Radiotherapy Treatment Planning

1. Radiotherapy Treatment Planning

2. Treatment planning is the task to make sure a prescription is put into practice in an optimized way Prescription Planning Treatment Anwar

3. 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 Anwar

4. Contents of the lecture A. Radiotherapy treatment planning concepts B. Computerized treatment planning Anwar

5. 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 Anwar

6. 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 Anwar

7. 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 Anwar

8. 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 tumor as well as normal structures - the irradiation of the latter can cause intolerable complications. Again, both volume and dose are important. Anwar

9. One needs to know • Target location • Target volume and shape • Secondary targets - potential tumor spread • Location of critical structures • Volume and shape of critical structures • Radiobiology of structures Anwar

10. Target delineation ICRU 50 & 62 • Gross Tumor Volume (GTV) = clinically demonstrated tumor • Clinical Target Volume = GTV + area at risk (eg. potentially involved lymph nodes) Anwar

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

12. Tumor: High dose to all Homogenous dose Critical organ Low dose to most of the structure The ideal DVH 100% 100% dose dose Anwar

13. 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? Anwar

14. In practice not always that clear cut • ICRU report 62 • Need to understand anatomy and physiology • A clinical decision Anwar

15. In many organs, dose and volume effects are linked - eg. 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. Anwar

16. In EBT practice • Need to know • where to direct beam to, and • how large the beam must be and how it should be shaped Anwar

17. 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 Anwar

18. 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 Anwar

19. Minimum patient data required for external beam planning • Target location • Patient outline Anwar

20. Diagnostic tools which could be used for patient data acquisition • 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! Anwar

21. Note on the role of simulation • Simulator is often used twice in the radiotherapy process • Patient data acquisition - target localization, contours, outlines • Verification - can the plan be put into practice? Acquisition of reference images for verification. • Simulator may be replaced by other diagnostic equipment or virtual simulation Anwar

22. Virtual simulation • All aspects of simulator work are performed on a 3D data set of the patient • This requires high quality 3D CT data of the patient in treatment position • Verification can be performed using digitally reconstructed radiographs (DRRs) Anwar

23. Virtual Simulation 3D Model of the patient and the Treatment Devices Anwar

24. Simulator Rotating gantry Diagnostic X-ray tube Radiation beam defining system Simulator couch Image intensifier and X-ray film holder Nucletron/Oldelft Anwar

25. Radiotherapy simulator • Obtain images and mark beam entry points on the patient Anwar

26. Isocenter Position CT Simulation (Thanks to ADAC) Marking the Patient already during CT Moveable Lasers CT images Isocenter Projection Anwar

27. Patient marking Marks on shell • Create relation between patient coordinates and beam coordinates Tattoos Skin markers Anwar

31. Beam placement and shaping DRR with conformal shielding simulator film with block Anwar

32. Choice of radiation quality Entry point Number of beams Field size Blocks Wedges Compensators Tools for optimization of the radiotherapy approach Anwar

33. Optimization approaches Choice of best beam angle beam beam target patient target patient wedge target Use of a beam modifier patient Anwar

34. Beam number and weighting Beam 1 beam 50% 100% 50% target patient Beam 2 patient 40% 30% 10% 20% Anwar

35. 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% Anwar

36. Use of wedges • Wedged pair • Three field techniques Isodose lines patient patient Typical isodose lines Anwar

37. Entry point Field size Blocks Wedges Compensators a two-dimensional approach? Beam placement and shaping Anwar

38. 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 Anwar

39. 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 Anwar

40. 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 Anwar

41. Simulator Film • Shows relevant anatomy • Indicates field placement and size • Indicates shielding • Can be used as reference image for treatment verification Field defining wires Anwar

42. 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 Anwar

43. 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... Anwar

44. Quick Question: Who is responsible for the preparation of beam data for the planning process?

45. Acquisition of machine data • …from vendor or publications (eg BJR 17 and 25) - this requires verification!!! • Done by physicist • Some dosimetric equipment must be available (water phantom, ion chambers, film, phantoms,…) • Documentation essential Anwar

46. 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 (eg. mu checks) - should be a completely independent set of data Anwar

47. 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 Anwar

48. Machine data summary • Need to include all beams and options (internal consistencyالقوام، اللزوجة, conventions, collisionتصادم protection, physical limitations) • Data can be made available for planning in installments as required • Some data may be required for individual patients only (eg. special treatments) • Only make available data which is verified Anwar

49. Quick Question:What data is available for physical wedges in your center? This should include at least: Wedge angle - and how it is defined Wedge output modification factor - and to which depth and field sizes it applies The field sizes for which the wedge can be used Beam hardening? Maybe a new beam must be defined by TMRs or percentage depth dose Profiles in both directions (wedged and un-wedged - the latter is affected by divergence related profile changes) Weight (eg for OHS restrictions on lifting)

50. 1 3 2 60 Gy 4 From single to multiple beams • Mainly an issue for megavoltage photons where we have significant contribution of dose to the target from many beams Beam weighting must be factored in !!! Anwar