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Dosimetry Measurement for Beam Commissioning

Dosimetry Measurement for Beam Commissioning. Outline. The radiotherapy dosimetry chain The radiation quantities and detectors The cavity theory for dose conversion The dosimetry protocols Dose measurement without CPE present Dose measurement for IMRT verification. The Dosimetry Chain.

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Dosimetry Measurement for Beam Commissioning

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  1. Dosimetry Measurement for Beam Commissioning

  2. Outline • The radiotherapy dosimetry chain • The radiation quantities and detectors • The cavity theory for dose conversion • The dosimetry protocols • Dose measurement without CPE present • Dose measurement for IMRT verification

  3. The Dosimetry Chain • Standards Labs - calibrate dosimeters Primary/secondary dosimetry standards • Reference dosimetry - calibrate treatment units national and international protocols on reference dosimetry • Relative dosimetry - obtain data for RT planning national and international protocols, textbooks • Daily, monthly, annual QA - keep data accuracy national and international protocols, regulations • Plan/patient dose QA - validate real patient dose national and international protocols, regulations

  4. Radiation Quantities • Exposure • Kerma • Collision Kerma • Absorbed Dose

  5. Kerma – kinetic energy transferred by indirect ionizing radiation per unit mass

  6. Energy Transferred, Etr e T hu1 hu2

  7. Collision Kerma– kinetic energy transferred from indirect ionizing radiation to charged particles per unit mass

  8. Net Energy Transferred, Etrn e T' T hu4 hu1 hu3 hu2

  9. Absorbed Dose -the expectation value of the energy imparted by ionizing radiation to matter per unit mass at a point To what materials???

  10. Energy Imparted, E e T' T hu4 hu1 hu3 hu2

  11. Exposure, X where dQ is the absolute value of the total charge of the ions of one sign produced in (dry) air when all the electrons liberated by photons (indirect ionizing radiation) in air of mass dm are completely stopped in air

  12. (W/e)air is the mean energy expended in air per ion pair formed. It gives the number of joules of energy deposited in the air per coulomb of charge released

  13. Exposure and Air Kerma

  14. Measurement of Exposure Free Air Chamber

  15. Correction factors Raw reading Cavity Chamber The basis of an air-wall chamber is that the air surrounding the active volume can be "condensed" into a "solid air" wall The definition of an air-wall chamber is a chamber whose walls interact with radiation in the same manner as air interacts For a typical ion chamber, the real charge :

  16. Ionization Chamber Dosimetry Farmer chamber: the thimble wall is made of graphite and the central electrode is made of aluminum. The collecting volume of the chamber is nominally 0.6 cm3. Energy response of a Farmer chamber

  17. EDR2 Detectors for Radiotherapy Dosimetry • Air-filled ion chambers are recommended for absolute dose measurements (and Fricke dosimeters, TLDs) • Diode, TLDs, film, and other solid/liquid detectors for relative measurements

  18. Dose to medium Conversion factor Dose to detector Cavity Theory – converts dose from one medium to another medium Large cavity theory(for "photon detectors"): If the detector size is much greater than the mean electron range in a phantom irradiated by a photon beam (and CPE exists), then

  19. Dose to medium Conversion factor Dose to detector Cavity Theory (cont.) Small (Bragg-Gray) cavity theory (for "electron detectors"): if the detector size is much smaller than the mean electron range in a phantom irradiated by a photon/electron beam (no need for CPE), then

  20. Dose to medium Conversion factor Dose to detector Cavity Theory (Cont.) Burlin cavity theory:for intermediate sized detectors

  21. Bragg-Gary Cavity Theory W F Dw g F Dg W F Dw 1st condition: cavity does not perturb electron fluence 2nd condition: dose deposited by electrons crossing it

  22. Primary only Bragg-Gary Cavity Theory Unrestricted stopping power for primary electrons CPE exists for knock-on electrons

  23. Spencer-Attix Cavity Theory Spencer-Attix theory explicitly takes into account all knock-on electrons above some energy threshold (traditionally called D) Primary & secondary Track-end effect Restricted stopping power

  24. Spencer-Attix vs. Bragg-Gray(S-A is more accurate than B-G)

  25. Charged Particle Equilibrium Charged Particle Equilibrium (CPE) exists for a volume v if each charged particle of a given type and energy leaving v is replaced by an identical particle entering. (Rin)c = (Rout) c i.e., energy carried in and out by charged particles is equal

  26. Break Down of CPE(CPE does not exit in many situations) For high-energy photon beams: the attenuation of the photon beam is significant for a full electron buildup, it is impossible for CPE to occur. For example, a 10 MeV photon beam is attenuated 7% in the maximum range of its secondary electrons.

