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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 2: Dosimetry

  2. Dose in radiotherapy • Is the therapeutic agent • Is high - radiotherapy means putting as much dose into the target as possible • Carries some risk of severe complications • Must be delivered very accurately Part 10, lecture 2: Dosimetry

  3. Required dose accuracy • Depends on steepness of the dose response curve • 5% difference in dose make a 15% difference in tumour control probability in head and neck patients - this is clinically detectable Part 10, lecture 2: Dosimetry

  4. Delivery of dose within +/-5% • Sources of uncertainty: • Absolute dosimetry/calibration • Relative dosimetry (%depth dose, profiles, output factors) • Treatment planning (estimated uncertainty of the order of +/- 2%) • Machine performance on the day (+/- 2%) • Patient set-up and movement (+/- 3%) Not much room for error in dosimetry... Part 10, lecture 2: Dosimetry

  5. Objectives • Understand the principles of beam calibration • Appreciate the objectives of clinical dosimetry • Identify methods for in vivo dose verification on patients undergoing external beam radiotherapy Part 10, lecture 2: Dosimetry

  6. Contents 1. Calibration 2. Clinical dosimetry • Beam data acquisition • Phantom measurements • In vivo dosimetry 3. External audits Part 10, lecture 2: Dosimetry

  7. Absolute and relative dosimetry • Absolute dosimetry is a technique that yields information directly on absorbed dose in Gy. This absolute dosimetric measurement is also referred to as calibration. All further measurements are then referenced to this standard geometry i.e. relative dosimetry is performed. In general no factors are required in relative dosimetry since it is only the comparison of two dosimeter readings, one of them being in reference conditions. Part 10, lecture 2: Dosimetry

  8. 1. Calibration • Determine absolute dose in Gy at one reference point in the beam • Determines the beam on time or the number of monitor units required to deliver a certain dose • Very important - if this is wrong, everything will be wrong • In the BSS framework part of the optimization of medical exposure Part 10, lecture 2: Dosimetry

  9. Optimization of protection in therapeutic exposure • BSS appendix II.18. “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; ... (e) the patient be informed of possible risks.” Part 10, lecture 2: Dosimetry

  10. Important note on optimization 1. The dose only to normal tissues shall be kept as low as reasonable achievable 2. In practice, the dose to the target in radical radiotherapy shall be as high as possible to maximize the chances of tumour control • The two requirements may be seen at times as incompatible - the key lies in the term “reasonable” • What is “reasonable” is a decision which the patient and the clinician must make Part 10, lecture 2: Dosimetry

  11. Important note 1. The dose only to normal tissues shall be kept as low as reasonable achievable 2. The dose to the target in radical radiotherapy shall be as high as possible to maximize the chances of tumour control • In practice usually the second objective takes precedence in radical treatments - if the tumour cannot be controlled, there is no point sparing normal tissues... • One must still protect normal tissues as much as possible... Part 10, lecture 2: Dosimetry

  12. Mis-calibration is an important contributor to accident in EBT • Calibration of beams • Accidents due to mistakes in the determination of the dose rate caused overdosage to as many as 115, 207, 426 patients… by as much as 60% • There were other accidents, related to misinterpretation of a calibration certificate, of a reported pressure value for correction, a change of physicist with poor information transfer; a wrong use of a plane-parallel ionization chamber Part 10, lecture 2: Dosimetry

  13. Accidents due to calibration mistakes • Contributing factors to accidents • Lack of understanding of beam calibration, certificates, conversion factors and dosimetry instruments… lack of training and expertise within radiotherapy physics • Lack of redundant and independent determination of the absorbed dose (mistakes were not detected) • Lack of formal procedures for communication and change of personnel Part 10, lecture 2: Dosimetry

  14. Accidents due to calibration mistakes • Contributing factors to accidents • In one of the cases, for 22 months, there was no verification of the beam; the physicist was devoted to a new accelerator and “ignored” the Co-60 unit (There was a lack of revision of the staff needs when a new accelerator was installed) Part 10, lecture 2: Dosimetry

