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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 11 Medical Exposure: Brachytherapy Lecture 2: Dosimetry, planning and verification

  2. Optimization in brachytherapy • First of all: getting the geometry right • Then: • choosing the best isotope and source form • determining the dose to the target - and following this the time for treatment • some scope for optimization in stepping source brachytherapy • supporting the patient to get through the treatment Part 11, lecture 2: Dosimetry, planning and verification

  3. Objectives • To be able to judge the geometry of an implant • To be able to understand the treatment planning process in brachytherapy • To understand dose prescriptions • To develop an understanding for the dose rate achieved in a particular implant • To appreciate the importance of treatment verification in brachytherapy Part 11, lecture 2: Dosimetry, planning and verification

  4. Contents 1. Calibration of sources 2. Dose measurements in brachytherapy 3. Treatment planning 4. Prescribing and reporting 5. Treatment verification Part 11, lecture 2: Dosimetry, planning and verification

  5. 1. Calibration of Brachytherapy Sources • Must be done on receipt and must be documented • Long half-life sources (Cs 137 etc.) • All sources must be calibrated • Short half-life sources • If only a few, do them all • If a large number (e.g. for prostate seed implants), check a sample of at least 10% Part 11, lecture 2: Dosimetry, planning and verification

  6. Different means to specify the ‘source strength’ • mgRa equivalent - old unit specifying the activity which would be equivalent to a radium treatment. This unit should not be used anymore • Activity in Bq or Ci (the SI unit Bq is preferred even if in practice Ci is still widely used). Due to source encapsulation some confusion may arise from the difference between true activity and apparent activity including self absorption and filtration • Air KERMA rate (Gy per hour) at a specified distance, typically 1m from the source. This is the preferable quantity to specify source strength Part 11, lecture 2: Dosimetry, planning and verification

  7. BSS appendix II.19 • Calibration: “Registrants and licensees shall ensure that: (c) sealed sources used for brachytherapy be calibrated in terms of activity, reference air kerma rate in air or absorbed dose rate in a specified medium, at a specified distance, for a specified reference date; ... (e) the calibrations be carried out at the time of commissioning a unit, after any maintenance procedure that may have an effect on the dosimetry and at intervals approved by the Regulatory Authority. “ Part 11, lecture 2: Dosimetry, planning and verification

  8. Calibration in radiotherapy practice • The quantity ‘activity’ is still widely in use - it should be discouraged in favor of air KERMA rate or absorbed dose in a specified medium • In any case, the manufacturers source certificate MUST be checked - this should be a traceable calibration (in particular for sources which will be in use for a long time, e.g. 137-Cs or HDR 192-Ir) • Protocols are available Part 11, lecture 2: Dosimetry, planning and verification

  9. Calibration chain (ICRU 58) Part 11, lecture 2: Dosimetry, planning and verification

  10. Determination of source strength • In a phantom - this may in practice be difficult due to strong dose gradients • HDR sources Hole for source Holes for cylindrical ion chamber 10cm Plugs for empty holes Perspex phantom MDS Nordion Part 11, lecture 2: Dosimetry, planning and verification

  11. Determination of source strength • In phantom - in practice difficult due to strong dose gradients, usually only applicable for HDR sources • In air at reference distance - in practice only possible for high activity sources • In well type chamber/source calibrator = or >10cm source Part 11, lecture 2: Dosimetry, planning and verification

  12. Brachytherapy Source Calibrators Well type ionization chamber • Source calibrators are isotope and source design specific • Calibration must be traceable to standard lab • Good reference: IAEA TECDOC 1079 Part 11, lecture 2: Dosimetry, planning and verification

  13. Tests for Brachytherapy Sources • Suggested calibration tolerances • Ideal for seeds • mean of batch (3%) • Deviation from mean (5%) • Practical • Review manufacturer’s documentation for tolerances • Review ALL the manufacturer’s documentation Courtesy Medtec Part 11, lecture 2: Dosimetry, planning and verification

