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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 4: Treatment verification and reporting

  2. The Problems: • The correct dose of radiation shall be delivered just to the target. • The dose to surrounding structures shall be as low as possible. • Must be achieved on many occasions (typically >30 treatment fractions) • It must be verifiable • Must be documented in a way that allows others to understand all important factors of the treatment performed Part 10, lecture 4: Treatment verification

  3. Objectives • To understand the different frames of reference used in radiotherapy • To be familiar with techniques which allow to verify that the treatment is delivered to the appropriate location • To appreciate the need for reporting dose AND volume in prescription and treatment reporting in radiotherapy • To be aware of the reports of the ICRU regarding reporting of radiotherapy treatments Part 10, lecture 4: Treatment verification

  4. Contents of lecture 4 in part 10 1. Sources of uncertainty 2. Methods to verify dose delivery • portal films • in vivo dosimetry 3. Prescription and reporting Part 10, lecture 4: Treatment verification

  5. 1. Sources of uncertainty • Patient localization • Organ motion • Imaging (resolution, distortions,…) • Definition of anatomy (outlines,…) • Beam geometry • Dose calculation • Dose display and plan evaluation • Plan implementation Part 10, lecture 4: Treatment verification

  6. Patient localization • The patient should be positioned identically during diagnostics (CT), in simulation and 30+ times during treatment • Sources of uncertainty: • motion • reliability of marks on the skin • couch sag Part 10, lecture 4: Treatment verification

  7. Organ motion • Affects most organs - particularly noticeable for lung cancer, liver, prostate and other pelvic malignancies • Shown here is the difference in CT scan between inhale and exhale position Part 10, lecture 4: Treatment verification

  8. Imaging issues • Partial volume effects • Distortion (MR) • Limited spatial resolution (PET) • More discussion on these issues in the companion course on diagnostic radiology Part 10, lecture 4: Treatment verification

  9. Target definition, outlining of organs • Decide where the organ is and what extend it has. • Are the seminal vesicles really where they are drawn here? Part 10, lecture 4: Treatment verification

  10. Beam geometry and positioning • Is the block exactly where it should be? • Is there gantry sag? Part 10, lecture 4: Treatment verification

  11. Dose calculation • There are many different dose calculation algorithms • All have limitations (and be it the long time required to calculate the dose) • Must know what to trust Part 10, lecture 4: Treatment verification

  12. Plan display and evaluation • Comparison of competing plans... Part 10, lecture 4: Treatment verification

  13. Implementation of the plan • Transfer of data between units Part 10, lecture 4: Treatment verification

  14. 2. Verification of dose delivery • The plan looks great... • However, one must ensure that during treatment everything is matching the treatment plan ADAC Part 10, lecture 4: Treatment verification

  15. Diagnostic tools Patient Treatment planning Treatment unit In practice there are many systems... Part 10, lecture 4: Treatment verification

  16. Diagnostic tools Patient Treatment planning Treatment unit Ensure different co-ordinate systems match... Part 10, lecture 4: Treatment verification

  17. Good practice • Verify data transfer • Use critical approach • Verify treatment by comparing an image taking during treatment with a reference image taken either during simulation or taken from the treatment planning system Part 10, lecture 4: Treatment verification

  18. Green Journal 1992: > 50 occasions of data transfer from one point to another for each patient! Part 10, lecture 4: Treatment verification

  19. Most important comparison • Reference from planning • Simulator film • DRR • Check film during treatment • Port film • EPID Part 10, lecture 4: Treatment verification

  20. Portal films • Taken during (or directly before) treatment with: • beam from the treatment unit • patient in treatment position • shielding in place Part 10, lecture 4: Treatment verification

  21. Port films • Usually taken before or after treatment • If the field itself does not show enough anatomy, a double exposure technique can be used: Expose treatment field Open collimators and expose the same film again Part 10, lecture 4: Treatment verification

  22. In radiotherapy practice • The risk from the additional exposure in dual exposure technique portal images is negligible compared to the risk not treating the correct area in the patient... Part 10, lecture 4: Treatment verification

  23. Electronic Portal Images • A film less way to verify field location • Mounted on the linac • Different systems: • ion chamber • fluoroscopic screen • semiconductor arrays Part 10, lecture 4: Treatment verification

  24. Offered by all major manufacturers Has several advantages: Easy use and positioning of the system Allows on line verification Multiple images (‘cine’) can be taken during one treatment Images available in digital format Electronic Portal Imaging Devices - EPIDs Part 10, lecture 4: Treatment verification

  25. Verification films/images • Two different aims: • Verify correct shielding • Verify patient location Part 10, lecture 4: Treatment verification

  26. Verify block shape and position • should be done for every treatment field at least once per course of radiotherapy Part 10, lecture 4: Treatment verification

  27. Verify patient location • Should be done using two images which give adequate information on the location of the target in respect to the treatment beam geometry. • These do not necessarily need to be treatment fields (e.g. in case of IMRT or arcs) • This verification should be repeated every week during treatment for radical treatments Part 10, lecture 4: Treatment verification

  28. Verify dose delivered to the patient • May be done in customized anthropomorphic phantoms • More likely using in vivo dosimetry Breasts modeled on a particular patient to verify skin dose using TLDs Part 10, lecture 4: Treatment verification

  29. IN VIVO DOSIMETRY ICRU report 24 (1976): “An ultimate check of the actual treatment given can only be made by using in vivo dosimetry.”

