Linear Accelerator

Dosimetry and Quality Assurance

Table of Contents

Overview

AAPM TG-51 is the current standard for clinical dosimetry for high-energy photon and electron fields. It is the direct predecessor of TG-21 with a focus on simplicity and human error reduction. This simplification was accomplish primarily through three important changes. First, the use of plastic phantoms in primary dosimetry was eliminated. This eliminated the need to transfer dose for the phantom to water using stopping powers and mass attenuation ratios. Second,  TG-51 used absorbed-dose-to-water calibration factor (\(N_{D,w}^{60_{Co}}\)), referred to in the addendum as the calibration coefficient, as directly in an accredited dosimetry calibration laboratory (ADCL). This avoided TG-21's need to transfer dose to chamber gas to dose to phantom using Bragg-Gray (BG) Cavity Theory. Third, the quality conversion factor (\(k_Q\)) greatly simplifies the process by rolling ratio of stopping powers as well as chamber corrections (wall, gradient, fluence, and central electrode) into a single quantify that may be looked up as a function of beam quality.

An addendum to TG-51 was published in March 2014 addressing flattening filter free (FFF) beams, updating some values based on new clinical data, an analysis of sources of error in TG-51, and providing guidance in choosing a reference class dosimeter. Changes introduced to the protocol are identified herein along with the original TG-51 values.

Farmer Chambers

Key Point: Farmer chambers are thimble ionization chambers widely used in reference dosimetry. Questions regarding farmer chamber design and operation are common on board exams.

  • Typical Sensitive Volume: 0.6cc (approximately cylindrical, 0.3cm radius, 2cm length)
  • Typical Response: 20nC/Gy
  • Effective Point of Measurement:
    • Photons: 0.6rcav (~0.18cm) upstream of central axis
    • Electrons: 0.5rcav (~0.15cm) upstream of central axis

Photon Dosimetry Procedure

1. Select an ADCL calibrated ion chamber and record ND,wCo-60

Recommendations for ion chamber performance are provided in TG-51 Addendum.

Reference Class Ionization Chamber Specifications

MeasureSpecification
Pleak<0.1% of chamber reading (0.999 < Ppol < 1.001)
Pion = 1 + Cinit + CgenDppCinit < 0.2%
Pion should be linear with pulse
Ppol<0.4% correction (0.996 < Ppol < 1.004)
Chamber Stability<0.3% change over calibration period of 2 years
Chamber Settling<0.5% change in chamber reading per MU from first irradiation to stabilization of the ion chamber

2. Determine beam quality conversion factor (kQ) using %DD(10)x.

Measure percent depth dose of the photon component of the beam at 10cm depth in a 10x10cm2 field (%DD(10)x). This measurement must be made at effective point of measurement with SSD=100cm.

Note that effective point of measurement is 0.6rcav for cylindrical chambers in photon beams.

kQ is found using the below table and is a function of both %DD(10)x and ionization chamber used.

Use of lead foil

      • TG-51 required 1mm lead foil be placed at 50cm (±5cm) or 30cm (±1cm) from the water surface for high energy fields (>10MV). This measurement, referred to as %DD(10)Pb, serves to filter out scatter electrons from the treatment head while introducing a known, and thus compensated for, quantity of electrons generated in the foil. %DD(10)Pb is corrected to %DD(10)x using the below equations.
      • TG-51 addendum removes the requirement to use lead foil for high energy beams and adds the requirement that FFF fields use lead foil. Note that use of lead foil will improve measurement accuracy.

kQ Key Points: The beam quality correction factor (kQ) corrects for more than just energy dependence of the ion chamber! kQ also accounts for:

  • Electron contamination from lead foil
  • Perturbations in electron fluence due to chamber design
  • Effective point of measurement
    • Chamber will later be placed at point of measurement rather than effective point of measurement.
kQ values at 10cm depth in accelerator photon beams. Source: AAPM TG-51 figure 4.

3. Find the correction factors and convert Mraw to M.

Note: All these measurements are taken at 10cm point of measurement in the 10x10cm2 reference field.