  14. In radiotherapy practice • Often calibration is difficult due to large number of sources (e.g. 125-I seeds), lack of equipment or lack of appropriate dose standard • A consistency check may need to suffice • Dose from one source compared to the previous one - this is a minimum requirement! PTW 125-I seed check device: check at least 10% of all seeds in a batch Part 11, lecture 2: Dosimetry, planning and verification

  15. Brachytherapy Source Calibrators • Must be commissioned like any dosimetric tool. Test are required for: • Precision • Scale factors and linearity • Ion collection efficiency • Geometry and length dependence • Energy dependence • Dependence on the wall of the source Part 11, lecture 2: Dosimetry, planning and verification

  16. Brachytherapy Source Calibrators • Quality Assurance essential • Use a long half-life encapsulated source with • a reliable mechanical integrity • manufacturer’s calibration certificate • Better to use two different sources to verify energy dependence • Cross checking with another Calibrator (if available - could be in another Hospital) Part 11, lecture 2: Dosimetry, planning and verification

  17. IAEA TECDOC 1113: Calibration Checklist Part 11, lecture 2: Dosimetry, planning and verification

  18. 2. Dose measurements in brachytherapy • 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 11, lecture 2: Dosimetry, planning and verification

  19. Brachytherapy dose measurements • In general, dose measurements in brachytherapy are • difficult • because of strong dose gradients • potential energy dependence of the detector • relatively low instantaneous dose rate • not often performed Part 11, lecture 2: Dosimetry, planning and verification

  20. Interstitial implant dose distribution Dose profiles Part 11, lecture 2: Dosimetry, planning and verification

  21. Other complexities • Source is not a point source • Anisotropy of the source due to shielding and encapsulation 125-I seed Part 11, lecture 2: Dosimetry, planning and verification

  22. Shielding included in applicators • e.g. vaginal applicators with rectal shielding • Good means to optimize the medical exposure - however dosimetry is difficult Part 11, lecture 2: Dosimetry, planning and verification

  23. Dose measurements in brachytherapy • In a phantom require rigid geometry • In a patient require meaningful detector placement • In any case require good localization of both the source and the detector Part 11, lecture 2: Dosimetry, planning and verification

  24. In vivo dosimetry in brachytherapy • Mostly done to verify the dose in critical organs • Common practice in some centers to verify in gynecological implants • rectum and/or • bladder dose applicator Part 11, lecture 2: Dosimetry, planning and verification

  25. In vivo dosimetry • Localization of the detector and the applicator is essential • Shown here is a rectal marker in a gynecological implant Part 11, lecture 2: Dosimetry, planning and verification

  26. In vivo dosimetry • In brachytherapy, the detectors needs to be • small to provide high spatial resolution • sensitive to determine relatively low dose in critical organs • Detectors of choice are TLDs, diodes and MOSFETs with the semiconductors having the advantage of immediate readout Part 11, lecture 2: Dosimetry, planning and verification

  27. Dosimetry • In practice, clinicians rely on treatment planning where the initial calibration of the source is used to calculate dose in reference points or volumes HDR source calibration phantom Part 11, lecture 2: Dosimetry, planning and verification

  28. 3. Brachytherapy treatment planning • Most treatment planning systems also offer a brachytherapy module for standard isotopes • Complex delivery techniques (stepped source HDR, endovascular brachytherapy) come with their own planning system Part 11, lecture 2: Dosimetry, planning and verification

  29. Patient flow in brachytherapy Treatment decision Ideal plan - determines source number and location Implant of sources or applicators in theatre Localization of sources or applicators (typically using X Rays) Treatment plan Commence treatment Part 11, lecture 2: Dosimetry, planning and verification

  30. Ideal plan - prior to implant • Pre-implant plan • Required for LDR implants • Determines number of sources required (e.g. if seeds or wire are to be ordered) • Determines the implant approach (number of needles, templates, other equipment required) Part 11, lecture 2: Dosimetry, planning and verification

