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ABR Part III > Treatment Planning > Flashcards

Treatment Planning Flashcards

1
Q

What is a typical lung treatment regimen?

A

36 Gy x 18 fxns (1.8-2 Gy/fxn) 3D palliative
48-60 Gy in 5 fxns (12 Gy/fxn) SBRT

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2
Q

According to QUANTEC on H&N txt using conventional fractionation to 70 Gy, what are the suggested mean dose guidelines for one parotid gland and total (combination) of both glands, respectively, to reduce or eliminate xerostomia?

A

20 Gy individual, 25 Gy combined

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3
Q

How do you calculate cumulative dose from a permanent brachytherapy implant?

A

D = (initial dose rate) * 1.44 * T1/2

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4
Q

How do you calculate average wedge transmission?

A

T_avg = 1/(mut)(1-e(-mut))

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5
Q

How much does an Ir-192 source decay per day?

A

~1%/day

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6
Q

How much more dense is lead than water?

A

11 times

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7
Q

What is the equation for conformity index?

A

CI = V_Rx/V_PTV

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8
Q

What is the equation for the Dose Gradient Index?

A

DGI = V_50%/V_100% - calculated relative to the Rx dose

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9
Q

What is the equation for the homogeneity index?

A

HI = (D2% - D98%)/D50%

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10
Q

How do you calculate effective SSD for an electron beam?

A

OF2 = OF1*(SSD+dmax)/(SSD+g+dmax)

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11
Q

What was the largest single factor affecting agreement between TPS and MU verification programs in TG114?

A

Surface irregularities

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12
Q

What are the 2 reasons you lose electronic equilibrium?

A

1) inhomogeneities 2) small fields

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13
Q

In what ways should MU verification be independent of primary MU calc?

A

1) different algorithm/method 2) independent data files, even if they draw from the same measured data 3) independent person running them

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14
Q

How do you calculate D90 for an electron beam?

A

D90 ~ E/3.2

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15
Q

How do you calculate D80 for an electron beam?

A

D80 ~ E/2.8

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16
Q

How do you find the practical range of an electron beam?

A

Rp ~ E/2

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17
Q

How does electron surface dose vary with energy?

A

Surface Dose goes up with increasing energy

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18
Q

How does electron surface dose vary with increasing SSD?

A

Surface dose goes up with increasing SSD

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19
Q

How does electron surface dose vary with increasing field size?

A

surface dose goes down with increasing field size, up to field size ~ Rp, then it is pretty constant.

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20
Q

How do you calculate R50 for an electron beam?

A

~E/2.33

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21
Q

What margin is used for SRS/SBRT?

A

<= 1 mm

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22
Q

How do you ensure patients receiving SRS/SBRT don’t move during treatment?

A

1) mid-txt imaging 2) surface imaging 3) bb on nose with infrared imaging 4) fast treatments with FFF beams

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23
Q

How do you get TERMA

A

ray tracing to get your fluence, then fluence * total mass attenuation coefficient (u/rho) in each voxel

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24
Q

What is convolution and what is convolved in a convolution superposition algorithm?

A

Convolution is a mathematical process between 2 functions to create a 3rd function. The result is how one function is modified by the other

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25
Q

What statistical uncertainty does TG105 recommend for treatment planning systems based on monte carlo models?

A

2% in your max dose voxel

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26
Q

What are some safety precautions for working with (or switching to) a Monte Carlo-based TPS?

A

1) need to be very precise in your model design (dimensions/materials) 2) physician may need to modify prescriptions because the same dose in the patient may look different on the computer now 3) make sure statistical uncertainty is low

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27
Q

What are some benefits of cone-based SRS/SBRT?

A
  • sharper penumbra because of less transmission and shorter collimator-to-target distance
  • Great for regularly shaped targets
28
Q

What are some benefits of MLC-based SRS/SBRT?

A
  • and conform to target so great for irregularly shaped targets
  • often faster deliveries and may require fewer arcs to achieve conformality
  • —MLCs require more QA (?)
29
Q

What is a standard prescription for trigeminal neuralgia?

A

85 Gy in 1 fraction

30
Q

What is an output factor?

A

It relates the dose from any sized field to that of the reference field size (10x10)

31
Q

What is the ICRU 62 definition of the treated volume?

A

The volume of tissue that receives the prescription dose or higher

32
Q

What is the ICRU 62 definition of the irradiated volume?

A

The volume of tissue that receives 50% of the prescription dose or higher

33
Q

What is the dose limit to the spinal cord?

A

45 Gy (2 Gy/fxn)

34
Q

What does TD5/5 mean?

A

the tolerance dose that corresponds to a 5% occurrence within 5 years

35
Q

Where to tolerance doses come from?

