Radiation Therapy Measurement Standards

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.

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.