Pion is found by comparing readings with a high and low voltage applied to the ion chamber using the below equations. The subscripts L and H indicate low or high voltage, typically 150V and 300V respectively.

Pion Key Points:

Pion corrects for loss of ion collection efficiency due to recombination. Therefor, Pion cannot be lower than 1!

Pion generally increases with increased dose per pulse.

Pion values for farmer chamber:

  • 1.003 for 6X
  • 1.006 for 18X
  • 1.014 for electrons

PPol is found by measuring a with a positive and negative voltage. 

PTP is found using local atmospheric pressure for air communicating chambers and temperature. Here P is given in kPa and T is given in Celsius.

Pelec accounts for measurement error in the electrometer. It is given by the ADCL and will be 1 if the ion chamber and electrometer were calibrated together.

Pleak  accounts for leakage current of the ion chamber. It is typically measured with equipment in place, accelerator on, but no beam on. If the ion chamber meets the TG-51 addendum specifications for a reference-class ion chamber (i.e. Pleak < 0.1%) then Pleak may be taken as 1.000.

Prp is computed by the average of the radiation profile over the dimensions of the active part of the ion chamber then correcting to the reading at the point of measurement. At the time of writing, it remains common to assume Prp=1 for flattened fields.

PPol Key Points:

PPol corrects for differences in collection efficiency with different polarities.

PPol is usually the smallest correction factor that is routinely applied.

TG-51 and the TG-51 addendum vary in their acceptable range and handling of Ppol.

        • TG-51 states that if Ppol correction is <0.997 or >1.003, substitute for .
        • TG-51 addendum states that a reference class ion chamber must have 0.996≤ Ppol ≤ 1.004 at any energy and that the total variation in Ppol across all energies must be less than 0.5%.

4. Dose at 10cm depth is computed.

Measurement is made at 10cm point of measurement in a 10x10cm2 field using either an SSD or SAD setup.

5. %DD is used to compute dose at calibration depth (usually dmax).

When computing dose at calibration depth, one may choose to use either a measured %DD curve or the %DD curve modeled in one's treatment planning system. Using a measured %DD will optimize accuracy at dmax while using the TPS value of %DD(10) will optimize TPS calculation accuracy at depth. While either measurement technique is valid, it is generally accepted that TPS accuracy at depth is most clinically significant.

Electron Dosimetry Procedure

1. Select a calibrated ion chamber and record ND,wCo-60

Cylindrical Chambers may be used for electron fields where R50≥2.6cm (≥6MeV) and should calibrated at an ADCL.

Plane-Parallel Chambers must be used for electron fields  where R50<2.6cm (<6MeV) and are recommended for fields with R50<4.3cm (<10MeV). Plane-Parallel chambers should be cross calibrated in the user's beam with an ADCL calibrated cylindrical ion chamber. Read the plane-parallel chamber cross calibration procedure here (local link).

Key Point: Plane-parallel chambers must be cross calibrated against an ADCL calibrated cylindrical chamber in the users beam. They are not calibrated directly at an ADCL.

2. Determine the reference field size (usually 10x10cm)

For beams with R50≤8.5cm (<20MeV) a 10x10cm2 field may be used.

For R50>8.5cm (>20MeV) a 20x20cm2 or greater field must be used.

Reference Class Ionization Chamber Specifications

MeasureSpecification
Pleak<0.1% of chamber reading (0.999 < Ppol < 1.001)
Pion = 1 + Cinit + CgenDppCinit < 0.2%
Pion should be linear with pulse
Ppol<0.4% correction (0.996 < Ppol < 1.004)
Chamber Stability<0.3% change over calibration period of 2 years
Chamber Settling<0.5% change in chamber reading per MU from first irradiation to stabilization of the ion chamber

3. Determine the electron quality conversion factor (k'R50)

i. Measure an ionization curve using the field size determined in step 2 with SSD = 100cm using effective point of measurement. Use this curve to determine the depth of 50% of maximum ionization (I50).

