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Radiation Protection in Radiotherapy

Radiation Protection in Radiotherapy. IAEA Training Material on Radiation Protection in Radiotherapy. Part 2 Radiation Physics Lecture 2: Dosimetry and Equipment. Rationale.

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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 2 Radiation Physics Lecture 2: Dosimetry and Equipment

  2. Rationale • Radiation dose delivered to the target and surrounding tissues is one of the major predictors of radiotherapy treatment outcome (compare part 3 of the course). It is generally assumed that the dose must be accurately delivered within +/-5% of the prescribed dose to ensure the treatment aims are met. Part 2, lecture 2: Dosimetry and equipment

  3. Objectives • To understand the relevance of radiation dose and dosimetry for radiotherapy • To be able to explain the difference between absolute and relative dosimetry • To be able to discuss the features of the most common dosimeters in radiotherapy: ionization chambers, semiconductors, thermoluminescence dosimeters (TLD) and film Part 2, lecture 2: Dosimetry and equipment

  4. Contents of lecture 2 1. Absolute and relative dosimetry 2. The dosimetric environment: phantoms 3. Dosimetric techniques • physical background • practical realization Part 2, lecture 2: Dosimetry and equipment

  5. 1. 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 compared to this known dose under reference conditions. This means … • relative dosimetry is performed. In general no conversion coefficients or correction factors are required in relative dosimetry since it is only the comparison of two dosimeter readings, one of them being in reference conditions. Part 2, lecture 2: Dosimetry and equipment

  6. Absolute dosimetry • Required for every radiation quality once • Determination of absorbed dose (in Gy) at one reference point in a phantom • Well defined geometry (example for a linear accelerator: measurements in water, at 100cm FSD, 10x10cm2 field size, depth 10cm • Follows protocols (compare part 10) Part 2, lecture 2: Dosimetry and equipment

  7. Absolute dosimetry • Required for every radiation quality once • Determination of absolute dose (in Gy) at one reference point in a phantom • Well defined geometry: Eg. water phantom, 100cm FSD, 10x10cm2 field size, depth 10cm • Follows protocols (compare part 10) Of tremendous importance: If the absolute dosimetry is incorrect EVERYTHING will be wrong Part 2, lecture 2: Dosimetry and equipment

  8. Quick Question A dose of 1Gy delivers a huge quantity of energy to the patient - is it true or false?

  9. Answer FALSE – 1Gy = 1J/kg. Delivering this amount of energy would raise the temperature of tissue by less than 0.001oC. Even for a 100kg person it is much less than the energy consumed with a bowl of muesli – please note the amount of energy in food is often listed on the package. Part 2, lecture 2: Dosimetry and equipment

  10. Relative dosimetry • Relates dose under non-reference conditions to the dose under reference conditions • Typically at least two measurements are required: • one in conditions where the dose shall be determined • one in conditions where the dose is known Part 2, lecture 2: Dosimetry and equipment

  11. Examples for relative dosimetry • Characterization of a radiation beam • percentage depth dose, tissue maximum ratios or similar • profiles • Determination of factors affecting output • field size factors, applicator factors • filter factors, wedge factors • patient specific factors (e.g. electron cut-out) Part 2, lecture 2: Dosimetry and equipment

  12. Percentage depth dose measurement • Variation of dose in a medium (typically water) with depth • Includes attenuation and inverse square law components Part 2, lecture 2: Dosimetry and equipment

  13. Percentage depth dose Relates dose at different depths in water (or the patient) to the dose at the depth of dose maximum - note that the y axis is relative!!! Part 2, lecture 2: Dosimetry and equipment

  14. TAR, TMR, TPR • Relative dosimetry for isocentric treatment set-up (compare part 5) • All can be converted into percentage depth dose • TAR = ratio of dose in phantom with x cm overlaying tissue to dose at the same point in air • TMR = ratio of dose with x cm overlaying tissue to dose at dose maximum (detector position fixed) • TPR as TMR but as a ratio to dose at a reference point (e.g. 10cm overlaying tissue) Part 2, lecture 2: Dosimetry and equipment

  15. TMR, TPR • Mimics isocentric conditions • TMR is a special case of TPR where the reference phantom depth is depth of maximum dose Part 2, lecture 2: Dosimetry and equipment

  16. PDD and TMR Strong ISL dependence • Percentage depth dose (PDD) changes with distance of the patient to the source due to variations in the inverse square law (ISL), TAR, TMR and TPR do not. Weak ISL dependence Part 2, lecture 2: Dosimetry and equipment

  17. Output factors • Compare dose with dose under reference conditions • different field sizes • wedge factor • tray factor • applicator factor • electron cutout factor Part 2, lecture 2: Dosimetry and equipment

  18. Example: wedge factor Dose under reference conditions Could also involve different field sizes and/or different depths of the detector in the phantom Part 2, lecture 2: Dosimetry and equipment

  19. Quick Question Is a Half Value Layer measurement for the determination of X Ray quality absolute or relative dosimetry?

