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Y3: Superficial Radiotherapy > Clinical Use of SXT and Orthovoltage > Flashcards

Clinical Use of SXT and Orthovoltage Flashcards

1
Q

Describe the features of an SXT machine.

A
  • 50-150kVp (peak energies)
  • Beam Quality:1-8mm Al Half Value Layer (HVL)
  • Typical Focus Skin Distance (FSD) is 15-30cm
  • Circular applicators
  • Usually no monitor chamber (due to attenuation hecne timer is used)
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2
Q

Describe the features of an Orthovoltage machine.

A
  • 150-500kVp
  • 0.5-4mm Cu HVL
  • 50cm FSD
  • Circular, square and rectangular applicators
  • Monitor chamber
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3
Q

Why is SXT or Orthovoltage chosen for clinical use?

A
  • Tight penumbra at low energies allowoing for small margins and tightly conformed treatment areas
  • Cheaper than electrons (unit, room design & footprint)
  • Rapid fall off
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4
Q

What does the SXT or Orthovoltage dose distribution look like?

A
  • Little or no build-up
  • Sharp discontinuity at geometric beam edge
  • Rounded isodose lines due to ISL effect over applicator
  • Significant low dose penumbra beyond geometric field edge
  • Heel effect in anode-cathode direction
    (for 200 kVp)
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5
Q

Name 2 clinical indications that SXT is used to treat?

A
  • basal cell carcinoma
  • squamous cell carcinoma
  • keloid scars
  • dermatological conditions
  • mycosis fungoides
    + others
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6
Q

What modifications can be done to alter the SXT clinical beam?

A
  • Applicator dimensions
  • Cut-outs
  • ISL
  • Internal & external shielding
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7
Q

What affects the dose rate on the surface from an SXT machine?

A

Applicator dimensions

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8
Q

Which machine requires a timer end error correction: SXT or Orthovoltage?

A

SXT

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9
Q

Which machine requires calculated MU or calculated time: SXT or Orthovoltage? (match the two couples)

A

SXT: time
Ortho: MU

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10
Q

Which is calibrated to reference conditions: an SXT or Orthovoltage unit?

A

Orthovoltage

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11
Q

Which factors are required to adjust the dose rates in SXT therapy?

A

ISL & BSF

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12
Q

Which machine contains a monitor chamber: SXT or Orthovoltage?

A

Orthovoltage

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13
Q

What is the purpose of a cut-out in superficial therapy?

A

Conform the beam and avoid a geometric miss with patient motion

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14
Q

Dose the cut-out increase or decrease the dose rate at the surface in superficial therapy?

A

Decrease (account for by the backscatter factor ratio)

BSFR = BSF for cut out / BSF for open field

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15
Q

What is the equation for the Stand Off Correction in superficial therapy?

A

SOC = ( FSD / FSD + d ) ^2

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16
Q

Define the BSF in superficial therapy.

A

The ratio absorbed dose to surface of water phantom / absorbed dose to same point in a water equivalent detector in air.
BSF = (primary dose + scattered dose) / (primary dose)
BSF is a function of the area irradiated and the beam quality.

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17
Q

How is BSF measured?

A
  1. Place a LiB* TLD flush with the surface of a water equivalent phantom. Obtain TLD reading per unit time of irradiation for a particular applicator.
  2. Rotate the same applicator so the open end is pointing up. Place a LiB TLD chip on a thin piece of mylar (e.g. 20 microns thick) across the applicator. Obtain TLD reading for same time as above.
  3. BSF = [Measurement to TLD at phantom surface (measurement 1)]/[Measurement to TLD in air (Measurement 2)]
  4. Repeat for a range of beam qualities and applicator diameters and FSDs
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18
Q

What effect on the BSF does increasing the irradiated area have?

A

Initially increases due to larger fraction of photons from edge of field contributing to dose on the Central Axis (CAX)
Plateaus at larger beam diameters when photons beyond a certain distance are far enough away that they don’t have enough energy to reach CAX

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19
Q

What effect on the BSF does increasing the beam quality (HVL) have?

A

Increases through superficial beam qualities because the Compton scatter-to-PE ratio increases
Decreases through orthovoltage beam qualities because scattered photons are more forward peaked

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20
Q

Where can BSF data be found?

A

BJR Supplement 25, London: British Institute of Radiology, 1996

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21
Q

What is BSF a function of?

A
  • HVL
  • FSD
  • Field diameter
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22
Q

How much dose does a cut out remove?

A

approx 5%

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23
Q

What applicator is used for eye shielding in superfiecial therapy?

A

Gold eye applicator (for under eyelid).
Au used as non-toxic. 1mm of 24 Carat for adequate shielding and thin.
Plastic cap goes between Au and eyelid to protect the inner eyelid from high intensity, low energy backscatter

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24
Q

What chamber is used to measure internal shielding in superficial therapy?

