Radiation Therapy Measurement Standards

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Questions and Answers

What is the maximum allowable beam flatness percentage according to standard linac specifications?

3% (correct)

10%

Which component contributes to the total penumbra but arises from photon scatter in the patient?

Effective penumbra

Transmission penumbra

Scatter penumbra (correct)

Geometric penumbra

What is defined as the separation between the 50% dose level points on the beam profile?

Central axis depth

Beam profile

Geometric penumbra

Field size (correct)

Beam symmetry is ideally measured at what point to achieve maximum sensitivity?

Depth of maximum dose Signup and view all the answers

Which statement best describes the characteristics of phantoms used for measuring radiation beams?

Homogeneous, unit density and flat surface Signup and view all the answers

How is beam flatness (F) mathematically defined?

F = (Dmax - Dmin) / (Dmax + Dmin) × 100 Signup and view all the answers

What is typically used to display planar and volumetric dose distributions?

Isodose curves and surfaces Signup and view all the answers

What is the total percentage of dose on the central axis referred to as umbra?

Less than 1% Signup and view all the answers

In SSD set-ups, at which point are the isodose values normalized to 100%?

At point P on the central beam axis Signup and view all the answers

What type of measurements are most commonly used for relative dose measurements?

Solid state detectors such as diodes Signup and view all the answers

Which parameters affect the single beam isodose distribution?

Beam quality and source size Signup and view all the answers

For SAD set-ups, what do the isodose values correspond to when normalized?

The isocentre Signup and view all the answers

What correction factors might need to be applied when using ionization chambers for dose measurement?

Chamber air temperature, pressure, and polarity Signup and view all the answers

Which method is often employed to generate isodose charts aside from direct measurements?

Calculations with treatment planning systems Signup and view all the answers

Which of the following is NOT a parameter that influences the isodose distribution?

Patient age Signup and view all the answers

What is the purpose of using different sizes and geometrical shapes of ionization chambers?

To cater to specific tasks in dose determination Signup and view all the answers

What does the Tissue-Air Ratio (TAR) measure?

The ratio of dose at a given point in the phantom to the dose in free space at that point Signup and view all the answers

Which of the following factors does NOT affect the Tissue-Air Ratio (TAR)?

Patient's anatomical structure Signup and view all the answers

In the context of scatter, what does the Scatter Air Ratio (SAR) represent?

The scatter component of TAR Signup and view all the answers

What is the prescribed dose for the Anterior field?

77 cGy Signup and view all the answers

How is the prescribed dose for an individual beam determined?

By dividing the tumor dose by the beam weight Signup and view all the answers

What is the Back Scatter Factor (BSF) also known as?

Peak Scatter Factor (PSF) Signup and view all the answers

What is the weight assigned to both the Right posterior field and the Left posterior field?

0.8 Signup and view all the answers

What is the relationship between PDD and TAR in terms of calculation?

PDD can be derived from TAR and field size using a specific formula Signup and view all the answers

Which setup remains constant regardless of the specific beam used?

Source-axis distance (SAD) Signup and view all the answers

Given a wedge angle of 60°, what is the weight for the fields mentioned?

0.8 Signup and view all the answers

For a 6MV beam at a depth of 10 cm with a prescribed dose of 180 cGy, what is required for calculating the monitor units?

Percentage depth dose, Sc, Sp, TMR, and OAF Signup and view all the answers

Which of the following correctly describes the concept of self-reading in the context of radiotherapy?

It refers to the Clarkson Method for irregular fields Signup and view all the answers

What does the calibration dose signify in this context?

Dose calibrated for a specific field size and SSD Signup and view all the answers

What is the impact of field size on the Peak Scatter Factor (PSF)?

PSF increases as field size increases Signup and view all the answers

What percentage depth dose is given for a dose at 3 cm and at a depth of 5 cm in the example?

95.1% at 3 cm and 87.1% at 5 cm Signup and view all the answers

What is the depth of maximum dose referred to in the context of a 6MV beam?

3 cm Signup and view all the answers

What comprises the dose at point Q in the patient?

The primary dose and the scatter dose Signup and view all the answers

Which dosimetric function is primarily used for photon energies above cobalt-60?

Tissue maximum ratio (TMR) Signup and view all the answers

What is the role of the scatter component at point Q?

It accounts for photons produced through Compton scattering Signup and view all the answers

What setup technique involves measuring from the source to the skin surface?

Source Skin Distance (SSD) Signup and view all the answers

Which factor is used specifically in the treatment of deep seated tumors with multiple beams?

Source Axial Distance (SAD) Signup and view all the answers

Which factor quantifies the relative contribution of scatter from the machine components?

Collimator factor (CF) Signup and view all the answers

Which dosimetric function is used at cobalt-60 and below?

