Week 3 - Tomotherapy

Question 1

List the indications for tomotherapy

Answer 1

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

Question 2

What is the maximum treatment width and length for tomotherapy?

Answer 2

Width - 40cm
Length - 135cm

Question 3

State 6 properties of binary MLCs

Answer 3

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)

Question 4

List 2 contraindications for tomotherapy

Answer 4

1. Must lie still for 20 minutes
2. Claustrophobic patients

Question 5

List 5 advantages of helical therapy

Answer 5

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

Question 6

Discuss how corrections work in positioning verification

Answer 6

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

Question 7

Discuss how changing the modulation factor influences dose and delivery

Answer 7

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

Question 8

Explain the main comparative features between tomotherapy and VMAT

Answer 8

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

Question 9

List 7 tomotherapy considerations

Answer 9

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

Question 10

What is the impact of smaller field widths in tomotherapy?

Answer 10

Improve dose modulation in longitudinal direction but increase treatment time

Question 11

Define pitch

Answer 11

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

Question 12

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

Answer 12

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

Question 13

What is threading and how is it reduced?

Answer 13

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

Question 14

Describe the difference between a directional and complete block

Answer 14

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

Question 15

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

Answer 15

Increase the importance of the structure

Question 16

State the 6 steps of DQA

Answer 16

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

Question 17

Discuss 3 main comparison metrics of tomotherapy and VMAT

Answer 17

1. Treatment time
2. Collision risks
3. Breathing control

Question 18

how does tomo work

Answer 18

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

Question 19

bore and FOV diameter

Answer 19

85cm Bore
40cm FOV

Question 20

helical delivery

Answer 20

• 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

Question 21

what do we treat with tomo

Answer 21

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

Question 22

tomo treatment room (bunker)

Answer 22

• 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

Question 23

tomo - machine considerations

Answer 23

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

Question 24

tomo image registration

Answer 24

• 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

Question 25

lateral volumed patients

Answer 25

must be offset and indexed on the bed lateral movement is limited to 2cm either way due to the bore size

Question 26

image registration process

Answer 26

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

Question 27

weight loss and volume change

Answer 27

- 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

Question 28

dealing with weight loss

Answer 28

- 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

Question 29

planning - structures close together

Answer 29

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

Question 30

planning - overlapping structures

Answer 30

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

Question 31

planning aims

Answer 31

- achieve coverage of PTV - reduced dose to OARs - maintain reasonable treatment time (beam on) H&N = 5-7mins TNI/CNS = 14mins

Question 32

planning - field width

Answer 32

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

Question 33

planning - pitch

Answer 33

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

Question 34

planning - modulation factor

Answer 34

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

Question 35

increasing modulation factor

Answer 35

- better coverage - longer treatment time (slower gantry rotation) - potentially higher hotspot (leaves open more/longer)

Question 36

planning - directional blocks

Answer 36

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

Question 37

planning - complete blocks

Answer 37

the primary beams cannot pass through the structure can be used for eyes in complex H&N or brain planning

Question 38

synchrony - tracking modes

Answer 38

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