Practical Radiotherapy: Beams and Dose Measurement

Short for anterior to posterior and posterior to anterior. Also called Ant and Post beams, these beams are respectively incident on the anterior and posterior side of the patient (under standard supine patient set up)

What are AP and PA beams? Alternative names?

Parallel opposed beams, or APPA beam for anterior to posterior and posterior to anterior

What is a POP beam? Alternative name?

In palliative cases for immediate pain relief (esp in on call and weekend cases) as they are easy to set up but not especially conformal. AP beams deposit dose more shallowly (e.g. good for upper back pain) and POP beams deposit dose deeper (e.g. lumbar back pain)

When are AP and POP beams used clinically today? Why pick one over the other?

Because of the two step process of depositing dose, secondary electrons deposit dose farther than those electrons themselves being incident on the surface

Why do photon beams spare dose at the surface compared to electron beams?

The dosimeter is moved from deep to shallow to avoid disturbances from surface tension

In water tank dosimetry, which direction is the ionization chamber moved to measure a PDD? Why?

The reference, typically in the target

Where is a PDD often normalized to?

The central axis

Where is a beam profile often normalized to?

Photons traverse matter with no interactions and no energy loss or a single or few "catastrophic" events and complete energy loss. Electrons interact with every single atom it passes by and lose energy as if via friction

Compare how photons and electrons lose energy in absorbing media

LET is a "restricted" stopping power that describes energy lost to locally absorbed dose. It is a fraction of collision stopping power that includes all soft collisions and the hard collisions that produce delta rays with energy less than some limit

How does linear energy transfer relate to stopping power?

Kinetic energy released in a medium (per mass). This is the total energy transferred to an electron by a photon (collision and radiative).

What is kerma?

Kerma is the energy fluence times the mass energy transfer averaged over the energy fluence spectrum

How does kerma relate to energy fluence?

The total charge of the ions of one sign produced in air when all the electrons that are liberated by photons in some mass of dry air are completely stopped in air

What is exposure?

The energy retained in some mass of the medium

What is dose?

1 Gy is 100 rad

How does Gy relate to rad?

There is some depth of build up, where dose lags kerma, then at d max electronic equilibrium occurs and dose equals kerma

What phenomenon begins at d max? What happens before and after d max?

Each secondary charged particle that leaves the volume with some kinetic energy is replaced by the same type of secondary charged particle that enters the volume with same kinetic energy and expends it inside the volume

What is charged particle equilibrium?

Atomic composition and density of medium is homogeneous; indirectly ionizing radiation is only negligibly attenuated through the volume; no homogeneous electric or magnetic fields are present

What are the conditions of charged particle equilibrium?

Higher dose deposited at depth with increasing energy, with shallower drop off. This is because higher energy photons and secondary electrons travel farther before depositing dose

How does photon depth dose curve change at depth with incident energy? Why?

Increases with increasing energy. This is because higher energy photons and secondary electrons travel farther before depositing dose

How does photon d max change with incident energy? Why?

Attenuation per path length (after electronic equilibrium)

What does the slope of a depth dose curve represent?

Decreases with increasing energy. This is because higher energy photons and secondary electrons travel farther before depositing dose, and electrons are less likely to backscatter

How does photon surface dose change with incident energy? Why?

Increasing field size increases surface dose due to more electron contamination from linac head, and increases dose at depth along central axis due to in field scatter. The d max also becomes shallower

How does depth dose curve change with field size?

It follows a law of diminishing returns: effects from the central axis cease to impact central axis depth dose due to the limited range of electrons

How does the change in dose depth curve due to field size change with further increases in field size?

Assuming the same field size at each depth measurement, increasing SSD increases dose at depth. Dose drop off is due to two factors: inverse square intensity drop off with distance; and attenuation which is the same across equivalent distance for different SSD. The proportional difference in inverse square drop off affects the dose at depth more for small field size

How does depth dose curve change with SSD? Why?

Theoretically, it requires the measurement of all ionizations produced by collision interactions in air by the electrons resulting from photon interactions in a known mass of air. Practically, this is not possible and instead ionizations are measured under charged particle equilibrium in a free air ionization chamber

How is exposure measured?

Source exposes local standard dosimeter at point D and beam continues into collection volume, where measurement is made at point P by collecting generated ionizations through applied bias voltage

How does a free air ionization chamber work?

They are a standard used by standards labs (NRC in Canada and NIST in USA) to which local standard ion chambers are calibrated

What are free air ionization chambers used for?

