Electron dosimetry

Question 1

What is the relevant documentation for small field electron dosimetry?

Answer 1

TG 70

Question 2

For a planar electron beam incident on a skin lesion that is above the flat skin surface, where would a dose hotspot be created?

Answer 2

lateral to the lesion (see Fig 32 of TG 25), beneath the nominal skin surface. The reason is due to the scatter generated in the above-skin lesion, scattering laterally into the air and towards the nominal skin surface.

Question 3

For oblique/curved surfaces irradiated by an electron beam, what affect does this have on the isodose curves?

Answer 3

The isodose curves roughly follow the curvature of the external contour. As the angle of incidence increases, dmax moves more towards the surface. This means the R90 coverage is going to be challenging to achieve in these oblique regions.

Question 4

How is electron beam quality specified?

Answer 4

R50

Question 5

For absolute dosimetry in photon beams, the chamber is positioned at a depth of 10cm. For electrons, how is the reference depth specified? Provide the equation.

Answer 5

dref (cm)=0.6R50-0.1

Question 6

What are the two protocols used for absolute absorbed dose to water for electrons and photons? What do we use in Australia?

Answer 6

AAPM TG 51, and IAEA TRS 398 rev1. In Australia we use TRS 398

Question 7

According to TG 70, the reference field size for electron absolute dosimetry is 10x10cm2. As the beam energy increases, R_50 increases. Above what beam energy/R50 value should the field size for absolute dosimetry be increased to 20x20cm2 or greater?

Answer 7

Beam energies above 20 MeV, corresponding to a R50 >8.5 cm

Question 8

When a air-filled ionisation chamber is used to measure a depth dose profile of an electron beam, what is this curve called? What correction needs to be applied to get PDD?

Answer 8

PDI, can be converted to PDD using stopping power ratio tables.

Question 9

If an electron depth dose curve is measured with a solid state device, is the measured data a PDI or PDD? Why?

Answer 9

PDD (diode represents the dose directly), as the relative stopping power ratio between water and silicon is essentially independent of electron energy, and hence depth.

Question 10

For a broad beam electron field PDD, what sort of chamber is used?

Answer 10

PPC

Question 11

For absolute dosimetry of a broad electron beam, what sort of chamber is used for absolute dosimetry?

Answer 11

Cylindrical ionisation chamber

Question 12

Are shielded or unshielded diodes recommended for electron beam dosimetry? Why?

Answer 12

Unshielded diodes are recommended for electron beam dosimetry. Shielded diodes have a high Z material around the sensitive region to attenuate low energy photons in a photon beam which contributes to an overresponse of the detector (remembering the mass energy absorption curve with energy of water: silicon)

Question 13

Does mass attenuation coefficient (also known as mass absorption coefficient) pertain to photons or electrons?

Answer 13

photons, is the rate at which photons are attenuated across a cross-section/density of the attenuating material. Quantity is used for shielding calculations

Question 14

what dosimetric quantity does mu_tr/p multiplied by photon energy fluence give?

Answer 14

Kerma= sum of all the kinetic energies of all primary charged particles released by photons, per unit mass.

Question 15

For depth dose beam scanning in water, should the scan direction be towards or away from the surface?

Answer 15

Scan direction should be towards the surface to reduce the effect of meniscus formation.

Question 16

For an electron field size, how is the output factor defined?

Answer 16

ratio of the dose per monitor unit on the CAX at dmax for a measurement field (M_field) to the dose per monitor unit for the reference field size taken at dmax.

Question 17

Before performing an output factor measurement, what information is needed?

Answer 17

PDD data

Question 18

What happens to dmax when the field size becomes too small to produce lateral electronic equilibrium at the central axis?

Answer 18

dmax shifts closer to the surface, hence dmax for the same beam energy at different field sizes is likely different. Hence if measuring OPFs, PDDs should be performed to determine dmax for each field size.

Question 19

What effects can using an extended treatment distance (SSD) for electron beams?

Answer 19

(1) decrease in output factor (inverse square)
(2) reduced dose uniformity within the field
(3) widening of the penumbra (relative to measurements at the calibrated SSD of 100 cm)

Question 20

Dosimetrically, what is observed for small electron fields (3 points)?

Answer 20

When field size is <= radius required for lateral scattering equilibrium:
- dmax shifts towards surface
-output decrease
- CAX PDD decreases

Question 21

As a rule of thumb, what is the equation given in TG 70 used to determine if an electron field can be considered small?

Answer 21

field radius, r,=0.88 x sqrt(E’_0)

Where average energy (E_0) can be determined using the PDD.

Question 22

If you were creating a library of small cut out information for electron fields, what information is relevant to include?

Answer 22

depth of dmax, depth dose data, output factor, and penumbral and isodose information.

Question 23

Tissue heterogeneities penetration of the beam and hence affect the dose distribution because of differences in what?

Answer 23

Stopping power, which are a result of different densities and tissue compositions

Question 24

what are the two types of electron beam dose calculation algorithms currently available in commercial TPS?

Answer 24

(1) pencil beam, (2) Monte carlo

Question 25

What condition is considered when selecting an appropriate electron energy for a treatment?

Answer 25

Electron energy is selected so it covers the deepest portion of the target, to be treated to at least 90% of the given dose.

Question 26

When using a cylindrical chamber for electron beam scanning, what is the generally accepted effective point of measurement equation used to place the chamber at the EPOM?

Answer 26

d=d_cent-0.5r where d=depth

Question 27

For electron beams, as the field size increases what happens to the PDD? (see Fig 14, TG 25)

Answer 27

dmax shifts closer to the surface, and drop off after dmax occurs quicker with depth. As field size decreases, the PDD drop off tangent moves to the LHS of the graph. All graphs meet at the practical range point. Reason for field dependence on depth dose: lateral scattering of electrons

Question 28

As the electron beam energy increases, what happens to the surface dose and why?