A

Grenz-ray chamber (parallel plate)

25
Q

What is the timer end error correction in superficial therapy?

A

The time taken for the machine to ramp up to full dose rate.

This can either be included in the prescription time (overall underdose) or excluded (overall overdose).

26
Q

How is the timer end error correction QA’d in superficial therapy?

A

Give 4 short pulses (t(1)) of radiation with the detector running to get measurement m(1). Then repeat with one long radiation pulse of the same length as the total input short pulses to get measurement M(2).

t(e) = [ 4 * t(1) * ( M(2) - m(1) ) ] / [ 4 * M(2) - m(1) ]

27
Q

How is the timer end error correction determined at commissioning in superficial therapy?

A

Input multiple timings between 0 - 5 in increments of 0.5 seconds and plot the dose. It should be linear. Extrapolate back from the first point that fits the trend at higher doses. The point at which this extrapolated line intercepts the time axis is the timer end error.

28
Q

What is the equation to calculate the prescribed treatment time on an SXT unit?

A

( Prescribed dose / Modified SDR ) + timer end correction factor

Ensure consistency in units: mins & Gy/min or secs & Gy/sec

29
Q

What is the equation to find the depth at which a hot spot may occur when there are two adjacent fields in superficial therapy?

A

d = ( 2 * S ) / [ ( L(1) / SSD(1) ) + ( L(2) / SSD(2) ) ]
where
d is depth
S is is the distance between the two fields at the surface x 2
L is length of each field at the surface
SSD is the source to skin distance

30
Q

How many degrees of freedom does a superficial/orthovoltage unit have?

A

6

31
Q

What are the basic tube components in a superficial unit?

A

System held in high vacuum to prevent electron deflection via stray atoms between cathode and anode.
Metal housing
Araldyte conductor, rubber insulator, ceramic insulator.
Cathode containing: Electrons generated by a filament and focussed by focussing cup.
H20 cooled target
Maintained within a Cu earthed anode
Exit through Beryllium window

32
Q

Describe the cathode within a superficial unit.

A

Electrically heated Tungsten (W) filament:

  • High melting point
  • High atomic number, Z(=74), therefore low binding energy of outer shell electrons allows…

Electron production via Thermionic Emission

Filament current determines:

  • Tube Current (mA) proportional to number electrons accelerated per unit time
  • X-ray intensity (no. photons per photon energy), I proprtional to mA

Focusing Cup at high potential - Peak kilovoltage (kVp):

  • Max energy attainable by electrons during acceleration
  • Higher kVp produces more X-rays - I proportional to kVp^2
33
Q

Describe the target/anode in the superficial unit.

A

Non-rotating reflection target at ground potential

De-accelerates beam of electrons

Results in energy loss via:

  • Collision (~99%)
  • Heating requires high melting point material (e.g. W)
  • Embedded in conductive anode
  • Anode is water or oil cooled

Radiation (~1%):

  • Characteristic X-rays proprtional to Z^3
  • Bremsstrahlung radiation proportional to Z^2
34
Q

Describe the heel effect in superficial therapy.

A

Inhomogeneous distribution along cathode-anode axis

Result of differential self-attenuation within anode

Extent dependent on:
Angle of anode
Initial X-ray spatial distribution

Typical target angles of:
45 degrees (superficial energies)
30 degrees (orthovoltage energies)
35
Q

What causes the self attenuation within the anode seen in superficial therapy?

A

The heel effect

36
Q

Describe the filtratin within the superficial unit.

A

Desirable to preferentially absorb ‘softer’ X-rays:

  • Not therapeutically useful
  • Only contribute unwanted skin dose
  • Additional filters act to ‘harden’ the beam

Inherent filtration:

  • Lowest energies removed within the anode itself
  • 2.2mm Beryllium window

Choice of materials a compromise to ensure:

  • Mechanical stability (rigidity)
  • Minimal reduction in overall intensity
  • Minimal production of characteristic (K-edge) energies

Generally medium-Z filters are used

  • Aluminium, Al (Z = 13) 0.5-3mm (for superficial (50-150kV))
  • Copper, Cu (Z = 29) 1-4mm (for ortho (highest Z nearest source with locked orientation for safety)

Overall intensity reduced but mean kV increases

37
Q

What is the maximum energy of a superficial beam?

A

kVp

38
Q

What is the mean energy of a superficial beam?

A

kVp / 3

39
Q

What is the energy range of a superficial beam?

A

kVp - min KeV

40
Q

What safety systems are available on a superficial unit?

A

Unsealed ionisation monitor chamber (orthovoltage)

Timers:

  • Primary
  • Back-up

Mechanical interlocks:

  • Filters
  • Applicators
  • Doors
41
Q

Where is the reference data for PDDs, TAR (tissue air ratio), and BSFs?