Distance between local standard dosimeter and measurement point P; temperature, pressure and humidity; air attenuation; recombination; ionization by scattered photons outside the beam area

Free air ionization chamber corrections

When ionizations that are to be measured through separation via applied bias voltage interact with an opposite signed ion before reaching the measurement electrode

What is recombination?

Electron range increases with photon energy which increases practical collecting volume and plate separation, but increasing plate separation causes more recombination and increases non-uniformities in electric field; exposure cannot be measured about 3 MeV

Free air ionization chamber limitations

Because to measure exposure in air, secondary charged particles must be generated through interactions with the chamber wall. Higher energies require impractically thick chamber walls to reach charged particle equilibrium

Why is there an energy limit to free air ionization chambers?

Co-60 because it is at the upper energy limit for measuring exposure, it has stable and predictable output, and discrete energy levels

What source is used for free air ionization chambers? Why?

Dose in medium is exposure times A which is the ratio of energy fluence in medium to air (usually close to 1), and the medium-specific f-factor which converts roentgen to cGy in the medium

How is dose in medium calculated from exposure?

It is 0.873 times the ratio of mass energy absorption coefficient in medium to air

What equation defines the f-factor?

Between 10 and 100 keV, bone has a much larger f-factor and muscle a slightly larger f-factor than water. In the range 100 keV to 10 MeV, f-factor for bone, muscle, and water are all roughly 1 Gy per R (bone remains slightly higher)

Compare f-factor in bone, muscle and water versus photon energy

A cavity ion chamber is used to measure exposure in air by multiplying the raw ionization measurement with a calibration factor. The exposure in air is then converted to dose as if there was a tiny volume of tissue floating in free space. This dose in free space is the exposure times A which is the ratio of energy fluence in tissue to air (usually close to 1), and the tissue f-factor which converts roentgen to cGy. We don't perform this, we use Bragg-Gray cavity theory instead

How is dose calibration with a measurement in air performed? When do we perform this?

The ionization produced in a gas-filled cavity placed in a medium is related to the energy absorbed in the surrounding medium, provided the cavity is sufficiently small that it does not alter the number or distribution of the electrons that would exist in the medium without the cavity

What is Bragg-Gray cavity theory?

Measurement in air cannot be done practically with photons more energetic than 3 MV because the cavity ion chamber walls must be so thick to generated secondary particles. It's also not applicable where charged particle equilibrium does not exist

Why do we use Bragg-Gray cavity theory to measure dose to a medium rather than via exposure from dose calibration with a measurement in air?

Dose in the medium as if there was no cavity is the product of: ionization charge of one sign produced per unit mass of the cavity gas; average energy needed to create one ion pair in dry air (=33.85 eV); ratio of mass stopping power in medium to cavity gas

What is the Bragg-Gray cavity theory equation?

small (better resolution); high sensitivity (can be smaller); tissue equivalent material; Accurate in the range of: doses and energies of interest; minimal recombination losses; waterproof; gas is manageable (air is easiest); Robust; Considerations for the cable and electrometer connection

Qualities of a good ionization chamber

Thimble type ionization chamber

What is another word for a Farmer's chamber?

There is a volume of air surrounding a central low Z material electrode, and the other electrode is the inner cylindrical surface of the chamber covered in tissue equivalent material. Applying a voltage bias separates ionizations which are measured as current. Accumulated charge is converted to dose via calibration factors

How does a Farmer's chamber work?

0.01 to 0.7 cc, larger volume gives more precise measurement but has less resolution which is unfavorable in steep dose gradients

Range of volume in Farmer's chambers and reasons to use small or large volume?

When the Farmer's ion chamber cable is irradiated and the ionization pair inappropriately contributes to measured charge

What is the stem effect?

Analogous to the stem effect in a Farmer's chamber. Radiation outside of the volume of interest causes inappropriate ionization measured as additional dose

What is an extracameral effect?

Thin gas layer ~0.5 mm allows good depth resolution; can be made with thin foils or plastic membranes causing minimal attenuation or scattering of incident electrons or soft x-rays; surface can be extrapolated

Advantages of parallel plate ion chambers

More complicate to design and build compared to Farmer's chambers; susceptible to extracameral ionization

Disadvantages of parallel plate ion chambers

Ionization chambers, Semiconductor Dosimetry (diodes); Calorimetry; Chemical Dosimetry (ex: Fricke gels); Thermoluminescent Dosimetry (TLDs); Radiographic Film Dosimetry; Radiochromic Film Dosimetry

What are different types of dosimeters?

