Dosimetry and calibration of Outline photon and electron beams • General considerations with cavity ion chambers • Calibration of ion chambers Chapter 13 – For photon beams F.A. Attix, Introduction to Radiological Physics and – For electron beams Radiation Dosimetry • Reference dosimetry of photon beams Almond et al., AAPM’s TG-51 protocol for clinical reference • Reference dosimetry of electron beams dosimetry of high-energy photon and electron beams, Med. Phys. 26, pp.1847-1870, 1999 McEwen at al., Addendum to the AAPM’s TG-51 protocol, Med. Phys. 41, pp. 041501-1-20, 2014 Introduction Ion chamber calibration • The success of radiation therapy depends on • Ion chamber can serve as an absolute dosimeter if its the accuracy of a prescribed dose delivery gas mass is known • This necessitates high accuracy in the • Most of the commercially manufactured ion chambers dosimetry of high-energy photon and are not constructed with exactly known sensitive electron beams volume, therefore they require calibration • Two aspects are involved: • National laboratories maintain standard ionization – proper calibration of the measuring instruments chambers and calibrated g-ray beams (ionization chamber and electrometer) • Regional calibration laboratories (ADCL - Accredited – characterization of clinical beams Dosimetry Calibration Laboratories in US) provide calibration services for general-use instruments for a fee Ion chamber calibration Ion chamber calibration • Starting from an ion chamber calibrated free-in-air • Three approaches to ion chamber calibration: for one quantity (exposure or air kerma) and – Exposure Nx transferring this information to obtain another – Dose in cavity gas N – old TG-21 protocol quantity, absorbed dose to water, based on a gas measurement in a phantom introduces complexity – Absorbed dose in water ND– new TG-51 protocol and possible errors • Beam dosimetry can be done • To overcome these complexities, primary – In free space standards laboratories have developed standards for absorbed dose to water in photon beams from – In water phantom (need correction for field 60Co and accelerator beams and these have an perturbation due to chamber insertion) uncertainty of 1% or less 1 TG-51 protocol General formalism • Prescribes a methodology for clinical reference • Given NQ (in Gy/C or Gy/rdg), the absorbed- dosimetry D,w dose to water calibration factor for an ion chamber • Applies to photon beams with nominal energies located in a beam of quality Q between 60Co and 50 MV, and electron beams • Under reference conditions: with nominal energies between 4 and 50 MeV Q Q • Uses ion chamber calibrated in terms of absorbed D w MN D ,w (Gy) 60 dose to water in a Co beam Q where D w is the absorbed dose to water (in Gy) at • Sets up certain well-defined reference conditions the point of measurement of the ion chamber when • Starting point: an ion chamber with calibration it is absent and M is the fully corrected factor directly traceable to national standards of electrometer reading in coulombs (C) or meter absorbed dose (may be done through ADCL) units (rdg) General formalism: kQ General formalism: kQ • Usually absorbed-dose calibration factors will be • Recall Spenser-Attix cavity theory, stating: 60 obtained for reference conditions in a Co beam med L M W • Define the quality conversion factor, kQ, such that D D P , D med air correction s air 60 m e Q Co air air air N D ,w k Q N D ,w (Gy/C or Gy/rdg) • The quality conversion factor k is chamber specific w P – accumulates Q L corrections P various chamber- correction s • Using kQ, gives Q related corrections 60 N air Q Co D ,w Q (wall, dose gradient, D Mk N (Gy) • Thus k Q 60 change in electron w Q D ,w Co w N D ,w L fluence inside the P correction s chamber, etc.) air 60 Co General formalism: kQ General formalism: kQ • For electron beams the quality conversion factor kQ contains two components: • For photon beams, the protocol provides Q k Q Pgr k R values of kQ for most cylindrical ion 50 Q chambers used in reference dosimetry • P gr is necessary only for cylindrical chambers – corrects for the ionization gradient at the measurement point (extended list in Addendum to TG-51) – depends on the radius of the chamber cavity and • Plane-parallel chambers are not included – must be measured by the user, the protocol provides a procedure for measuring PQ in the user’s electron beam because there is insufficient information gr • kR50 is a chamber-specific factor, a function of electron about wall correction factors in photon beam quality as specified by R50 (depth in water where beams other than 60Co beams dose falls off to 50% of maximum dose) 2 General formalism: kQ General formalism • In an electron beam, the dose is given by • The factor kR50 is