A

BJR Supplement 25 (1996)

42
Q

Describe the PDDs for superficial photons.

A

Little to no build-up i.e. dmax at or near surface

Steep fall-off with depth

Measured via:
Ionisation chambers
GafC film (radiographic unsuitable due to high Z components)

43
Q

Describe the dose distributions for superficial photon.

A

Little to no build-up i.e. dmax at or near surface

Steep fall-off with depth

Measured via:
Ionisation chambers
GafC film (radiographic unsuitable due to high Z components)

44
Q

Describe the term beam quality for superficial photons.

A

Defined in terms of Half Value Layer (HVL)

HVL is a property of the emergent x-ray spectrum

Thickness of material (usually Al or Cu) required to reduce intensity to 50% of un-filtered beam

Accurate values required to determine:
Water-to-air mass energy absorption coefficient ratios
Backscatter factors

‘Good Geometry’ for accurate measurements:
Narrow beam
Scatter free conditions

45
Q

How is the homogeneity factor determined? What is it?

A

1st/2nd HVL
Measure of energy spread:
= 1 for monenergetic beam
< 1 for polyenergetic beam

46
Q

What is the backscatter factor?

A

‘the ratio of water collision kerma at the surface of the patient on the Central AXis (CAX) to that at the same point without the patient’
(Dose at the patient surface will be greater than that measured in air due to backscattered photons)

It is a function of both field size and HVL:

  • As beam quality increases: Bw increases initially due to compton-PE ratio increasing, then it decreases as scattered photons become more forward peaked (>300kV or > 4mm Cu)
  • As applicator increases: Bw increases initially due to larger fraction of photons from edge of field making it to the CAX, then it plateaus as photons do not have enough energy to reach the CAX

Measurement via TLD or parallel-plate chambers

When a (Pb) cut-out is applied, Bw is determined by a ratio of: Bw(cutout) / Bw(applicator)

47
Q

What are the 3 sections in the kV CoP?

A 
Medium kV (0.5-4mm Cu HVL, 180-220kV)
Low kV (1-8mm Al HVL, 50-180 kV)
Very Low kV (0.035-1mm Al HVL)
48
Q

What is the equation to determine the dose to water in the kV CoP?

A

D(w) = X * [( W / e )air] * [( μ(en) / ρ )w/air]
= K * ( 1 - g ) * [( μ(en) / ρ )w/air]
= M * N(k) * [( μ(en) / ρ )w/air]
where
X is charge per unit mass
(W/e) is the average energy required to produce one ion pair in dry air
K(1-g) is the collision kerma = 0 in kV range

49
Q

What type of chamber is the primary stanard for kV energies?

A

Free air ionisation chamber

50
Q

What are the accepted chambers for medium energy orthovoltage)?

A

Secondary Standard: NE2561, NE2611 or NE2571

Field instrument: NE2505/3A, NE2571, NE2581, PTW30002

51
Q

What are the accepted chambers for Low kV (superficial)?

A

Secondary Standard: NE2561, NE2611 or NE2571
Field instrument: Any thimble ionization chamber with volume ≤1.0cm3 for which NK varies smoothly and by ≤5% over the energy range of interest

52
Q

What are the accepted chambers for very low kV?

A

Secondary Standard: PTW23342 (0.02 cm3) or PTW23344 (0.2 cm3) soft-x-ray ionization chamber
Field Instrument: Any parallel-plate ionization chambers with volume 0.02–0.8 cm3. These should have an NK which varies smoothly and by ≤5% over the energy range of interest

53
Q

What is the setup for cross-calibrations in medium energy kV?

A

Measurements at 2cm in full scatter
Chambers side by side, then swap
D(w) = M * N(k) * k(ch) * [( μ(en) / ρ )w/air] * f(elec)

54
Q

What is the setup for cross-calibrations in low energy kV?

A

Measurements in air
Dose / dose rate defined at patient surface (i.e. end of applicator)
Chambers side by side, then swap
D(w) = M * N(k) * [( μ(en) / ρ )w/air] * B(w) * ISL * f(elec)

55
Q

What kV QC should be carried out weekly? What is the tolerance?

A

Calibration constancy (5%)

56
Q

What kV QC should be carried out daily? What is the tolerance?

A 
Door interlocks (functional)
Filter interlocks (functional) 
Other interlocks (functional)
Calibration constancy (5%)
57
Q

What kV QC should be carried out monthly? What is the tolerance?

A

Timer/Monitor Accuracy (2%)
Homogeneity (-10%)
Collimator/radiation axis (2mm)

58
Q

What kV QC should be carried out yearly? What is the tolerance?

A 
Absolute calibration (3%)
Timer/Monitor linearity/End effect (1%)
Beam quality (10%)
Applicator ratios (3%)

Y3: Superficial Radiotherapy (5 decks)

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