Diodes are solid state semi-conductors with a 0.2-0.3 mm3 collecting volume made of silicon crystal doped with p-type (electron receptor or hole) and n-type (electron donor) impurities under a dielectric field. Under ionization by an incident particle, the electrons and holes move. This radiation induced current in measured

How do diodes measure dose?

Compared to ion chambers, diodes have higher sensitivity, instantaneous response; small size; “rugged” but they have energy and directional dependence

Advantages and disadvantages of diodes?

For relative measurement of electron beams; output constancy checks; in vivo dose monitoring

when are diodes used?

It assumes energy deposited in the medium (m times c times delta T) is the energy contributing to dose. The small change in temperature (1e-4 degrees per Gy) is measured by a thermistor (semiconductor)

How does calorimetry measure dose?

This is the only way to measure absolute dose, but very challenging: must electrically isolate thermistor from conductivity of water; small temperature change for large dose; heat conduction of water and surrounding materials; if water has impurities, they can cause chemical reactions, which adds noise (1-3%); carbon is more accurate (0.1%), but requires a conversion of dose to water

Advantages and disadvantages of calorimetry?

At national labs for absolute dose measurement

when is calorimetry used?

Absorbed dose causes a chemical change that can be read via optical density with spectrophotometer or laser CT

How does chemical dosimetry measure dose?

Hypothetically excellent for 3D visualization, but not used outside of research

Advantages and disadvantages of chemical dosimetry?

Never oof

when is chemical dosimetry used?

Fricke xylenol orange gelatin gels; polyacrylamide polymer gels; leuco-crystal-violet micelle gel

Examples of chemical dosimetry?

Diodes are solid state semi-conductors made of crystal dielectric doped with p-type (electron receptor or hole) and n-type (electron donor) impurities. For TLDs in particular, ionized charges and holes are trapped in a potential until heating releases them. Then, luminescence centres emit light when the liberated electrons and holes recombine, which is measured

How do thermoluminesent diodes measure dose?

wide useful dose range; dose rate independence; small size; passive energy storage; economical and commercially available; reusability; readout convenience; accuracy through calibration to 1-2%; precision of 1-2%

Advantages of thermoluminesent diodes?

lack of uniformity; batch dependence requires batch calibration; storage instability; can drift over time so anneal before use; fading; light and heat sensitivity; susceptible to mechanical damage; will eventually maintain a radiation and thermal history “memory”; reader instability; heat cycle can damage; loss of a reading when read

Disadvantages of thermoluminesent diodes?

Radiation worker personal dosimeters, in vivo dosimetry

when are thermoluminesent diodes used?

LiF doped with Mg and Ti; CaF2 doped with Mn

Examples of thermoluminesent diodes?

Similar to TLDs but with light stimulation instead of heat (ex: stimulate with green, emits blue wavelength)

How do Optically Stimulated Luminescence dosimeters measure dose?

Compared to TLDs, no need to anneal and restimulation is possible to confirm does measurement

Advantages of Optically Stimulated Luminescence dosimeters?

AL2O3 doped with C

Example of Optically Stimulated Luminescence dosimeter?

Radiographic Film comprises a transparent film base (cellulose acetate or polyester resin) coated with silver bromide (AgBr) crystals. Radiation produces a latent image, as attenuated beam appears bright in areas where silver tarnish forms when film is developed to fix the transformation in irradiated areas. Dose is proportional to optical density

How does Radiographic Film measure dose?

Resolution is related to crystal size (1-3 nm); good for electron dosimetry because film sensitivity does not depend on electron energy; stores dose until development (can be used for personal dosimeters)

Advantages of Radiographic Film?

film development requires dedicated facilities to manage the chemicals and variation in film processing leads to inconsistency; artifacts from air pockets; planar dose distributions; photon dosimetry is compromised by scatter

Disadvantages of Radiographic Film?

treatment verification; exposure; dose distribution QA; light vs x-ray field QA; MLC QA; head leakage QA

when is Radiographic Film used?

radiosensitive dye sandwiched between two layers of clear polyester film and ionising radiation leads to the polymerisation and a colour change (clear to blue), which is measured by a spectrophotometer

How does Radiochromic Film measure dose?

no developer needed; insensitive to visible light; useful across a large dose range (0.01 – 106 Gy)

Advantages of Radiochromic Film?

small fields; surface or skin dose; brachytherapy; monitoring dose to food (for sterilisation)

when is Radiochromic Film used?