Week 3 - Tomotherapy Flashcards
List the indications for tomotherapy
- Full CNS
- Nasopharynx
- Full nodal irradiation
What is the maximum treatment width and length for tomotherapy?
Width - 40cm
Length - 135cm
State 6 properties of binary MLCs
- Pneumatically driven
- Open/close time of 20 ms
- Leaf width is 6.25cm at isocentre
- 10 cm thick
- Interleaf transmission is 0.5% in field and 0.25% out of field
- Can change every 7-degrees (51 dynamic arc segments in 1 arc)
List 2 contraindications for tomotherapy
- Must lie still for 20 minutes
- Claustrophobic patients
List 5 advantages of helical therapy
- Ability to bend the dose
- Narrow rotating beam
- With high speed MLC
- Multiple angles around the target
- Conforms dose to PTV and avoid critical structures
Discuss how corrections work in positioning verification
Tomotherapy couch is not robotic, therefore, corrections in yaw + pitch can not be corrected for
Discuss how changing the modulation factor influences dose and delivery
If the modulation factor is increased, better coverage is achieved, longer treatment time (because of slower gantry rotation), potentially higher hotspot
Explain the main comparative features between tomotherapy and VMAT
- Tomo = 20-30 mins, VMAT 30 mins
- Tomo - daily imaging with no collision
- Tomo = free breathing treatments
List 7 tomotherapy considerations
- CT simulation
- Reproducible and stable
- Visually straight
- Good shell fit for HN patients
- Vac lock bags for all pts
- Dosimetry on tomo planning system
- DQA
What is the impact of smaller field widths in tomotherapy?
Improve dose modulation in longitudinal direction but increase treatment time
Define pitch
Distance of couch travel per rotation of the gantry divided by the field width
What is the impact on treatment if pitch is increased (tighter)? And why would we do this?
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
What is threading and how is it reduced?
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
Describe the difference between a directional and complete block
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
How would you use a priority if the max and min dose were unacceptable?
Increase the importance of the structure
State the 6 steps of DQA
- Calculated on phantom
- Ion chamber measurement
- Film evaluates geometric conformity
- Ion chamber and film positioned in phantom
- Phantom receives fraction
- Optical density of film measured
Discuss 3 main comparison metrics of tomotherapy and VMAT
- Treatment time
- Collision risks
- Breathing control
how does tomo work
radiation delivered in a helical way (couch moving while gantry rotates)
bore and FOV diameter
85cm Bore
40cm FOV
helical delivery
- 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
what do we treat with tomo
- complex tumours (@ Royal)
- H&N
- brain
- lung
- breast
- prostate
tomo treatment room (bunker)
- 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
tomo - machine considerations
- new radixact larger machine –> limited space in the room eg. GA team
- taller couch - patient mobility
tomo image registration
- 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
lateral volumed patients
must be offset and indexed on the bed
lateral movement is limited to 2cm either way due to the bore size
image registration process
firstly automatic match
- sagittal - check for pitch across treatment volume. correct positioning sup-inf and ant-post
- coronal - check for yaw. correct for left-right.
- transverse. check for roll. make final adjustments in left-right and ant-post directions across whole volume
weight loss and volume change
- 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
dealing with weight loss
- 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
planning - structures close together
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
planning - overlapping structures
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
planning aims
- achieve coverage of PTV
- reduced dose to OARs
- maintain reasonable treatment time (beam on)
H&N = 5-7mins
TNI/CNS = 14mins
planning - field width
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
planning - pitch
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
planning - modulation factor
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
increasing modulation factor
- better coverage
- longer treatment time (slower gantry rotation)
- potentially higher hotspot (leaves open more/longer)
planning - directional blocks
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
planning - complete blocks
the primary beams cannot pass through the structure
can be used for eyes in complex H&N or brain planning
synchrony - tracking modes
- fiducial tracking = prostate only, no nodes
- respiratory tacking = lung
- combined = abdomen (liver)