  20. Answer • Relative dosimetry: • we relate the dose with different aluminium or copper filters in the beam to the dose without the filters to determine which filter thickness attenuates the beam to half its original intensity • the result is independent of the actual dose given - we could measure for 10s or 20s or 60s each time, as long as we ensure the irradiation is identical for all measurements Part 2, lecture 2: Dosimetry and equipment

  21. 2. The dosimetric environment • Phantoms • A phantom represents the radiation properties of the patient and allows the introduction of a radiation detector into this environment, a task that would be difficult in a real patient. • A very important example is the scanning water phantom. • Alternatively, the phantom can be made of slabs of tissue mimicking material or even shaped as a human body (anthropomorphic). Part 2, lecture 2: Dosimetry and equipment

  22. Scanning water phantom Part 2, lecture 2: Dosimetry and equipment

  23. Slab phantoms Part 2, lecture 2: Dosimetry and equipment

  24. Tissue equivalent materials • Many specifically manufactured materials such as solid water (previous slide), white water, plastic water, … • Polystyrene (good for megavoltage beams, not ideal for low energy photons) • Perspex (other names: PMMA, Plexiglas) - tissue equivalent composition, but with higher physical density - correction is necessary. Part 2, lecture 2: Dosimetry and equipment

  25. Anthropomorphic phantom Whole body phantom: ART Part 2, lecture 2: Dosimetry and equipment

  26. Allows placement of radiation detectors in the phantom (shown here are TLDs) Includes inhomogeneities Part 2, lecture 2: Dosimetry and equipment

  27. RANDO phantom torso CT slice through lung Head with TLD holes Part 2, lecture 2: Dosimetry and equipment

  28. Pediatric phantom Part 2, lecture 2: Dosimetry and equipment

  29. Some remarks on phantoms • It is essential that they are tested prior to use • physical measurements - weight, dimensions • radiation measurements - CT scan, attenuation checks • Cheaper alternatives can also be used • wax for shaping of humanoid phantoms • cork as lung equivalent • As long as their properties and limitations are known - they are useful Part 2, lecture 2: Dosimetry and equipment

  30. 3. Radiation effects and dosimetry Part 2, lecture 2: Dosimetry and equipment

  31. Principles of radiation detection • Ionization chamber • Geiger Mueller Counter • Thermoluminescence dosimetry • Film • Semiconductors Part 2, lecture 2: Dosimetry and equipment

  32. Detection of Ionization in Air Ion chamber Adapted from Collins 2001 Part 2, lecture 2: Dosimetry and equipment

  33. Detection of Ionization in Air Adapted from Metcalfe 1998 Part 2, lecture 2: Dosimetry and equipment

  34. Ionization Chamber 200-400V Measures exposure which can be converted to dose not very sensitive Geiger Counter >700V Every ionization event is counted Counter of events not a dosimeter very sensitive Ionometric measurements Part 2, lecture 2: Dosimetry and equipment

  35. Ionization Chambers 600cc chamber Thimble chambers Part 2, lecture 2: Dosimetry and equipment

  36. Cross section through a Farmer type chamber (from Metcalfe 1996) Part 2, lecture 2: Dosimetry and equipment

  37. Ionization Chambers • Farmer 0.6 cc chamber and electrometer • Most important chamber in radiotherapy dosimetry Part 2, lecture 2: Dosimetry and equipment

  38. Electrometer From the chamber Part 2, lecture 2: Dosimetry and equipment

  39. Ionization chambers • Relatively large volume for small signal (1Gy produces approximately 36nC in 1cc of air) • To improve spatial resolution at least in one dimension parallel plate type chambers are used. Part 2, lecture 2: Dosimetry and equipment

  40. Parallel plate chambers From Metcalfe et al 1996 Part 2, lecture 2: Dosimetry and equipment

  41. Parallel Plate Ionization Chambers • Used for • low energy X Rays (< 60 KV) • Electrons of any energy but rated as the preferred method for energies < 10 MeV and essential for energies < 5 MeV • Many types available in different materials and sizes • Often sold in combination with a suitable slab phantom Part 2, lecture 2: Dosimetry and equipment

  42. Markus chamber small designed for electrons Holt chamber robust embedded in polystyrene slab Parallel Plate Ionization Chambers - examples Part 2, lecture 2: Dosimetry and equipment

  43. Well type ionization chamber • For calibration of brachytherapy sources Brachytherapy source Part 2, lecture 2: Dosimetry and equipment

  44. Ionization chamber type survey meters • not as sensitive as G-M devices but not affected by pulsed beams such as occur with accelerators • because of the above, this is the preferred device around high energy radiotherapy accelerators Part 2, lecture 2: Dosimetry and equipment

  45. Geiger-Mueller Counter • Not a dosimeter - just a counter of radiation events • Very sensitive • Light weight and convenient to use • Suitable for miniaturization Part 2, lecture 2: Dosimetry and equipment

  46. Geiger-Mueller (G-M) Devices • Useful for • area monitoring • room monitoring • personnel monitoring • Care required in regions of high dose rate or pulsed beams as reading may be inaccurate Part 2, lecture 2: Dosimetry and equipment

  47. Thermoluminescence dosimetry (TLD) • Small crystals • Many different materials • Passive dosimeter - no cables required • Wide dosimetric range (Gy to 100s of Gy) • Many different applications Part 2, lecture 2: Dosimetry and equipment

  48. Various TLD types Part 2, lecture 2: Dosimetry and equipment

  49. Simplified scheme of the TLD process Part 2, lecture 2: Dosimetry and equipment

  50. TLD glow curves Part 2, lecture 2: Dosimetry and equipment

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