Note: effective point of measurement for a cylindrical chamber in an electron beam is 0.5rcav and is the upstream face of the chamber for a plane-parallel chamber.

ii. I50 is converted to depth of 50% maximum dose (R50) using the below equation.

iii. k'R50 is found as a function of R50 and chamber model using table 5 from TG-51.

Calculated k'R50 values at dref Image credit: AAPM TG-51 figure 5.

Key Point: Conversion from depth ionization curve to depth dose curve is required for electron measurements but not for photon measurements.

Why?

Photon beams are liberating electrons continually along the beam path. As a result, although the electron fluence may vary with depth, the spectrum of electron energies is roughly constant. Since stopping power varies as a function of energy, this means that stopping power doesn't change with depth for electrons generated by a photon beam. Therefor depth dose and depth ionization curves both have the same shape.

Electron beams in contrast are constantly losing energy. This means that the stopping power for electron beams does vary with depth and the shape of a depth ionization curve is not the same as a depth dose curve. Failure to correct from this effect would result in a 3-5% error in PDD(dref)!

4. Determine the photon-electron conversion factor (kecal)

Kecal is needed to convert the ADCL supplied calibration factor (ND,wCo-60) from a photon reference to an electron-beam absorbed-dose calibration factor.

Kecal is determined from tabulated values presented in tables  III of TG-51.

Tabulated k<sub>ecal</sub> values for plane-parallel chambers. Image credit: AAPM TG-51 table II.
Tabulated kecal values for plane-parallel chambers. Image credit: AAPM TG-51 table II.
Tabulated kecal values for cylindrical chambers. Image credit: AAPM TG-51 table III.

5. Determine reference depth (dref)

dref is chosen as the measurement depth rather than 10cm (as in photon beams) because 10cm depth is in or beyond the region of rapid dose fall off for many clinical beams. dref is also preferred over simply using dmax because of significant scatter electron contribution at dmax in higher energy fields (<12MeV).

Typical %DD(dref) values:

<12E: dref ≅ 100%
16E: dref ≅ 99%
18E: dref ≅ 98%
20E: dref ≅ 95.5%

6. Determine the gradient correction factor (PgrQ) for cylindrical chambers

Measurement of Mraw should be taken at point of measurement.

Key Point: It's not necessary to physically measure Pgr! The same correction can be achieved by using effective point of measurement rather than point of measurement in subsequent measurements.

 

Typical PgrQ  values: 

Plane-parallel chambers: PgrQ = 1.00

Cylindrical chamber:

E < 12 MeV: PgrQ > 1.00
E = 12 MeV: PgrQ ≅ 1.00
E > 12 MeV: PgrQ < 1.00

Why this trend?

Because for low energies dref ≅ dmax placing the effective point of measurement for Mraw(dref) measurement in the buildup region.  At higher energies dref > dmax which means that shallower depths will have greater dose.

7. Find the correction factors and convert Mraw to M.

Note: All these measurements are taken at reference depth (drefpoint of measurement as determined in step 5.

Pion is found by comparing readings with a high and low voltage applied to the ion chamber using the below equations. The subscripts L and H indicate low or high voltage, typically 150V and 300V respectively.

Pion Key Points:

Pion corrects for loss of ion collection efficiency due to recombination. Therefor, Pion cannot be lower than 1!

Pion generally increases with increased dose per pulse.

Pion values for farmer chamber:

  • 1.003 for 6X
  • 1.006 for 18X
  • 1.014 for electrons

PPol is found by measuring a with a positive and negative voltage. 

PTP is found using local atmospheric pressure for air communicating chambers and temperature. Here P is given in kPa and T is given in Celsius.

Pelec accounts for measurement error in the electrometer. It is given by the ADCL and will be 1 if the ion chamber and electrometer were calibrated together.

Pleak  accounts for leakage current of the ion chamber. It is typically measured with equipment in place, accelerator on, but no beam on. If the ion chamber meets the TG-51 addendum specifications for a reference-class ion chamber (i.e. Pleak < 0.1%) then Pleak may be taken as 1.000.

