Bushberg Diagnostic

http://www.rad.washington.edu/academics/academic-sections/diagphys/diag-phys-in-med-imaging/diagnostic-physics/2007-2008-lectures-1/2007-2008-lectures-1#week-1-5-july

Equation for permanent implant

Dose Volume Constraints for Organs at Risk

Dose Volume Constraints for OARs
Conventional fractionation

Spinal Cord – 40 Gy (max)

Brainstem – 54 Gy (max)

Optic nerves – 54 Gy (max)

Parotid Glands – 35 Gy (max)
25 Gy (<50%)

Brain – 50 Gy (minimize volume above 30 Gy as much as possible)

Mandible – 70 Gy (max)

Cochlea/Middle Ear – 30 Gy (max)

Oral Cavity – 30 Gy (mean)

Brachial Plexus – 60 Gy (max)

Posterior Neck Avoidance – 45 Gy (mean)

Lens – 5 Gy (max)

Eyes – 45 Gy (max)

Optic Chiasm – 54 Gy (max)

Optice Nerves – 54 Gy (max)

Lung – 20 Gy (<35%)

Heart – 40 Gy (max)

Esophogus – 60 Gy (<50%)

Kidney – 20 Gy (max)
< 50% of combined both kidneys
< 75% of one side of kidney if another kidney is not functional

Liver – 30 Gy (<40%)

Femoral heads – 45 Gy (max)

Small Bowel – 48 Gy (max)

Rectum –
40 Gy (<60%)
45 Gy (<50%)
60 Gy (<40%)
70 Gy (<20%)
75.6 Gy (<15%)
78 Gy (<5%)

Bladder – 70 Gy (<20%)

Femoral Heads – 45 Gy (<50%)

Pituitary gland – 36 Gy (max, adults), 18 Gy (max, children), very safe


Dose Volume Constraints for OARs
SRS, single fraction
Brainstem – 12 Gy (max)
Optic nerves – 12 Gy (max)
Optic Chiasm – 12 Gy (max)
Optice Nerves – 12 Gy (max)
Retina – 12 Gy (max)
Normal Brain – 20 Gy (max)
Lens – 2 Gy (max)
Skin – 5 Gy (max)


Dose Volume Constraints for OARs
SBRT, 3-5 fractions

Spinal Cord – 18 Gy (max), 10 Gy (<10%) single fraction
3 Gy per fraction always safe, only draw cord within 4 mm of PTV

Lung – 20 Gy (<15% of total lung capacity) total

Heart – 6-8 Gy (max) per fraction

Esophogus – 6 Gy (max), 4 Gy (<75%) per fraction

Stomach – 8 Gy (max) per fraction

Small bowel – 4 Gy (max) per fraction

Kidney – 1 functioning kidney: 2 Gy (max), 1 Gy (<75%) per fraction
2 kidneys: 3 Gy (max) per fraction to one, test to make sure both work

Liver – 15 Gy (<700 cc) total

Bronchii and trachea – 8 Gy (max) per fraction

Skin – 4 Gy (max) per fraction

Mayneord F Factor

Mayneord Factor
* Overestimates the increase in PDD with increase in SSD
* Overestimates for small field sizes
* MF = ((f2+ dm) /(f1 + dm))squared x ((f1 + d) / (f2+ d))2

PDD (Percentage Depth Dose) Key Points.

PDD

Increases with FS (less dependent with higher E)
Increases with E
Increases with SSD (due to ISL)
Decreases with Depth (exponentially) beyond build up region

FILM: EDR VS XV

The two films are different in their response to dose:
XV films being to saturate at about 30 cGy
EDR2 films begin to saturate at about 300 cGy

The difference between the two films originates from their differences in the content of silver bromide crystals and grain size.

The grain size of EDR2 is smaller.

DOSIMETRIC PERFORMANCE
Compared with XV film, EDR2 film showed better agreement with calculations and measurements of dose. (olch2002)

EDR 2 film is less sensitive to low energy photons.

XV requires less amount of dose and thus the irradiation will be much quicker. On the other hand it will saturate much quicker than EDR2 film.

Source Film dosimetry (Yeo and Kim)

OBI Imager QA

Nov 2006 Medical Physics has a quality assurance program for the on-board imager.

Travelling vs Standing waves

· Accelerate electrons using a microwave using either a traveling wave, which absorbs the residual power with a dummy to prevent “backward reflected waves”, or a stationary wave accelerator which reflects the waves from each end towards each other to produce “standing waves”.

Mnemonic, when one is travelling they don't look backward (i.e. prevent backward reflecting waves)

When one is stationary or standing they can reflect back on their life (reflect waves to from each end)

Photon Interactions as a function of energy

Why use dref for electrons in TG-51

TG-51 gives a reference for the reference depth for electron beam, which is Ref.#17.Quote: For electron beam reference dosimetry in radiotherapy, it is shown that by choosingthe reference depth as dref=0.6R50 -0.1 cm, ..., the Spencer-Attix water-to-airstopping-power ratioat dref is given by (L/p)(water to air) = 1.2534-0.1487(R50)exp(0.2144).This is the magic.

REF YAHOO STUDY GROUP 4/23/08

Photon Interactions Part III

As far as the photon fate after the interaction with an
atom is concerned there are two possible outcomes:
• Photon disappears (i.e., is absorbed completely) and a portion
of its energy is transferred to light charged particles (electrons
and positrons in the absorbing medium).
• Photon is scattered and two outcomes are possible:
• The resulting photon has the same energy as the incident photon and no
light charged particles are released in the interaction.
• The resulting scattered photon has a lower energy than the incident photon
and the energy excess is transferred to a light charged particle (electron).

Photon Interaction (Tightly Bound)

1.4.1 Slide 5 (125/194)
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 A tightly bound electron is an electron whose binding
energy is comparable to, larger than, or slightly smaller
than the photon energy .
• For a photon interaction to occur with a tightly bound electron, the
binding energy of the electron must be of the order of, but
slightly smaller, than the photon energy
• An interaction between a photon and a tightly bound electron is
considered an interaction between photon and the atom as a
whole.

Photoelectric effect (tightly bound)

Loosely bound electrons (Compton)

1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 A loosely bound electron is an electron whose binding
energy to the nucleus is small compared to the
photon energy
􀀁 An interaction between a photon and a loosely bound
electron is considered to be an interaction between a
photon and a free (unbound) electron.

Photon Interactions part 2

1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 Interactions of photons with nuclei may be:
• Direct photon-nucleus interactions (photodisintegration)
or
• Interactions between the photon and the electrostatic field of the
nucleus (pair production).
􀀁 Photon-orbital electron interactions are characterized as
interactions between the photon and either
• A loosely bound electron (Compton effect, triplet production)
or
• A tightly bound electron (photoelectric effect).