Answer 28

Surface dose increases

Question 29

For internal shielding, a lead shield can be placed beyond the target to protect the underlying normal structures. e.g. treating lip or eyelid. Electron backscatter produces significant dose enhancement near the tissue-lead interface. How can this dose enhancement be minimised?

Answer 29

Using low atomic number material between lead and overlying tissue to absorb the backscattered electrons. Layer order: tissue, wax, lead, tissue

Question 30

Photon beam question: What is the Mayneord F factor equation used for?

Answer 30

Inverse square law correction of PDDs from one SSD to another. For example, you have PDD data obtained at 100cm SSD but wish to treat the patient at 115cm SSD. Given the Mayneord F factor equation, the relative increase or decrease in % will be calculated for a given depth, e.g. 10cm. Mayneord’s F Factor= [(SSD1+dmax)^2* (SSD2+d)^2]/[(SSD1+dmax)^2* (SSD2+d)^2)] Quantitative example: for 6MVbeam (dmax=1.5cm) changing SSD from 100 to 150cm will result in PDD(10cm) increase by 5%

Question 31

What is the 2,3,4 rule of thumb when it comes to electron beam range concepts?

Answer 31

x2: dmax/R100 x3: R90 x4: R50 x5: R10

Question 32

When lateral scatter equilibrium exists (broad beam), how does the depth dose curve change for a fixed electron beam energy?

Answer 32

PDD is essentially independent of field size

Question 33

A electron field can be considered 'small' when the field dimensions are smaller than the what?

Answer 33

Practical range of electrons (Rp)

Question 34

What is the approximate energy loss per cm in water for electron beams?

Answer 34

2 MeV/cm

Question 35

What is the equation 'rule of thumb' for estimating the practical range (Rp) depth for an electron beam of energy E?

Answer 35

Rp (cm)= E (Mev)/2 E.g. Rp for 10 MeV beam = 10/2=5 cm

Question 36

For low-energy beams all the isodose curves buldge. For higher energy beams what happens to (a) lower isodose levels and (b) higher isodose levels?

Answer 36

(a) in higher energy electron beams, low energy isodose levels bulge (b) higher isodose levels laterally constrict for isodose levels >80%

Question 37

For low energy electron beams (4-6 MeV), surface dose generally around what percentage?

Answer 37

80%

Question 38

For higher energy beams, >20 MeV, what is the approximate surface dose % range?

Answer 38

90-100%

Question 39

Explain the reasoning for the change in surface dose between low and high-energy electron beams in terms of fluence (# particles/unit area), scattering angle, and the ratio of surface dose to dose at dmax. (see Figure 14.10 in Khan, 3rd ed.)

Answer 39

As the beam energy increases surface dose increases. The reasons for this are: - fluence at the surface is dictated by relatively forward-directed electrons (i.e. tangential to the surface, electron paths approx parallel). - At low energies, electrons are scattered more easily & through larger angles (e.g. 45 deg) from the original incident direction, within the phantom. Comparatively, higher energy beams experience less lateral scatter and coincidence with smaller scatter angles. - consider the relative fluence of electrons at the surface is equivalent for both high and low energy beam scenarios. At depth, the low-energy beam is going to have a greater fluence (more particles in a set volume) than that of a higher-energy beam. - Hence, ratio of surface dose to dmax dose for lower energy electron beams is less - for higher energy beams, fluence at depth and surface is approx equivalent, hence ratio of surface dose to dose at dmax is greater

Question 40

For electron beam energies (a) less than 4 MeV and (b) less than 20 MeV, what is the relative % contribution (normalised to dmax) of bremmstrahlung contamination in the tail section?

Answer 40

(a) 1% (b) 4%

Question 41

At what depth is flatness and symmetry assessed for electron beams? What document states this and what is the tolerance?

Answer 41

dref (cm)=0.6R50-0.1 1% deviation from baseline is tolerance

Question 42

Energy losses of charged particles are described by what quantity? What are the two components of this quantity?

Answer 42

stopping power consists of two components: (1) mass collision stopping power, resulting from electron-orbital electron interactions (atomic excitations and ionization), (2) mass radiative stopping power resulting from electron-nucleus interactions (bremsstrahlung production)

Question 43

Of the two components of stopping power, which has an important role in radiation dosimetry? Give the equation for dose in terms of this component of stopping power

Answer 43

Collisional stopping power, (S/p)_c, where Dose (D)= e- fluence x (s/p)_c

Question 44

How would you verify the accuracy of the PDI to PDD conversion for an electron scan performed with an ion chamber?

Answer 44

Use a diode and perform measurements at selected positions. Diode measurements will be dose.

Question 45

TG 106: How would you determine the effective SSD of an electron beam energy?

Answer 45

measurements at dmax for various air gaps between electron cone and water surface, to create plot of the sqrt(I0/I) vs gap size. Gradient of line=effective SSD

Question 46

Per TG 106, what are ideal features of a detector used for beam scanning?

Answer 46

high sensitivity, small dimensions, low noise, minimum dose rate and energy dependence

Question 47

What detector type is preferred for relative dosimetry in photon beams?

Answer 47

Ion chambers with small volume

Question 48

What detector type is preferred for relative dosimetry in electron beams?

Answer 48

electron diode

Question 49

What portion of the electron depth dose curve does an electron diode no longer become the preferred detector choice? (TG 106)

Answer 49

Bremsstrahlung portion. For accurate measurement of Bremsstrahlung region, an ion chamber should be used.

Question 50

What does TG 106 (2008) relate to?

Answer 50

Accelerator beam data commissioning equipment and procedures

Question 51

which has greater surface dose, low or high energy electrons?

Answer 51

high