiotherapy treatment planning

  24. Minimum patient data required for external beam planning • Target location • Patient outline Part 10, lecture 3: Radiotherapy treatment planning

  25. Diagnostic tools which could be used for patient data acquisition • Ruler, calipers, many homemade jigs… • CT scanner, MRI, PET scanner, US,… • Simulator including laser system, optical distance indicator (ODI) • Many functions of the simulator are also available on treatment units as an alternative - simulator needs the same QA! (compare part 15) Part 10, lecture 3: Radiotherapy treatment planning

  26. Simulator Rotating gantry Diagnostic X Ray tube Radiation beam defining system Simulator couch Image intensifier and X Ray film holder Nucletron/Oldelft Part 10, lecture 3: Radiotherapy treatment planning

  27. Radiotherapy simulator • Obtain images and mark beam entry points on the patient Part 10, lecture 3: Radiotherapy treatment planning

  28. Patient marking Marks on shell • Create relation between patient coordinates and beam coordinates Tattoos Skin markers Part 10, lecture 3: Radiotherapy treatment planning

  29. Beam placement and shaping DRR with conformal shielding simulator film with block Part 10, lecture 3: Radiotherapy treatment planning

  30. Choice of radiation quality Entry point Number of beams Field size Blocks Wedges Compensators Tools for optimization of the radiotherapy approach Part 10, lecture 3: Radiotherapy treatment planning

  31. Optimization approaches Choice of best beam angle beam beam target patient target patient wedge target Use of a beam modifier patient Part 10, lecture 3: Radiotherapy treatment planning

  32. Beam number and weighting Beam 1 beam 50% 100% 50% target patient Beam 2 patient 40% 30% 10% 20% Part 10, lecture 3: Radiotherapy treatment planning

  33. A note on weighting of beams Different approaches are possible: 1. Weighting of beams as to how much they contribute to the dose at the target 2. Weighting of beams as to how much dose is incident on the patient These are NOT the same 25% 40% 25% 25% 30% 10% 20% 25% Part 10, lecture 3: Radiotherapy treatment planning

  34. Use of wedges Isodose lines • Wedged pair • Three field techniques patient patient Typical isodose lines Part 10, lecture 3: Radiotherapy treatment planning

  35. Entry point Field size Blocks Wedges Compensators a two-dimensional approach? Beam placement and shaping Part 10, lecture 3: Radiotherapy treatment planning

  36. Entry point Field size Blocks Wedges Compensators Multiple beams Dynamic delivery Non-coplanar Dose compensation (IMRT) not just missing tissue Biological planning Beam placement and shaping This is actually a 3D approach Part 10, lecture 3: Radiotherapy treatment planning

  37. Target Localization • Diagnostic procedures - palpation, X Ray, ultrasound • Diagnostic procedures - MRI, PET, SPECT • Diagnostic procedures - CT scan, simulator radiograph Allows the creation of Reference Images for Treatment Verification: Simulator Film, Digitally Reconstructed Radiograph Part 10, lecture 3: Radiotherapy treatment planning

  38. Simulator image • During ‘verification session’ the treatment is set-up on the simulator exactly like it would be on the treatment unit. • A verification film is taken in ‘treatment’ geometry Part 10, lecture 3: Radiotherapy treatment planning

  39. Simulator Film • Shows relevant anatomy • Indicates field placement and size • Indicates shielding • Can be used as reference image for treatment verification Field defining wires Part 10, lecture 3: Radiotherapy treatment planning

  40. iii. Machine data requirements for treatment planning • Beam description (quality, energy) • Beam geometry (isocentre, gantry, table) • Field definition (source collimator distance, applicators, collimators, blocks, MLC) • Physical beam modifiers (wedges, compensator) • Dynamic beam modifiers (dynamic wedge, arcs, MLC IMRT) • Normalization of dose Part 10, lecture 3: Radiotherapy treatment planning

  41. Machine data required for planning • Depends on • complexity of treatment approaches • resources available for data acquisition • May be from published data or can be acquired • MUST be verified... Part 10, lecture 3: Radiotherapy treatment planning

  42. Quick Question: Who is responsible for the preparation of beam data for the planning process in your center?

  43. Acquisition of machine data • …from vendor or publications (e.g. BJR 17 and 25) - this requires verification!!! • Done by physicist • Some dosimetric equipment must be available (water phantom, ion chambers, film, phantoms,…) • Documentation essential Part 10, lecture 3: Radiotherapy treatment planning

  44. Machine data availability • Hardcopy (isodose charts, output factor tables, wedge factors,…) - for emergencies and computer break downs • Treatment planning computer (as above or beam model) - as standard planning data • Independent checking device (e.g. MU checks) - should be a completely independent set of data Part 10, lecture 3: Radiotherapy treatment planning

  45. Machine data availability • Hardcopy (isodose charts, output factor tables, wedge factors,…) • Treatment planning computer (as above or beam model) • Independent checking device (eg. mu checks) The data must be dated, verified in regular intervals and the source (including the person responsible for it) must be documented Part 10, lecture 3: Radiotherapy treatment planning

  46. Machine data summary • Need to include all beams and options (internal consistency, conventions, collision protection, physical limitations) • Data can be made available for planning in installments as required • Some data may be required for individual patients only (e.g. special treatments) • Only make available data which is verified Part 10, lecture 3: Radiotherapy treatment planning

  47. Quick Question: What data is available for physical wedges in your center?

  48. iv. Basic dose calculation • Once one has the target volume, the beam orientation and shape one has to calculate how long a beam must be on (60-Co or kV X Ray units) or how many monitor units must be given (linear accelerator) to deliver the desired dose at the target. Part 10, lecture 3: Radiotherapy treatment planning

  49. Normalization • Specifies what absolute dose should be given to a relative dose value in a treatment plan - e.g. deliver 2Gy per fraction to the 90% isodose • Often the reason for misunderstanding • Should follow recommendation of international bodies (compare e.g. ICRU reports 39, 50, 58 and 62) Part 10, lecture 3: Radiotherapy treatment planning

  50. Components of dose calculation for a single beam • Calibration method - what is the reference condition? • Dose variation with depth and field size - covered in percentage depth dose or TPR/TMR data • Off axis ratio - if the normalization point is not on central axis Part 10, lecture 3: Radiotherapy treatment planning