  27. Kc D Transient Charged Particle Equilibrium (TCPE)(D is proportional to Kc)

  28. Kilovoltage x-ray dosimetry- a review • ICRU Report 23 (1973) significant changes made 40-150 kV in-air method, >150 kV in-phantom • NCRP Report 69 (1981) only protocol for N. Ame. 10 kV and above, in-air method, no BSF given • IAEA Report 277 (1987) significant changes made 10-100 kV in-air method, >100 kV in-phantom

  29. Kilovoltage x-ray dosimetry- a review • IPEMB Code of Practice (1996) with three ranges Very low- (< 1mmAl) in-phantom, low- (1-8mmAl) in-air, medium-energy (>0.5mmCu) in-phantom • NCS Code of Practice (1997) two energy ranges 50 - 100 kV in-air method, 100 - 300 kV in-phantom • IAEA Report 398 (2000) - new recommendations Absorbed dose based, consistent with other beams

  30. Kilovoltage X-Ray Beam Calibration (The AAPM TG-61 Protocol, C Ma et al) • Use of both in-air and in-phantom methods for tube potentials 100 - 300 kV • More complete data (for water, tissue & bone) • Recommendations for relative measurements • Recommendations for QA and consistency check

  31. Formalism for kV x-ray dosimetry • The backscatter method Dose to detector Conversion factor

  32. Formalism for kV x-ray dosimetry • The in-phantom method Dose to detector Conversion factor

  33. Megavoltage Photon & Electron Calibration (The AAPM TG-51 Protocol) • TG-51 applies to clinical reference dosimetry for external beam radiation therapy using ion chambers. • Beam quality range: 60Co - 50 MV for photons 4 - 50 MeV for electrons • A water phantom (at least 30cm x 30cm x 30cm) for clinical reference dosimetry, other phantom materials for routine checks and relative dosimetry measurements.

  34. Megavoltage Photon & Electron Calibration (The AAPM TG-51 Protocol) • Simplification compared to TG-21 (less tabulated data). • TG-21: • TG-51: • for photons: • for electrons:

  35. Beam Specification Photon beam specification: %dd(10)x %dd(10) : measured PDD at 10 cm depth in water for a 10cm x 10cm field at 100cm SSD %dd(10)x : the photon component of the PDD at 10 cm depth in water for a 10cm x 10cm field at 100cm SSD %dd(10)pb : the PDD at 10 cm depth in water for a 10cm x 10cm field at 100cm SSD with a 1 mm lead foil at about 50 cm from the phantom surface (or 30cm if 50cm clearance is not available)

  36. SSD SAD 10 x 10 • Reference Conditions • Photon beam measurements: The reference depth: dref = 10 cm depth in water for a 10cm x 10cm field at 100cm SSD or SAD.

  37. dmax dref R50 • Reference Conditions • Electron beam measurements: The reference depth: dref = 0.6 R50 – 0.1 cm depth in water The field size is 10x10 for E  20 MeV or 20x20 for E > 20 MeV SSD = 90-110cm are allowed

  38. Equipment: • Ion chamber and electrometer (calibration traceable to national standards laboratories). • Equipment for two independent checks. • Voltage supply (two voltages, both signs) • Waterproofing for ion chamber (if needed): < 1 mm PMMA • Water phantom: at least 30cm x 30cm x 30cm • Lead foil for photons 10 MV and above: 1 mm + 20% • System to measure temperature and pressure

  39. Step-By-Step Photon Calibration Procedure • Obtain a traceable for the ion chamber. • Measure %dd(10)pb with a lead foil. • Deduce %dd(10)x from %dd(10)pb for an open beam. • Measure Mraw at 10 cm water equivalent depth with a 10cm x 10cm field defined at 100 SSD or SAD. • M = Pion PTP PelecPpol Mraw. • Look up kQ (Table I or Figure 4 in TG-51 report) for the chamber. • Finally, (Gy) • Derive dose at other depths using PDD, TPR or TMR.

  40. Other Dosimetry Protocols • TG-25: clinical dosimetry protocols for electron beams (absolute and relative dosimetry). • TG-40: radiotherapy QA (linacs, TPS, special procedures). • TG-53: commissioning and QA for treatment planning systems • TG-65: inhomogeneity corrections for RT dose determination

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