Tomotherapy Flashcards
What is tomo?
- form of CT guided IMRT
How does tomo work?
- 6MV photon
- radiation delivered in helical manner
- 64 binary MLC
- max treatment width = 40cm
- max treatment length = 160cm
What is the slip-ring gantry?
- continous radiation through 360 degrees
- MLC’s change every 7 degrees
- max 64 beamlets per projection
What are the advantages of helical delivery?
- 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
What are the sim considerations for tomo?
- all patient have CT sim
- reproducible and stable
- visually straight
- good fit shell
- vacbag for all patients
What are the disadvantages inside the treatment room?
- noisy 78dB
- cold 20C
- pinch hazzard at top of bed
What are the advantages inside the treatment room?
- personal control panel touch screens, auto load convenience
- no applicators or heavy equipment thus no lifting
- no gantry to rotate so eliminate crash hazard
What IGRT is used for tomo?
- MVCT
- mean energy 1MV
- 0.5 to 2.5cGy per scan
- slice thickness 2, 4, 6mm
What is a disadvantae of MVCT?
- loss of distinction between soft tissue and bone
What is an adavantage of MVCT?
- improved imaging of high atomic number material (artifact)
What is the image registration process?
- sagittal: check pitch and sup-inf, ant-post
- coronal: check yaw and left-right
- transverse: check roll, left-right and ant-post
What 4 directions can be corrected for?
- sup-inf
- ant-post
- left-right
- roll
How is roll corrected for?
- starting gantry angle changed
How is treatment delivered?
- 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
What are the considerations for paediatrics?
- low dose wash is not ideal
- most require GA
- able to offer retreatment of brain tumours previosly treated
What is a treatment consideration for long treatment fields?
- need to take several MVCT as whole length not possible
What setup changes are commonly made due to weight loss?
- foam added where neck fat used to be
- daily MVCT to verifty foam and patient position
How is tomo planned?
- inverse planning
- a lot of contouring required
How does tomo deliver treatment?
- set number of beamlets available for the plan
- tomo assigns number of beamlets to each structure according to tolerance
What is the average treatment time for H and N?
- 5 to 7 mins
What is the average treatment time for TNI/CNS?
- 14 mins
What are 6 planning aspects?
- field width
- pitch
- modulation factor
- compensation
- blocks (directional vs complete)
- priotities/importance /penalties
What are some considerations with field width?
- deinfed by primary collimator jaw
- three settings: 1, 2.5, 5cm
- smaller field widths improve dose modulation in longitudinal direction but increase treatment time
What is pitch?
- distance the couch travels per rotation of the gantry divided by the field width
What does a tighter pitch increase?
- number of active rotations that treat over a target length
- more beamlets available
What does a pitch <1.0 mean?
- sup and inf edged of the primary beam overlp
Why is a tighter pitch commonly used?
- 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
What is modulation factor?
- referes to the limit of range of leaf intensity values allowed (how long MLC are open)
- 1.0 = equal intensity value for all beamlets
What does increasing the modulation factor allow?
- better coverage
- longer treatment time (slower gantry rotation)
- potentially higher hotspot
What are the two types of block?
- directional
- complete
What is a directional block?
- 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
What is a complete block?
- primary beams cannot pass through the structure
- can be used for eyes in complex h and n or brain planning
Once you begin planning what can you not change?
- field wight
- pitch
- contour of any structure
- overlap prioriy
- use of structure unselected
What is the optimal gantry period?
- 20 seconds
- indicates tomo working efficiency
- allows reasonable treatment time
What happens when a significant change is observed on an MVCT?
- planner notified
- plan adaptive calculation
- RO reveiw
- either replan or continue treatment
What is the dose for a breast?
- 50Gy in 25#
What are the dose tolerances for combined lung, heart and spinal cord?
- Lung: V30<13%
- heart V27<12%
- SC: max 35Gy
