Lecture 5 Dose Calculation Algorithms And Helical Tomography Flashcards

1
Q

What are the features of helical tomotherapy

A
  • fast binary MLC
  • continuous rotation
  • continuous couch movement (can treat up to 135 length)
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2
Q

What are the components of a tomo machine?

A
  • linac gun
  • jaws (one set, 23 cm tungsten)
  • MLC (64 in total, 32 on each side)
  • detector
  • beam stop (12.7cm Tungsten = x2 10VL)
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3
Q

What is the SAD and bore size?

A

Both 85cm

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4
Q

How many beam angles are optimised in tomo?

A

51 beam angles

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5
Q

The couch velocity is __________.

A

Constant

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6
Q

What are the dimensions of tomorrow MLC?

A

10cm thick
2mm wide on linac side
3mm wide on patient side

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7
Q

What is the time required for tomo MLC to switch between open/closed states?

A

10 msec

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8
Q

What are the jaw options?

A

1.0 / 2.5 / 5.0 = 3 different settings

Smaller jar size, longer treatment time
Larger jaw size, less conformality

2.5 is the most common

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9
Q

What are the planning parameters can be manipulated in the tomo planning system?

A
  • jaw width
  • tightness of helix (ie. Pitch)
  • modulation factor
  • DVH goals
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10
Q

What is the dose of MVCT image acquisition?

A

1-3 cGy

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11
Q

What is the benefit of radixact couch over traditional tomo couch?

A

Radixact couch have independent axes which can eliminate cobra motion and reduce couch sagging
(5mm vs 2mm)

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12
Q

With reference to TG148, what are the QA parameters?

A
  • 85 SSD
  • 1.5cm depth
  • FS = 5cm x 40cm
  • 10s warmup added on delivery side
  • configured to 1000MU/min
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13
Q

What is the equation for pitch in tomo?

A

Couch travel distance per rotation divided by field width

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14
Q

What do pitch <1 means:

A

Overlap from one rotation to the next
(More tightly wound helix)
Usually must smaller than one

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15
Q

How many beamlets do a single gantry rotation have?

A

51 (projections) x 64 (beamlets per projection)
= 3264 beamlets

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16
Q

What is a leaf sinogram?

A

A leaf sinogram s a 2D representation of the leaf pattern necessary to deliver dose to the tumour

17
Q

What are the setting user input?

A
  • patient image set
  • IVDT
  • display and use options for structures
  • field width
  • pitch
  • blocking
  • modulation factor
  • dose calculation grid
18
Q

What do the minimum planning scan distance beyond the target account for?

A
  • irradiation region
  • divergence
  • penumbra
19
Q

What is IVDT?

A

Image value to density table
= the IVDT converts CTT numbers to mass density

20
Q

How can minimise thread effect?

A
  1. Set up patient with target closer to the central axis
  2. Use smaller numbers for the pitch (0.86/n)
21
Q

What is modulation factor (MF)?

A

= the modulation factor I a number that the user enters to influence the balance between efficiency and conformality

= upper limit on the range of open times for ‘used’ beamlets

Conformality: unequal weighting of beamlets for treating the target and avoid sensitive structures

Efficiency: the most efficient way to deliver all the dose is to weight all the beamlets equally

22
Q

What is the formula for modulation factor

A

Max open time divided by average open time

23
Q

What are typical MF value

A

Prostate = 1.8
HN = 2.4

24
Q

What are the effects when adjusting the MMF

A

Increasing the MF increases delivery time, decrease efficiency

Decrease the MF may degrade dose conformity

25
How to reduce delivery times?
- use the largest field width that produce an acceptable plan - use the smallest modulation factor that produces an acceptable plan
26
Will adjusting the pitch affecting the treatment time?
Th pitch does not affect the treatment time as long as 12 sec
27
What should we do before machine quality asssurance
- turn the machine on and wait until the machine water temperature reaches 40 degree - run a 5 min warm up - run an air scan