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CSB069 - RT Treatment 4 > Week 3 - Tomotherapy > Flashcards

Week 3 - Tomotherapy Flashcards

1
Q

List the indications for tomotherapy

A
  1. Full CNS
  2. Nasopharynx
  3. Full nodal irradiation
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2
Q

What is the maximum treatment width and length for tomotherapy?

A

Width - 40cm
Length - 135cm

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3
Q

State 6 properties of binary MLCs

A
  1. Pneumatically driven
  2. Open/close time of 20 ms
  3. Leaf width is 6.25cm at isocentre
  4. 10 cm thick
  5. Interleaf transmission is 0.5% in field and 0.25% out of field
  6. Can change every 7-degrees (51 dynamic arc segments in 1 arc)
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4
Q

List 2 contraindications for tomotherapy

A
  1. Must lie still for 20 minutes
  2. Claustrophobic patients
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5
Q

List 5 advantages of helical therapy

A
  1. Ability to bend the dose
  2. Narrow rotating beam
  3. With high speed MLC
  4. Multiple angles around the target
  5. Conforms dose to PTV and avoid critical structures
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6
Q

Discuss how corrections work in positioning verification

A

Tomotherapy couch is not robotic, therefore, corrections in yaw + pitch can not be corrected for

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7
Q

Discuss how changing the modulation factor influences dose and delivery

A

If the modulation factor is increased, better coverage is achieved, longer treatment time (because of slower gantry rotation), potentially higher hotspot

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8
Q

Explain the main comparative features between tomotherapy and VMAT

A
  1. Tomo = 20-30 mins, VMAT 30 mins
  2. Tomo - daily imaging with no collision
  3. Tomo = free breathing treatments
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9
Q

List 7 tomotherapy considerations

A
  1. CT simulation
  2. Reproducible and stable
  3. Visually straight
  4. Good shell fit for HN patients
  5. Vac lock bags for all pts
  6. Dosimetry on tomo planning system
  7. DQA
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10
Q

What is the impact of smaller field widths in tomotherapy?

A

Improve dose modulation in longitudinal direction but increase treatment time

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11
Q

Define pitch

A

Distance of couch travel per rotation of the gantry divided by the field width

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12
Q

What is the impact on treatment if pitch is increased (tighter)? And why would we do this?

A

Increased number of active rotations therefore more beamlets helps with lateral targets as angles are usually limited and you want to maximise MLC movement it also reduces threading

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13
Q

What is threading and how is it reduced?

A

Threading is the result of helical beam junctioning causes dose variation it can be reduced by making the pitch an integer divisible by 0.86

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14
Q

Describe the difference between a directional and complete block

A

A beam may pass through a directional block if it first passes a target however a complete block means the primary beam cannot pass through the structure at all

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15
Q

How would you use a priority if the max and min dose were unacceptable?

A

Increase the importance of the structure

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16
Q

State the 6 steps of DQA

A
  1. Calculated on phantom
  2. Ion chamber measurement
  3. Film evaluates geometric conformity
  4. Ion chamber and film positioned in phantom
  5. Phantom receives fraction
  6. Optical density of film measured
17
Q

Discuss 3 main comparison metrics of tomotherapy and VMAT

A
  1. Treatment time
  2. Collision risks
  3. Breathing control
18
Q

how does tomo work

A

radiation delivered in a helical way (couch moving while gantry rotates)

19
Q

bore and FOV diameter

A

85cm Bore
40cm FOV

20
Q

helical delivery

A
  • in a narrow rotating beam
  • with high speed MLCs
  • from multiple angles around the target

we are able to “bend” the dose to conform tightly to the PTV or avoid nearby critical structures

21
Q

what do we treat with tomo

A
  • complex tumours (@ Royal)
  • H&N
  • brain
  • lung
  • breast
  • prostate
22
Q

tomo treatment room (bunker)

A
  • noisy (78dB)
  • cold room (max 20 degrees)
  • autoload/unload
  • no applicators/heavy equipment (workplace health and safety)
  • no gentry to rotate (no crash hazard)
  • couch weight limit = 200kg
23
Q

tomo - machine considerations

A
  • new radixact larger machine –> limited space in the room eg. GA team
  • taller couch - patient mobility
24
Q

tomo image registration

A
  • 6 DOF can be reviewed in registration but only 4 can be corrected
    • sup inf
    • ant post
    • left right
    • roll

if pitch and yaw are incorrect, the patient must be repositioned

25
Q

lateral volumed patients

A

must be offset and indexed on the bed

lateral movement is limited to 2cm either way due to the bore size

26
Q

image registration process

A

firstly automatic match

  1. sagittal - check for pitch across treatment volume. correct positioning sup-inf and ant-post
  2. coronal - check for yaw. correct for left-right.
  3. transverse. check for roll. make final adjustments in left-right and ant-post directions across whole volume
27
Q

weight loss and volume change

A
  • weight loss is the most common problem
  • lots of nursing, dietician and speech pathology support offered to these patients as standard to help control weight loss
  • PEGs offered to high risk patients
28
Q

dealing with weight loss

A
  • foams can be added to areas of vag bag where weight loss has occurred
    • eg. back of the neck –> affecting the pitch –> foams can be added under the neck
  • with a shell not fitting, the patient will need a replan with the new shell
  • daily imaging will ensure the foam has been placed correctly and achieves the desired goal
29
Q

planning - structures close together

A

when structures are very close together, there is decreased control over the dose. this is because beamlets from one structure can be affected by the other

30
Q

planning - overlapping structures

A

when structures are overlapping, the number assigned to them dictates their overlap priority

the smaller the number, the higher the priority, therefore external should be the last in your list

31
Q

planning aims

A
  • achieve coverage of PTV
  • reduced dose to OARs
  • maintain reasonable treatment time (beam on)

H&N = 5-7mins
TNI/CNS = 14mins

32
Q

planning - field width

A

three settings = 1cm, 2.5cm and 5cm

field width is selected by planner according to length of treatment volume and degree of variation in the target contour in the sup/inf aspects

smaller field widths improve dose modulation in the longitudinal direction, however, will increase your treatment time

33
Q

planning - pitch

A

distance the couch travels per rotation of the gantry divided by the field width

tighter pitch = increased number of active rotations that treat over a target length. more beamlets available

pitch < 1 means the superior and inferior edges of the primary beam overlap

34
Q

planning - modulation factor

A

refers to the limit of the range of leaf intensity values allowed ie. the amount your MLCs leaves remain open for

measure of how hard your MLCs are working

35
Q

increasing modulation factor

A
  • better coverage
  • longer treatment time (slower gantry rotation)
  • potentially higher hotspot (leaves open more/longer)
36
Q

planning - directional blocks

A

the primary beams may pass through a directional-blocked structure if they pass through a target first

often used for shoulders in H&N planning

37
Q

planning - complete blocks

A

the primary beams cannot pass through the structure

can be used for eyes in complex H&N or brain planning

38
Q

synchrony - tracking modes

A
  • fiducial tracking = prostate only, no nodes
  • respiratory tacking = lung
  • combined = abdomen (liver)

CSB069 - RT Treatment 4 (7 decks)

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