  15. BSS appendix II.19. • “Registrants and licensees shall ensure that: (a) the calibration of sources used for medical exposure be traceable to a Standards dosimetry laboratory; …” Part 10, lecture 2: Dosimetry

  16. The IAEA/WHO SSDL Network Part 10, lecture 2: Dosimetry

  17. Traceability of calibration Part 10, lecture 2: Dosimetry

  18. Traceability • National Strategy • Frequency established by the Regulatory Authority • If there is no SDL in the country, the national strategy should include institutional arrangements to facilitate quick import/export and additional arrangements among several countries • Redundancy in the calibration of new sources and beams Part 10, lecture 2: Dosimetry

  19. BSS appendix II.19. • “Registrants and licensees shall ensure that: ... (b) radiotherapy equipment be calibrated in terms of radiation quality or energy and either absorbed dose or absorbed dose rate at a predefined distance under specified conditions, e.g. following the recommendations given in IAEA Technical Reports Series No. 277 [20]; …” Part 10, lecture 2: Dosimetry

  20. Calibration • Determination of the dose at a reference point - correlation of treatment time or ‘monitor units’ with absolute dose • Absolute dosimetry required: • Const. must be well known and fundamental: • Calorimetry • Ionometry W/e • Chemical dosimetry g Dose = const * Detector Signal Part 10, lecture 2: Dosimetry

  21. Calibration protocols • Calibration is a complex process requiring an expert in radiation oncology physics • There are many protocols which can provide guidance • international (e.g. IAEA TRS 277 or TRS 398) • national (usually developed by the national medical physics association) – e.g. AAPM TG 21, AAPM TG 51, DIN 68, ... Part 10, lecture 2: Dosimetry

  22. Calibration protocols • It is essential to follow ONE protocol • It is essential to follow the protocol BY THE LETTER - there is no room for error... Part 10, lecture 2: Dosimetry

  23. Forms are available • Very helpful for guidance • Available for most protocols • Here shown for IAEA TRS 398 Part 10, lecture 2: Dosimetry

  24. Calibration protocols • There has been a development from protocols based on calibrations at the national standard lab in air as air KERMA or exposure, to calibration in terms of absorbed dose to water… • This development has been in parallel at the IAEA and many national associations (e.g. AAPM) Part 10, lecture 2: Dosimetry

  25. Move to absorbed dose to water calibration • Follows improved capability of national standard labs • Same in US by moving from AAPM TG21 (1983) to AAPM TG51 (2000) Part 10, lecture 2: Dosimetry

  26. Which protocol to use? • Depends on how the ionization chamber has been calibrated in the standards lab. If one has an air KERMA calibration factor (NK) or an exposure factor (NX), TRS-398 CANNOT be used… • If also the dose to water factor (NDw) can be provided by the laboratory, TRS 398 can be used. Part 10, lecture 2: Dosimetry

  27. Advantages of absorbed dose calibration The exposure/ KERMA way • Easier for the user • Less factors required • Get NDw directly - only conversion for beam quality required Part 10, lecture 2: Dosimetry

  28. A note on calibration • The process is beyond the scope of the present course • Calibration is a very critical process • Calibration (in particular using exposure/KERMA formalism) is complex (>10 factors) • It should always be checked by an independent person Part 10, lecture 2: Dosimetry

  29. A second note: Calibration can link the absolute dose to a variety of different reference conditions It is essential to know what your reference conditions are. (They are typically linked to the treatment planning system in use) Part 10, lecture 2: Dosimetry

  30. BSS appendix II.19. • “Registrants and licensees shall ensure that: ... (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.” The maximum interval in practice for re-calibration is 1 year - less if there is any indication of problems Part 10, lecture 2: Dosimetry

  31. Absolute dosimetry • Can be done in principle using calorimetry, chemical dosimetry or ionization chambers • For radiotherapy practice all protocols are based on ionization chambers Farmer type chamber Part 10, lecture 2: Dosimetry

  32. Tools required for calibration • In practice a Farmer type ionization chamber - air volume 0.6cc for photons and high energy electrons Part 10, lecture 2: Dosimetry