Prp is computed by the average of the radiation profile over the dimensions of the active part of the ion chamber then correcting to the reading at the point of measurement. At the time of writing, it remains common to assume Prp=1 for flattened fields.

PPol Key Points:

PPol corrects for differences in collection efficiency with different polarities.

PPol is usually the smallest correction factor that is routinely applied.

TG-51 and the TG-51 addendum vary in their acceptable range and handling of Ppol.

        • TG-51 states that if Ppol correction is <0.997 or >1.003, substitute for .
        • TG-51 addendum states that a reference class ion chamber must have 0.996≤ Ppol ≤ 1.004 at any energy and that the total variation in Ppol across all energies must be less than 0.5%.

8. Dose at reference depth (dref) is computed.

Note: SSD may be chosen in the range of 90-110cm.

9. %DD is used to compute dose at calibration depth (usually dmax).

When computing dose at calibration depth, one may choose to use either a measured %DD curve or the %DD curve modeled in one's treatment planning system. Using a measured %DD will optimize accuracy at dmax while using the TPS value of %DD(dref) will optimize TPS calculation accuracy at depth. While either measurement technique is valid, it is generally accepted that TPS accuracy at depth is most clinically significant.

Plan-Parallel Ion Chamber Cross Calibration

Because Plane-Parallel chambers are sensitive to very small changes in their construction, it is recommended by TG-21, TG-39, and TG-51 that they be cross calibrated against an ADCL calibrated cylindrical chamber in a high energy electron field. Steps for this process are given below.

1. Determine dose at reference depth for a high energy electron field using the cylindrical chamber (Dw)Cyl.

2. Measure M at the reference depth for the cylindrical chamber.

3. Look up k'R50 for the Plane-Parallel chamber in figure 5 of TG-51 (internal link).

4. Look up kecal for the Plane-Parallel chamber using table II of TG-51 (internal link).

5. Compute (kecal ND,wCo-60)PP and use this factor in place of (kecal ND,wCo-60) for future plane-parallel chamber measurements.

TG-21: Historic Photon and Electron Calibration

Downloads

AAPM TG-21: A protocol for the determination of absorbed dose from high-energy photon and electron beams (External link)

Overview

Released in 1983, TG-21 sought to standardize the calibration of absorbed dose for high energy photon and electron fields. TG-21 is the predecessor to TG-51 and the TG-51 Addendum which have superseded it in clinical practice in the US. Although TG-51 greatly simplified the calibration of clinical photon and electron beams, it does so by hiding much of the physics that TG-21 explicitly states. Thus understanding the TG-21 methodology is critical to understanding current calibration standard of practice for clinical beams.

TG-21 Simplified Procedure

  1. The ion chamber to be used is calibrated by an Accredited Dosimetry Calibration Laboratory (ADCL). The ADCL may return a cavity-gas calibration factor, \(N_{gas}\), for use in the protocol or an exposure calibration factor, \(N_X\), which may be converted to \(N_{gas}\) by the site physicist. \(N_{gas}\) is defined simply as in the below equation where \(D_{gas}\) is the dose to chamber fill gas, \(A_{ion}\) is the ionization collection efficiency during calibration at the ADCL, and \(M\) is the electrometer measurement. \begin{equation} \label{eq:N_gas} N_{gas}= \frac{D_{gas} A_{ion}}{M} \end{equation}
  2. Measurement, \(M\), is made in a phantom made of either water, acrylic, or polystyrene. The product of this measurement and the cavity-gas calibration factor is the dose to gas in the chamber at the point of measurement. \(P_{ion}\) corrects for collection electron collection inefficiency during measurement. \begin{equation} \label{eq:D_gas} D_{gas} = MN_{gas}P_{ion} \end{equation}
  3. Dose to the phantom material at the point of measurement (in absence of ion chamber) is obtained by the product of the dose to gas and the ratio of stopping powers from air to the medium, \((\frac{\overline{L}}{\rho})_{gas}^{med}\). \(P_{repl}\) and \(P_{wall}\) correct for ion chamber influences on the dose distribution. As per Bragg-Gray Cavity Theory, this step requires measurement of beam quality to accurately asses ratio of stopping powers between air and the medium. Stopping power for a material is dependent upon the photon energy. \begin{equation} \label{eq:D_med simple} D_{med} = D_{gas}  (\frac{\overline{L}}{\rho})_{gas}^{med} P_{repl}P_{wall} \end{equation}
  4. Dose to water is obtained as the product of dose to the phantom medium by the ratio of mass energy-absorption coefficients between water and the medium. This measurement is taken in the exponentially decreasing portion of the percent depth dose (%DD) curve for photons and at depth of maximum dose (\(d_{max}\)) for electrons. Excess scatter correction (ESC) and electron fluence correction (\(\phi_{med}^{water}\)) correct for phantom effects on fluence relative to water. More detail on these terms is given below.  \begin{equation} \label{eq: D_Water photon} D_{water} = D_{med} \times ESC \times (\frac{\overline \mu}{\rho})_{med}^{water} \quad \textrm{for photon fields} \end{equation} \begin{equation} \label{eq:D_Water electron} D_{water} = D_{med} \times \phi_{med}^{water} \times (\frac{\overline L}{\rho})_{med}^{water} \quad \textrm{for electron fields} \end{equation}
  5. For photons, the dose at reference depth is scaled using %DD to \(d_{max}\).