  30. Why do in vivo dosimetry • Quality Assurance – Treatment Verification • Measure because we don’t know • Limitations of dose planning • Patient movement • Verify dose for the record • Critical organs • Legal aspects • Clinical trials Part 10, lecture 4: Treatment verification

  31. Challenges for in vivo dosimetry • Typically low dose where the detectors can be located • Variation of detector response with • Temperature • Radiation Quality • Direction of radiation • Need to not interfere with therapeutic objective Part 10, lecture 4: Treatment verification

  32. Potential detectors • Needed: High sensitivity, tissue equivalence, high spatial resolution • TLD • Semiconductors (diodes, MOSFETs) • Radiochromic film • Others (alanine, gel dosimetry, …) Part 10, lecture 4: Treatment verification

  33. In vivo exit dosimetry • calculate exit fluence • determine what the portfilm/EPID should look like • verify dose by projecting back Part 10, lecture 4: Treatment verification

  34. 3. Prescription and reporting • Prescription may be at the discretion of individual clinicians, depending on equipment available, experience and training • Reporting must be uniform - any adequately educated person must be able to understand what happened to the patient in case of: • need for a different clinician to continue treatment • re-treatment of the patient • clinical trials • potential litigation Part 10, lecture 4: Treatment verification

  35. Recommendations by the ICRU • International Commission on Radiation Units and Measurements • ICRU reports provide guidance on prescribing, recording and reporting Part 10, lecture 4: Treatment verification

  36. The required dose information • Should describe the treatment concisely and unambiguously • Gets more and more complex as treatment approaches become more complex Part 10, lecture 4: Treatment verification

  37. The required dose information • 1950: Patient X: x Gy in n fractions to y tumour • 1993: ICRU 50, appendix I 1999: ICRU 62 …plus the same for OAR... Part 10, lecture 4: Treatment verification

  38. Important • Information includes dose AND volume • Information is provided for target AND normal structure • Information includes dose AND fractionation Part 10, lecture 4: Treatment verification

  39. Dose Volume Histograms • Dose and Volume information • Example from ICRU report 62: prostate cancer Part 10, lecture 4: Treatment verification

  40. Normal tissue information rectum bladder femoral heads Part 10, lecture 4: Treatment verification

  41. Normal tissue information rectum bladder femoral heads Quick Question: Why is there a ‘kink’ in the DVH for the femoral heads at about half of the dose to the prostate? Part 10, lecture 4: Treatment verification

  42. 1 3 2 60 Gy 4 Answer • In a ‘four field box’ treatment two of the four fields go through the femoral heads... Part 10, lecture 4: Treatment verification

  43. Record keeping required... • BSS.appendix II.31 “… in radiation therapy, a description of the planning target volume, the dose to the centre of the planning target volume and the maximum and minimum doses delivered to the planning target volume, the doses to other relevant organs, the dose fractionation, and the overall treatment time; …” Part 10, lecture 4: Treatment verification

  44. A final point • The best localization is void if it is the wrong patient!!! • Always check patient identification prior to treatment • In practice this is a surprisingly common problem - there are many patients called ‘Mr Smith’ and patients may be confused when presenting for treatment. A photo in the treatment sheet may also be useful Part 10, lecture 4: Treatment verification

  45. Summary • Optimization of radiotherapy includes the optimization of radiation beam design • Treatment verification using portal imaging is essential • In vivo dosimetry is a useful complementation of portal imaging • A large amount of information - including dose AND volume - is required to describe a radiotherapy treatment concisely • Radiotherapy treatments should be reported following international conventions Part 10, lecture 4: Treatment verification

  46. Where to Get More Information • International Commission on Radiation Units and Measurements. ICRU report 50: Prescribing, recording, and reporting photon beam therapy. Bethesda. 1993. • International Commission on Radiation Units and Measurements. ICRU report 62: Prescribing, recording, and reporting photon beam therapy (Supplement to ICRU report 50). Bethesda 2000. Part 10, lecture 4: Treatment verification

  47. Any questions?

  48. Question: Please discuss the advantages (and disadvantages) of electronic portal imaging as compared to portal films for verification of patient positioning during radiotherapy.

  49. Advantages No additional dose required for most images Easy positioning of the system Allows on line verification Multiple images (‘cine’) can be taken during one treatment Images available in digital format Disadvantages High initial investment costs Dual exposures may be more difficult Image quality of many systems inferior to film The answer should include: Part 10, lecture 4: Treatment verification