  33. Plane parallel chamber • Required for low energy electrons (<5MeV) and recommended for electrons with energy below 10MeV due to steep dose gradients PTW Markus chamber Part 10, lecture 2: Dosimetry

  34. Plane parallel chamber 2mm Adapted from Kron in VanDyk 1999 Part 10, lecture 2: Dosimetry

  35. Ionization Chamber reading require correction for • Air pressure: require an accurate barometer for calibration purposes • an error of 10 mBar will give an error of 1% in the calibration • Temperature: accurate thermometer • an error of 3 degrees centigrade will give an error of 1% in the calibration Part 10, lecture 2: Dosimetry

  36. Calibration Records • BSS appendix II.32. “Registrants and licensees shall keep and make available, as required, the results of the calibrations and periodic checks of the relevant physical and clinical parameters selected during treatments.” Part 10, lecture 2: Dosimetry

  37. 2. Clinical dosimetry • In the context of BSS, dosimetry has two components: a) dose measurements (dealt with in the present lecture) and b) dose planning discussed more extensively in the fourth lecture of part 10 Part 10, lecture 2: Dosimetry

  38. There are multiple objectives for dose measure-ments in radiotherapy practice Part 10, lecture 2: Dosimetry

  39. Roles of clinical dose measurements in radiotherapy • Data collection for treatment planning in general • Data collection for individual patients • Dose verification Part 10, lecture 2: Dosimetry

  40. 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 2: Dosimetry

  41. In radiotherapy practice: • This means dose measurements are required as • as dose determination for the treatment of individual patients • input for treatment planning systems Part 10, lecture 2: Dosimetry

  42. Dose measurement for individual patients • In vivo dosimetry • Determination of output for electron cut-outs or compensators • Assessment of dose distribution in complex treatments (e.g. IMRT) Part 10, lecture 2: Dosimetry

  43. Dosimetry as part of commissioning of equipment • In the past this has been more the determination of unknown dose rather than verification, however, these days most beam parameters are within tight specifications and known prior to commissioning. • Commissioning affects both: • Treatment units • Treatment planning Part 10, lecture 2: Dosimetry

  44. Aspects: Safety Verification that specs are met Other bits and pieces required for planning Many protocols and guidelines available Usually done using a water phantom and slab phantoms Significant time commitment - however, access is usually not a problem Treatment unit commissioning Part 10, lecture 2: Dosimetry

  45. Tools for commissioning • Mainly scanning water phantom • Determine all properties of all radiation beams • depth dose, TPR • profiles • wedges • blocks,... Part 10, lecture 2: Dosimetry

  46. Phantoms • In radiotherapy the term "phantom" is used to describe a material and structure which models the radiation absorption and scattering properties of human tissues of interest. • There are many different phantoms for a variety of purposes available in radiotherapy dosimetry. Phantoms are an essential part of the dosimetric process. Part 10, lecture 2: Dosimetry

  47. Commissioning of treatment planning • Non-dose related components • Photon dose calculations • Electron dose calculations • Brachytherapy • Data transfer • Special procedures Compare lecture 4 in the present part 10 Part 10, lecture 2: Dosimetry

  48. Typical dosimetric accuracy required (examples) • Square field CAX: 1% • MLC penumbra: 3% • Wedge outer beam: 5% • Buildup-region: 30% • 3D inhomogeneity CAX: 5% From AAPM TG53 Part 10, lecture 2: Dosimetry

  49. Square field CAX: 1% MLC penumbra: 3% Wedge outer beam: 5% Buildup-region: 30% 3D inhomogeneity CAX: 5% The required accuracy depends on situation and purpose Uncertainty has two components: dose uncertainty AND spatial localization uncertainty Typical accuracy required (examples) Part 10, lecture 2: Dosimetry

  50. Requirements for dosimetry • Required accuracy depends on situation and purpose • Uncertainty has two components: dose uncertainty AND spatial localization uncertainty Part 10, lecture 2: Dosimetry