A

Emami, QUANTEC

36
Q

What things do you need to consider when introducing a new treatment technique?

A
  • get your method validated by something like IROC (imaging and radiation oncology core)
  • decide on your general method (3D, arc, tomo)
  • ensure you have the proper immobilization/IGRT
  • consider your TPS and its limitations for calculating your chosen technique
37
Q

What is the PDD at a depth of 10cm in a Co-60 beam?

38
Q

What is the PDD at a depth of 10 cm in a 6 MV beam?

39
Q

What is the PDD at a depth of 10 cm in a 10 MV beam?

40
Q

What is the PDD at a depth of 10 cm in an 18 MV beam?

41
Q

What is the attenuation in water of a 10 MV beam?

42
Q

What is the attenuation in water of an 18 MV beam?

43
Q

What is the attenuation in water of a Co-60 beam?

44
Q

What QA would you perform for a cone-based SRS treatment?

A
  • Safety: SRS interlocks (cone size, backup jaws, etc.)
  • Mechanical: collimator size indicator correctness
    • radiation & mechanical isocenter coincidence (winston & lutz)
  • Dose: output factor constancy
45
Q

What is the definition of a hounsfield unit?

A

HU = (muvoxel - muwater)/muwater * 1000

46
Q

Why is a CT calibration curve (HU to physical or electron density) nonlinear?

A

Because there is a higher proportion of high-z materials in high density tissues. This increases the incidence of photoelectric interaction, and therefore the HU more rapidly for equal changes in physical/electron density.

47
Q

What are some variance reduction methods in monte carlo treatment planning?

A

Bremsstrahlung splitting, phase space files, range rejection

48
Q

What is the largest field size the Cyberknife can make and where is it defined? (SAD)

A

6 cm diameter circle at 80 cm. There is not a small MLC that can do 10x10, but the circular cones are still considered standard

49
Q

By how much does a collimator-shaped wedge affect out-of-field dose? what about a physical wedge?

A

physical wedge can increase by a factor of 2, collimator-shaped wedge doesn’t do much because it attenuates the full beam when it is present where as a physical wedge is working by transmission so is introducing a lot of scatter

50
Q

What general things do you check in a treatment plan (EBRT or brachy)?

A
  • correct patient
  • correct prescription
  • correct image details (orientation, immobilization, accessories, etc.)
  • target coverage
  • OAR
  • plan integrity (needle loading via radiograph, plan parameters)
51
Q

What are the action levels for diode in vivo dosimetry?

A

<5%, a ok
5-10%, 2nd therapist review of setup, notify physics
>10%, immediate physics review, typically measure with physics at the next fraction

52
Q

What are the average dimensions of a APBI balloon?

A

4-5 cm diameter

53
Q

what is the WAHA equation?

A

WA = 90 - HA/2

54
Q

What is the dose limit to a pacemaker/implanted cardiac defibrillator from TG34?

A

2 Gy - try to keep 3 cm from the radiation field edge
- if < 2 Gy, low level cardiac monitoring
- if > 2 Gy or highly dependent patient, high-level cardiac monitoring
- if > 10 Gy, consider getting it moved

55
Q

THE PHYSICAL ASPECTS OF TOTAL AND HALF BODY PHOTON IRRADIATION

56
Q

TOTAL SKIN ELECTRON THERAPY: TECHNIQUE AND DOSIMETRY

57
Q

At what depth do you match electron fields (whether electron/electron or electron/photon)?

A

at the surface

58
Q

By how much does ignoring the presence of a metal prosthetic affect your delivered dose from a 4-field box technique?

A

reduced target dose by 5-10%

59
Q

What is the difference between KERMA and TERMA?

A

TERMA includes rayleigh scatter (which is negligible in MV beams, so KERMA ~ TERMA)

60
Q

Why does dmax not necessarily correspond to the point where KERMA and dose cross?

A

Because of electron contamination and backscatter contributions to dose at that point

61
Q

About how much larger is dose than kerma when TCPE exists?

62
Q

What is the definition of penumbra?

A

the distance between the 80/20 or 90/10 isodose levels assessed typically at a depth of 10cm for photons and dmax for electrons

63
Q

What are the contributing components of penumbra?

A
  • geometric
  • transmission
  • scatter
64
Q

What measurements do you need to take when treating a pregnant patients?

A

3 points:
fundus
umbilicus
pubis

65
Q

What is a nomogram?

A

kind of like knowledge-based planning, it was a plot of the total source strength (or activity) that was required to treat a given tumor versus the tumor volume in prostate seed implants. It was useful for speeding up real-time planning and also as a sanity check that what you were doing was consistent with past cases

66
Q

Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

ABR Part III (13 decks)

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