written as the product of: Q Q 60 Co k R k R k ecal D MP k k N (Gy) 50 50 w gr R50 ecal D ,w • kecal is the photon-electron conversion factor (fixed • The reference depth for electron-beam dosimetry is for a given chamber model), it is the value needed to at d = 0.6R – 0.1 cm, which is essentially at the 60 Q ref 50 Co N ecal convert N Dw , into Dw , , the absorbed-dose depth of dose maximum for beams with energies <10 MeV but is deeper for higher-energy beams calibration factor in an electron beam of quality Qecal • At this depth the protocol can make use of stopping- • k´R50, is the electron beam quality conversion factor, beam quality dependent, and converts into N Q power ratios, accounting for the realistic (not mono- D ,w energetic) energy distributions of electron beams General formalism General formalism • Cylindrical chambers are preferred dosimeters • To use this formalism one starts by obtaining • The protocol allows and provides data to carry through the above approach using plane-parallel an absorbed-dose to water calibration factor chambers, although there is evidence that minor for an ion chamber in a 60Co beam construction details significantly affect the response • The next step is to determine the quality of these detectors in 60Co beams, making the conversion factor, k , for the chamber being measurements or calculations of kecal more uncertain Q • Plane-parallel chambers should be cross calibrated used in high-energy electron beams against calibrated – This step requires characterization of the beam cylindrical chambers quality Q Obtaining an absorbed-dose to Obtaining an absorbed-dose to water calibration factor water calibration factor • The absorbed-dose calibration factor is defined as 60 Co • The ion chamber should be checked for any 60 Co D w N Dw, (Gy/C or Gy/rdg) problems before it is sent for calibration M 60 Co • The ion chamber and the electrometer with where D w is the absorbed dose to water (in Gy) in the calibration laboratory’s 60Co beam at the point of measurement which it is to be used should both be of the ion chamber in the absence of the chamber calibrated, possibly as a single unit • It applies under standard environmental conditions of 22 °C, 101.33 kPa, and relative humidity between 20% and 80%, • All ranges of the electrometer that are respectively (in the US and Canada) routinely used for clinical reference • It must be traceable to the user’s national primary standard for absorbed dose to water dosimetry should be calibrated 3 Chamber waterproofing Measurement phantoms • A chamber is calibrated and used clinically in water • Clinical reference measurements must be performed • Equivalent waterproofing techniques must be used in a water phantom with dimensions of at least 30 for measurements in the user’s beam and in the 30 30 cm3 (non-water phantoms are prohibited) calibration laboratory • If the beam enters through the • If a chamber is not inherently waterproof (preferred plastic wall of the water phantom method) it requires extra waterproofing sleeves and the wall is >0.2 cm thick, all depths should be scaled to water- • A waterproofing sleeve should minimize air gaps near the chamber wall (0.2 mm) and should be equivalent depths by measuring made of PMMA 1 mm thick from the outside face of the wall with the phantom full of water and accounting for the wall density Charge measurement Shutter timing error – Co-60 only • The fully corrected charge reading from an ion • Any shutter timing error must be accounted for if needed chamber, M, is given by • If a beam shutter is used with a timer that closes the shutter M P P P P M (C or rdg) when a preset time has elapsed, the t ion TP elec pol raw measured by the timer may not agree exactly with the t´ representing the where Mraw is the raw ion chamber reading in coulombs, C, or the instrument’s reading units (rdg) shutter-open period • This can be detected by making two – PTP is the temperature–pressure correction; measurements X1 and X2 for different – Pion corrects for incomplete ion collection efficiency timer settings, t1 and t2 and – Ppol corrects for polarity effects calculating beam shutter timing error: – P takes into account the electrometer’s calibration factor elec X 2 t1 X 1t 2 if the electrometer and ion chamber are calibrated separately X 2 X 1 Polarity corrections Electrometer correction factor • Polarity effects vary with beam quality and other conditions such as cable position • If the electrometer is calibrated separately from the • It is necessary to correct for these effects each time clinical ion chamber, the electrometer correction factor, Pelec, reference dosimetry is performed is just the electrometer calibration factor, correcting + - • 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