Important Points

Phantoms

G-21 allowed reference measurements to be made in Acrylic or Polystyrene phantoms as well as water.

Beam Quality (Nominal Accelerating Potential and Mean Incident Energy)

Determination of stopping powers in BG cavity theory requires knowledge of the beam quality. Beam quality is specified as Nominal Ionizing Potential for photon beam and Mean Incident Energy for electron beams.

Nominal Ionizing Potential is determined by computing the ratio of measurement at a depth of 10cm to a depth of 20cm with an SSD of 100cm in a 10x10\(cm^2\) field of water. This measurement can be made in polystyrene or acrylic phantoms, in which case the depth of overlying material is decreased and the SSD is increased such that the source to detector distance (SDD) remains unchanged.

\begin{equation} \label{eq:ionization ratio} \textrm{Ionization Ratio} = \frac{M_{SDD = 10cm}}{M_{SDD = 120cm }} \end{equation}

Mean Incident Energy is determined by the below equation where \(d_{50}\) is the depth in water in which the ion chamber reading is 50% of its maximum reading, and \(f\) is a scaling factor with a value of 0.965, 1, or 1.11 for polystyrene, water, or acrylic respectively. It is recommended that \(d_50\) be measured in a large enough field size that further increasing the field size does not influence depth of maximum dose.

\begin{equation} \label{eq: mean incident energy} \bar{E}_0 = 2.33 \times d_{50} \times f \end{equation}

TG-21 fig. 3. Nominal ionization potential.
TG-21 fig. 3. Nominal ionization potential.

Depth of Calibration

For photon beams the depth of calibration is dependent upon the depth of maximum dose \(d_{max}\) and the ion chamber inner diameter but is taken in the exponentially decreasing portion of the percent dept dose (%DD) curve. Increasing portions of the %DD curve should not be used because CPE is not established. Depth of maximum dose is not an optimal position because \(P_{repl}\) is not well quantified in this region.

Electron beams should be calibrated at \(d_{max}\).

Gas Cavity Calibration Factor \({N_{gas}}\)

Ngas is a factor unique to each ionization chamber that is related to the volume of the chamber, where \( (\frac{\bar{W}}{e}) = 33.97 \frac{J}{C}\) and \( \rho_{air} = 1.2 \frac{kg}{m^3} \).

\begin{equation} \label{eq: N_gas definition} N_{gas} = \frac{(\frac{\bar{W}}{e})}{\rho_{air}V_{chamber}} = \frac{2.83 \times 10^7}{V_c (cm^3)} GyC^{-1} \end{equation}

The historical primary measurement made at an ADCL was Exposure, \(X\) and so it is useful to understand how \(N_{gas}\) may be obtained from the exposure calibration factor \(N_x\).  

