Tomotherapy

Question 1

What is tomo?

Answer 1

• form of CT guided IMRT

Question 2

How does tomo work?

Answer 2

• 6MV photon
• radiation delivered in helical manner
• 64 binary MLC
• max treatment width = 40cm
• max treatment length = 160cm

Question 3

What is the slip-ring gantry?

Answer 3

• continous radiation through 360 degrees
• MLC’s change every 7 degrees
• max 64 beamlets per projection

Question 4

What are the advantages of helical delivery?

Answer 4

• by delivering dose in narrow rotating beam, high speed MLC and from multiple angles around the target
• able to bend dose to conform tightly to the PTV and avoid OAR

Question 5

What are the sim considerations for tomo?

Answer 5

• all patient have CT sim
• reproducible and stable
• visually straight
• good fit shell
• vacbag for all patients

Question 6

What are the disadvantages inside the treatment room?

Answer 6

• noisy 78dB
• cold 20C
• pinch hazzard at top of bed

Question 7

What are the advantages inside the treatment room?

Answer 7

• personal control panel touch screens, auto load convenience
• no applicators or heavy equipment thus no lifting
• no gantry to rotate so eliminate crash hazard

Question 8

What IGRT is used for tomo?

Answer 8

• MVCT
• mean energy 1MV
• 0.5 to 2.5cGy per scan
• slice thickness 2, 4, 6mm

Question 9

What is a disadvantage of MVCT?

Answer 9

• loss of distinction between soft tissue and bone

Question 10

What is an advantage of MVCT?

Answer 10

• improved imaging of high atomic number material (artifact)

Question 11

What is the image registration process?

Answer 11

1. sagittal: check pitch and sup-inf, ant-post
2. coronal: check yaw and left-right
3. transverse: check roll, left-right and ant-post

Question 12

What 4 directions can be corrected for?

Answer 12

• sup-inf
• ant-post
• left-right
• roll

Question 13

How is roll corrected for?

Answer 13

• starting gantry angle changed

Question 14

How is treatment delivered?

Answer 14

• couch moves applied automatically
• treatment time is set and small MU variations can occur day to day
• patient monitored via CCTV
• patient movement detected by couch wil stop treatment

Question 15

What are the considerations for paediatrics?

Answer 15

• low dose wash is not ideal
• most require GA
• able to offer retreatment of brain tumours previosly treated

Question 16

What is a treatment consideration for long treatment fields?

Answer 16

• need to take several MVCT as whole length not possible

Question 17

What setup changes are commonly made due to weight loss?

Answer 17

• foam added where neck fat used to be

- daily MVCT to verifty foam and patient position

Question 18

How is tomo planned?

Answer 18

• inverse planning

- a lot of contouring required

Question 19

How does tomo deliver treatment?

Answer 19

• set number of beamlets available for the plan

- tomo assigns number of beamlets to each structure according to tolerance

Question 20

What is the average treatment time for H and N?

Answer 20

5 - 7 mins

Question 21

What is the average treatment time for TNI/CNS?

Answer 21

14 mins

Question 22

What are 6 planning aspects?

Answer 22

• field width
• pitch
• modulation factor
• compensation
• blocks (directional vs complete)
• priorities/importance /penalties

Question 23

What are some considerations with field width?

Answer 23

• defined by primary collimator jaw
• three settings: 1, 2.5, 5cm
• smaller field widths improve dose modulation in longitudinal direction but increase treatment time

Question 24

What is pitch?

Answer 24

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

Question 25

What does a tighter pitch increase?

Answer 25

- number of active rotations that treat over a target length | - more beamlets available

Question 26

What does a pitch <1.0 mean?

Answer 26

- sup and inf edged of the primary beam overlap

Question 27

Why is a tighter pitch commonly used?

Answer 27

- helps with targets that are offset laterally as there is limited entry angles - allows you to maximise the MLC movement within the amount of treatment angles you use - reduces threading

Question 28

What is modulation factor?

Answer 28

- refers to the limit of range of leaf intensity values allowed (how long MLC are open) - 1.0 = equal intensity value for all beamlets

Question 29

What does increasing the modulation factor allow?

Answer 29

- better coverage - longer treatment time (slower gantry rotation) - potentially higher hotspot

Question 30

What are the two types of block?

Answer 30

- directional | - complete

Question 31

What is a directional block?

Answer 31

- the primary beam may pass through a directional-blocked structure if they pass through a target first - often used for shoulders in h and n

Question 32

What is a complete block?

Answer 32

- primary beams cannot pass through the structure | - can be used for eyes in complex h and n or brain planning

Question 33

Once you begin planning what can you not change?

Answer 33

- field weight - pitch - contour of any structure - overlap priority - use of structure unselected

Question 34

What is the optimal gantry period?

Answer 34

- 20 seconds - indicates tomo working efficiency - allows reasonable treatment time

Question 35

What happens when a significant change is observed on an MVCT?

Answer 35

- planner notified - plan adaptive calculation - RO Review - either replan or continue treatment

Question 36

What is the dose for a breast?

Answer 36

-50Gy in 25#

Question 37

What are the dose tolerances for combined lung, heart and spinal cord?

Answer 37

- Lung: V30<13% - heart V27<12% - SC: max 35Gy