Lecture 5 Dose Calculation Algorithms And Helical Tomography Flashcards
What are the features of helical tomotherapy
- fast binary MLC
- continuous rotation
- continuous couch movement (can treat up to 135 length)
What are the components of a tomo machine?
- linac gun
- jaws (one set, 23 cm tungsten)
- MLC (64 in total, 32 on each side)
- detector
- beam stop (12.7cm Tungsten = x2 10VL)
What is the SAD and bore size?
Both 85cm
How many beam angles are optimised in tomo?
51 beam angles
The couch velocity is __________.
Constant
What are the dimensions of tomorrow MLC?
10cm thick
2mm wide on linac side
3mm wide on patient side
What is the time required for tomo MLC to switch between open/closed states?
10 msec
What are the jaw options?
1.0 / 2.5 / 5.0 = 3 different settings
Smaller jar size, longer treatment time
Larger jaw size, less conformality
2.5 is the most common
What are the planning parameters can be manipulated in the tomo planning system?
- jaw width
- tightness of helix (ie. Pitch)
- modulation factor
- DVH goals
What is the dose of MVCT image acquisition?
1-3 cGy
What is the benefit of radixact couch over traditional tomo couch?
Radixact couch have independent axes which can eliminate cobra motion and reduce couch sagging
(5mm vs 2mm)
With reference to TG148, what are the QA parameters?
- 85 SSD
- 1.5cm depth
- FS = 5cm x 40cm
- 10s warmup added on delivery side
- configured to 1000MU/min
What is the equation for pitch in tomo?
Couch travel distance per rotation divided by field width
What do pitch <1 means:
Overlap from one rotation to the next
(More tightly wound helix)
Usually must smaller than one
How many beamlets do a single gantry rotation have?
51 (projections) x 64 (beamlets per projection)
= 3264 beamlets
What is a leaf sinogram?
A leaf sinogram s a 2D representation of the leaf pattern necessary to deliver dose to the tumour
What are the setting user input?
- patient image set
- IVDT
- display and use options for structures
- field width
- pitch
- blocking
- modulation factor
- dose calculation grid
What do the minimum planning scan distance beyond the target account for?
- irradiation region
- divergence
- penumbra
What is IVDT?
Image value to density table
= the IVDT converts CTT numbers to mass density
How can minimise thread effect?
- Set up patient with target closer to the central axis
- Use smaller numbers for the pitch (0.86/n)
What is modulation factor (MF)?
= the modulation factor I a number that the user enters to influence the balance between efficiency and conformality
= upper limit on the range of open times for ‘used’ beamlets
Conformality: unequal weighting of beamlets for treating the target and avoid sensitive structures
Efficiency: the most efficient way to deliver all the dose is to weight all the beamlets equally
What is the formula for modulation factor
Max open time divided by average open time
What are typical MF value
Prostate = 1.8
HN = 2.4
What are the effects when adjusting the MMF
Increasing the MF increases delivery time, decrease efficiency
Decrease the MF may degrade dose conformity
How to reduce delivery times?
- use the largest field width that produce an acceptable plan
- use the smallest modulation factor that produces an acceptable plan
Will adjusting the pitch affecting the treatment time?
Th pitch does not affect the treatment time as long as 12 sec <gantry period<60 sec / rotation (gantry speed)
What should we do before machine quality asssurance
- turn the machine on and wait until the machine water temperature reaches 40 degree
- run a 5 min warm up
- run an air scan