\begin{equation} \label{eq: N_X} N_x = \frac {X}{M} \end{equation}

Ignoring corrections conversions for the chamber itself, \(N_{gas}\) may be computed as:

\begin{equation} \label{eq:N_gas from X} N_{gas} = \frac{D_{gas} N_x A_{ion}}{X} \end{equation}

\(D_{gas}\) and \(X\) may be removed by noting that, under charged particle equilibrium (CPE), they are related by the below equation where \(k\) is a conversion factor equal to \(2.58 \times 10 ^{-4}\frac{C}{kg \ R}\) and \(\frac{W}{e}= 33.97 \frac{J}{C}\) is the mean energy required to ionize air.

\begin{equation} \label{eq: D_gas and X} D_{gas} = X  k  (\frac{W}{e}) \end{equation}

This yields the simplified conversion equation given below. The full conversion equation, which includes factors accounting for fluence perturbation by the ion chamber materials, is given as equation 5 in TG-21.

\begin{equation} \label{eq: N_gas conversion simple} N_{gas} = N_x  k (\frac{W}{e}) A_{ion} \end{equation}

Normalized Ion Chamber Measurement (M)

TG-21 corrects the measurement only to standard temperature and pressure. This corrects for the difference in mass of air between current conditions and the conditions for which \(N_{gas}\) was provided. Note that this is slightly difference from TG-51 which also corrects for chamber collection inefficiency, polarity effects, and electrometer efficiency.

\begin{equation} \label{eq: TP correction} M = M_{raw} \frac{T + 273^\circ C}{295 ^\circ C} \times \frac{760mmHg}{P} \end{equation}

TG-142: Quality Assurance of Medical Accelerators

Downloads

AAPM TG-142: Quality assurance of medical accelerators (External link)

Overview

TG-142 updates TG-40 recommendations for linear accelerator quality assurance recommendations. By setting different tolerances based on the intended use of the accelerator (non-IMRT, IMRT, SRS/SBRT), TG-142 attempts to balance the costs of implementing a QA program against it's costs. TG-142 also adds information on Asymmetric jaws, multileaf collimators (MLCs), and dynamic/virtual wedges. The report also gives specific recommendations regarding setup of a QA program regarding QA team members, procedures, training, and end-to-end system checks.

Machine Check Tables

Daily Checks

Category Procedure Non-IMRT IMRT SRS/SBRT
Dosimetry X-ray output constancy (all energies) 3% 3% 3%
Dosimetry Electron output constancy (weekly test) 3% 3% 3%
Mechanical Laser Localization 2mm 1.5mm 1mm
Mechanical Distance Indicator (ODI) @ iso 2mm 2mm 1mm
Mechanical Collimator size indicator 2mm 2mm 1mm
Safety Door Interlock Functional Functional Functional
Safety Door Closing Safety Functional Functional Functional
Safety Audiovisual monitors Functional Functional Functional
Safety Stereotactic area monitor NA NA Functional
Safety  Radiation area monitor Functional Functional Functional
Safety Beam on indicator Functional Functional Functional

Monthly Checks

Category Procedure Non-IMRT IMRT SRS/SBRT
Dosimetry X-ray output constancy 2% 2% 2%
Dosimetry Electron output constancy 2% 2% 2%
Dosimetry Backup monitor chamber constancy 2% 2% 2%
Dosimetry Typical dose rate constancy NA 2% 2%
Dosimetry Photon beam profile constancy 1% 1% 1%
Dosimetry Electron beam profile constancy 1% 1% 1%
Dosimetry Electron beam energy constancy 2%/2mm 2%/2mm 2%/2mm
Mechanical Light/Radiation field coincidence 2mm/1% an a side 2mm/1% an a side 2mm/1% an a side
Mechanical Light/Radiation field coincidence 1mm/1% an a side 1mm/1% an a side 1mm/1% an a side
Mechanical Distance check device for lasers compared with front pointer 1mm 1mm 1mm
Mechanical Gantry/collimator angels indicator 1.0° 1.0° 1.0°
Mechanical Accessory trays 2mm 2mm 2mm
Mechanical Jaw position indicators (symmetric) 2mm 2mm 2mm
Mechanical Jaw position indicators (asymmetric) 1mm 1mm 1mm
Mechanical Cross-hair centering (walkout) 1mm 1mm 1mm
 Mechanical Treatment couch position indicators 2mm/1.0°  2mm/1.0°  1mm/0.5°
Mechanical Wedge placement accuracy 2mm 2mm 2mm
Mechanical Compensator placement accuracy 1mm 1mm 1mm
Mechanical Latching of wedges/blocking tray Functional Functional Functional
Mechanical Localization lasers 2mm 1mm 1mm
Safety Laser guard interlock Functional Functional Functional