Peak scatter factor (PSF) Signup and view all the answers

What does the primary dose in a phantom include?

Dose directly from the source and collimator scatter Signup and view all the answers

What is the definition of tissue-phantom ratio (TPR)?

The ratio of the dose rate at a given depth in a phantom to the dose rate at reference depth. Signup and view all the answers

Which of the following parameters does the tissue-maximum ratio (TMR) depend on?

Field size Signup and view all the answers

What is the range of values for TMR?

0 to 1 Signup and view all the answers

How does TMR behave with increasing depth while keeping energy and field size constant?

It decreases with depth. Signup and view all the answers

What is the scatter-phantom ratio (SPR) analogous to in its use?

Tissue-air ratio (TAR) Signup and view all the answers

In a megavoltage photon energy setup, what does TPR overcome?

Limitations of using TMR for high energy photon Signup and view all the answers

What describes the relationship between TMR and percentage depth dose (PDD)?

PDD can be calculated using TMR. Signup and view all the answers

Why does a field size of 0 x 0 cm show the steepest drop-off with depth in dose distributions?

Due to the lack of scatter. Signup and view all the answers

Study Notes

Photon Beams: Physical Aspects Part 2

The dose at a point in a patient has two components: primary and scatter. DT = Dp + Ds.
The primary dose contribution comes directly from the source. This is determined by extrapolating depth dose versus field size data for a 0x0 cm² field.
For megavoltage photon beams, collimator scatter is often considered part of the primary beam.
Scattered radiation contributes to the total dose. Scattered photons result from Compton scattering within the patient, the machine collimator, flattening filter, or air.

Dosimetry Quantities

Dosimetric functions for photon energy range include percentage depth dose (PDD) and relative dose factor (RDF).
Dosimetric functions for cobalt-60 and below include peak scatter factor (PSF), collimator factor (CF), scatter factor (SF), scatter function (S), tissue air ratio (TAR), and scatter air ratio (SAR).
Dosimetric functions for cobalt-60 and above include tissue maximum ratio (TMR), tissue phantom ratio (TPR), and scatter maximum ratio (SMR).

Dosimetry Quantities in terms of setup techniques

Source Skin Distance (SSD)
Percentage Depth Dose (PDD)
Source Axial Distance (SAD)
Tissue air ratio (TAR)
Tissue maximum ratio (TMR)
Tissue phantom ratio (TPR)

Dosimetry Quantities: SAD Setups

SAD setups are more practical for multiple beams or rotational beams used in treating deep seated tumors.
They rely on dose functions like tissue-air ratio (TAR), tissue-phantom ratio (TPR), and tissue-maximum ratio (TMR).
SSD varies between beams, but SAD remains constant.

Tissue Air Ratio: SAD Setup

Tissue-air ratio (TAR) is the ratio of dose in a phantom to the dose in free space.
"Dose in free space" relates to a tissue equilibrium mass in air. This mass has a significant radius.
TAR depends on depth, beam energy, field size, and field shape. It is independent of SSD. TAR is traditionally used for low-energy beams (up to 60Co).

Relationship between TAR and PDD

The relationship is mathematical. PDD(z, A, f) = TAR(z, A₂) × 1(f + Zm/f + z)2 × 100. PSF(A)

Scatter Air Ratio (SAR)

The scatter air ratio (SAR) represents the scatter component of TAR.
It's useful for irregularly shaped fields, such as those used in the Clarkson technique.
SAR = TAR(z, A₂) – TAR(z,0).

Radiation Treatment Parameters - Backscatter Factor

Backscatter Factor (BSF) or Peak Scatter Factor (PSF) are special cases of TAR, determined at the reference depth of maximum dose on the central axis.
PSF(A, hv) = Dp(Zmax, A. f, hv)/Dp(A, hv)
BSF or PSF is a substantial factor for beams in the orthovoltage range of energies.
PSF depends on field size. The larger the field size, the larger PSF usually is.
PSF also depends on photon energy. PSF decreases with increasing energy, except at very low energy photons.

Tissue Phantom Ratio: SAD Setup

Tissue phantom ratio (TPR) is the ratio of dose rate in the phantom to the dose rate at the same point at the reference depth.
TPR depends on z, A, and hv.
TPR is defined as TPR(z, Aq, hv) = Dq/ Doref.

Tissue-phantom ratio TPR and Tissue-maximum ratio TMR

Tissue-maximum ratio (TMR) is a special case of TPR where the reference depth is the depth of maximum dose.
TMR = Dq/Dmax.
TPR and TMR depend on depth, field size, and photon energy and are nearly independent of SSD.
TPR overcomes the limitation of using TAR for high-energy photons.
Constant Aq and hv, TMR decreases with increasing z.
Constant z and hv, TMR increases with increasing A.
Constant z and A, TMR increases with increasing hv.