  31. After the implant • Need to determine the location of the needles, catheters or applicators which determine the geometry of the implant • This is often done using two orthogonal X Rays (e.g. lateral and AP ) Part 11, lecture 2: Dosimetry, planning and verification

  32. Orthogonal views for reconstruction of a 3D implant Part 11, lecture 2: Dosimetry, planning and verification

  33. After the implant • In permanent seed implants this is typically done a couple of weeks after the implant to let swelling of the implanted area resolve • If a CT scanner is available it is a good tool to localize the seeds in 3D Part 11, lecture 2: Dosimetry, planning and verification

  34. Planning computer • Enter information of the implant geometry via digitizer • The system reconstructs the 3D geometry from the orthogonal views Part 11, lecture 2: Dosimetry, planning and verification

  35. Geometry of the implant • Can also be reconstructed from a CT scan after the implant • Allows delineation also of target and critical structures for better dosimetry • Shown here is a gynaecological applicator Part 11, lecture 2: Dosimetry, planning and verification

  36. Dose calculation • Not too difficult as the dose depends mainly on the inverse square law D/4 D Double the distance - the dose is down to 1/4 Part 11, lecture 2: Dosimetry, planning and verification

  37. Please note: • ISL only applicable to point sources • For line sources there is a linear dose fall off if the distance to the source is small compared to the length of the source 1/distance Part 11, lecture 2: Dosimetry, planning and verification

  38. Dose calculation • In many planning systems the ‘Meisberger’ algorithm is used - this is a modification of the ISL allowing for attenuation • More recently (1995) AAPM task group 43 has recommended a dose calculation system for 192-Ir, 125-I and 103-Pd Part 11, lecture 2: Dosimetry, planning and verification

  39. Dose calculation • Usually no contours of organs or the patient surface are considered • Usually no inhomogeneity corrections are applied • Usually no correction is made for • different applicator materials • shielding incorporated in some applicators Part 11, lecture 2: Dosimetry, planning and verification

  40. Output of treatment planning • Isodose plots • Point doses Part 11, lecture 2: Dosimetry, planning and verification

  41. High Dose Rate (HDR) Brachytherapy • Small high activity source (about 10Ci) - these days nearly exclusively 192-Ir • Source moves through implanted catheters and/or needles step by step • The dwell times determine the dose distribution Part 11, lecture 2: Dosimetry, planning and verification

  42. Patient flow in brachytherapy Treatment decision Ideal plan - determines source number and location Implant of sources or applicators in theatre Localization of sources or applicators (typically using X Rays) Optimization of dwell times Treatment plan Commence treatment Part 11, lecture 2: Dosimetry, planning and verification

  43. Comments on HDR planning • Best ‘optimization’ is a good implant • However, stepped source brachytherapy allows a certain degree of optimization by giving freedom to choose dwell times of the source in any location within the implant Part 11, lecture 2: Dosimetry, planning and verification

  44. Optimization of dose distribution • Can be achieved by altering the dwell times in all positions Part 11, lecture 2: Dosimetry, planning and verification

  45. Example with isodose lines Part 11, lecture 2: Dosimetry, planning and verification

  46. Another example Part 11, lecture 2: Dosimetry, planning and verification

  47. Dose distributions from four different HDR source movements as determined using film Nucletron Part 11, lecture 2: Dosimetry, planning and verification

  48. Comments on HDR planning • Optimization procedure needs extensive checking • How are dwell times transferred to the treatment unit? • Where is the source strength correction applied? • How is transfer time modeled? Part 11, lecture 2: Dosimetry, planning and verification

  49. Issues with the optimization • Can get rather complex when multiple catheters or needles are used • Is a three dimensional process • Requires in practice computerized treatment planning Part 11, lecture 2: Dosimetry, planning and verification

  50. Computerized treatment planning • Allows the calculation of isodose lines which can be laid over the original X Rays • Point doses can be calculated easily • Did not replace the reporting according to systems (as yet) Part 11, lecture 2: Dosimetry, planning and verification