Annual Checks

 Category  Procedure  Non-IMRT IMRT SRS/SBRT
 Dosimetry X-ray & Electron Flatness Change from Baseline 1% 1% 1%
Dosimetry X-ray and Electron symmetry change from baseline 1% 1% 1%
Dosimetry SRS arc rotation mode; MU setting vs delivered NA NA 1.0MU or 2%
Dosimetry SRS arc rotation mode; Gantry arc setting vs delivered NA NA 1.0 degree or 2%
 Dosimetry X-ray/electron output calibration (TG-51) 1% 1% 1%
Dosimetry  Spot check of field size dependent output factors 2% for < 4x4cm2 

1% for ≥ 4x4cm2

2% for < 4x4cm2 

1% for ≥ 4x4cm2

2% for < 4x4cm2 

1% for ≥ 4x4cm2

 Dosimetry Output factors for electron applicators  2% 2% 2%
 Dosimetry X-ray beam quality (PDD10 or TMR2010) 1% 1% 1%
Dosimetry  Electron beam quality (R50) 1mm 1mm 1mm
Dosimetry Physical Wedge Transmission Factor 2% 2% 2%
Dosimetry X-ray MU linearity (output constancy) 2% ≥ 5MU 5% (2-4MU), 

2% ≥ 5MU

5% (2-4MU), 

2% ≥ 5MU

Dosimetry Electron MU linearity (output constancy) 2% ≥ 5MU 2% ≥ 5MU 2% ≥ 5MU
Dosimetry X-ray output constancy vs dose rate 2% 2% 2%
Dosimetry X-ray output constancy vs gantry angle 1% 1% 1%
Dosimetry Electron output constancy vs gantry angle 1% 1% 1%
Dosimetry Electron and X-ray off-axis factor constancy vs gantry angle 1% 1% 1%
Dosimetry Arc Mode (expected MU, degrees) 1% 1% 1%
Dosimetry TBI/TSET mode Functional Functional Functional
Dosimetry PDD or TMR and OAF constancy 1% (TBI) or 1mm PDD shift (TSET) 1% (TBI) or 1mm PDD shift (TSET) 1% (TBI) or 1mm PDD shift (TSET)
Dosimetry TBI/TSET output calibration 2% 2% 2%
Dosimetry TBI/TSET accessories 2% 2% 2%
Mechanical Collimator rotation isocenter 1mm 1mm 1mm
Mechanical Gantry rotation isocenter 1mm 1mm 1mm
Mechanical Couch rotation isocenter 1mm 1mm 1mm
Mechanical Electron applicator interlocks Functional Functional Functional
Mechanical Coincidence of radiation and mechanical isocenter 2mm 2mm 1mm
Mechanical Table top sag 2mm 2mm 2mm
Mechanical  Table angle
Mechanical Table travel maximum range 2mm 2mm 2mm
Mechanical Stereotactic accessories, lockouts, etc NA NA Functional
Safety Follow manufacturer’s test procedures Functional Functional Functional

Knowledge Test

1. Neglecting corrections, which equation correctly converts dose to gas to dose to the medium in the TG-21 formalism?

Question 1 of 3

2. In TG-142, the majority of safety QA tests are performed at what frequency?

Question 2 of 3

3. What does the Prp correction to Mraw correct for in the AAPM TG-51 formalism?

Question 3 of 3


 

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