Off-Axis Ratios and Beam Profiles

Dose distributions along the central beam axis are used with off-axis beam profiles for accurate dose description inside the patient.
Off-axis data are typically presented as beam profiles measured perpendicular to central axis at a given phantom depth (typically z = Zmax and z = 10 cm).
Megavoltage beam profiles have three regions: central, penumbra, and umbra.
- The central region is from central beam axis to 1-1.5cm from the geometric edges.
- The penumbra region is close to the field edge, showing rapid dose changes, which depend on the finite size of the focal spot and collimator.
- The umbra region is outside the radiation field with very low dose.

Off-Axis Ratios and Beam Profiles: Specifications

Central region must meet flatness and symmetry specifications.
Penumbra region should have a rapid falloff with distance from the central axis (narrow penumbra) to optimize beam sharpness.
Umbra region should be close to zero dose to minimize dose outside the target volume.
Geometric field size is typically defined by optical light field separation from 50% dose points.

Total Penumbra

The total penumbra is the physical penumbra and has three components: geometric, scatter, and transmission.

Beam Flatness and Symmetry

Beam flatness (F) is assessed by finding the maximum (Dmax) and minimum (Dmin) dose point values on the beam profile.
F = 100 x (Dmax - Dmin) / (Dmax + Dmin).
Standard linac specifications require F ≤ 3% at a depth of 10 cm with SSD = 100 cm for the largest field size available.
Beam symmetry (S) is determined at Zmax and represents the area under the beam profile.

Isodose Distributions in Water Phantoms

Isodose distributions are measured in phantoms under standard conditions (homogeneous, unit density, perpendicular beam incidence).
Dose distributions are complete 2D and 3D information about a radiation beam. Shown with isodose curves and surfaces (equal dose points in a volume of interest).
Isodose curves are drawn at regular dose intervals, expressed as a percentage of the reference dose.

Isodose Distributions in Water Phantoms: Conventions

SSD setups normalize all isodose values to 100% at a point on the central beam axis.
SAD setups normalize values to 100% at the isocentre.

Delivery of Dose with a Single External Beam: X-Ray and/or Radioisotope Devices

Outputs are usually given in cGy/min at Zmax of a phantom under nominal SSD.
Linac outputs are usually given in cGy/MU at Zmax of phantom under nominal SSD.
Transmission ionization chambers' adjustments in linacs cause beam output to correspond to:
- 1 cGy/MU,
- at Zmax in phantom (point P)
- for a 10 x 10 cm² field
- at SSD = 100 cm.
The dose rate (Dp(Zmax, A, 100, hv)) for a given arbitrary beam size 'A' for an SSD of 100 cm is found by multiplying (Dp(Zmax, 10,100,hv)) by (RDF(A, hv)).

Delivery of Dose: PDD Formalism

The number of monitor units (MUs) needed to deliver a tumor dose (TD) at a point (Q) using a 100-cm SSD and field A is:
MU = TD / (Dp(Zmax, 10,100, hv) x RDF(A,hv) x F x PDD(z, A, f, hv) x WF(A, z, x) x TF x OAR(z, x) x ISQ).
- Note: TD is tumor dose rate, and Dp (Zmax, 10,100, hv) = 1cGy/MU.

Delivery of Dose: TMR Formalism

The number of monitor units (MUs) required for a single 100cm SAD field, A is:
MU = TD / [Dref (Zref, A, 100, hv) × F × TMR(z, A, hv) × WF (A, z, x) × TF × OAR(z, x)]
Note: Dref (Zref, A, 100, hv) ≈ Dp (Zmax,10,100ssd, hv) x RDF (A, hv) x (f + zref/f)2

Absolute and Relative Dose Measurement with Ionization

Dose parameters are often measured with ionization chambers in various shapes and sizes.
Ionization chambers are designed for specific tasks.
Measurements need correction factors for variables like air temperature & pressure, chamber polarity and voltage, and photon beam energy.
Relative dose measurements using solid state detectors are common.

Absolute and Relative Dose Measurement: Additional Details

Doses and dose rates at reference points are measured using large-volume cylindrical ionizing chambers (0.6 cm³).
This helps achieve a better signal to noise ratio compared to other measurements.
Small volume chambers (0.1 cm³) are used for obtaining better spatial resolution for relative dose distributions beyond zmax.

Assignment: Radiotherapy Correction Methods

Report on the most common correction methods in radiotherapy, encompassing contour irregularities, oblique beam incidence, tissue inhomogeneity, wedge use, compensator use, and bolus use.

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Description

Test your knowledge on the specifications and measurements used in radiation therapy, particularly those related to beam flatness, dose distribution, and isodose values. This quiz covers essential concepts crucial for understanding radiation therapy physics and quality assurance practices.