Tomo Flashcards
What is tomo
CT guided Helical IMRT
Radioactive units with Precision planning system
How does Tomo work
Radiation delivered in helical way (couch moving while gantry rotates) -> layered effect
360 degree rotation
FFF 6MV photon beam — dose rate 1180MU/Min
85cm bore (40cm imaging diameter FOV)
MLC window width (sup-inf)
Max treatment width - 40cm, max treatment length 135cm
TomoTherapy helical delivery
Conformal dosimetric is achieved through:
- bends the dose to conform tightly to the ptv
- in a narrow rotating beam
- with high speed MLCs
- from multiple angles around the target
Slip ring gantry
Continuous radiation through 360 degrees
MLCs can change every 7 degrees = 51 dynamic arc segments
64 beamlets per projection
Binary MLC
Pneumatically driven leaves
Transverse motion control
10cm thick
Open/close time = 20ms
Leaf width = 6.25cm at iso
Interleaf transmission = 0.5% in field and 0.25% out of field
What sites are often treated
Head and neck
Prostate and pelvis - due to better imaging now
Treatment bunker
Noisy - 78 decibels (85 can cause permanent damage)
Cold room
Control panel (touch screens)
Couch weight limit - 200kgs
Auto load and unoad
Machine considerations
Tall couch causes issues with patient mobility
Couch catcher - reduce sag and increased weight limit
Daily scanning
Time to complete scan - full CNS 90sec
Whole treatment area included daily
Sticky scan limits - saves parameters
Only Sag axis to choose scan limits
TomoTherapy image registration
6 DOF can be reviewed but only 4 can be corrected
- sup/inf
- ant/post
- left/right
- roll
Imaging tolerance
Pitch and yaw incorrect —> patient must be repositioned
Patients with lateral volumes must be offset and indexed on the bed as lateral movement is limited to +/-20mm
Weight loss and volume changes
Most common problem as Head and neck is biggest patient load
Requires MDT- nursing, dietitian, speech pathologist
PEGs are also offered - can lead to increase in recovery time
How to deal with weight loss
Foam can be added to areas of vac lock bag
Plan adaptive may be required- depending on where the foam is placed
TomoTherapy planning
Form of IMRT
Lots of contouring - can take between 1-6hrs
Planners check RO volumes before commencing outlining for errors
RO delineated contours
GTV, CTV, PTV
Optic chasm, brain stem, pituitary, cochlea, lacrimal gland
Pharyngeal constrictors, larynx, trachea, salivary glands, brachial plexus
RT contouring
PRV
Tuning/avoid structures
Ring volumes
OAR OPT
PTV eval
Shoulder blocks
Beamlets
Set number of beamlets available in a plan
Beamlets assigned to each structure depending on how it is meeting tolerances
When it has achieved constraints to a structure, it will divide that structure’s remaining beamlets over any remaining structures that are not achieving constraints
Planning structures close together
Decreased control over the dose —> beamlets from one structure cna be affected by the other
Planning overlapping structures
When two structures of the same kind overlap, the number assigned to them dictates their overlap priority
Smaller the number —> higher the priority
External should be last in your list
Planning aims
Coverage of PTV
Reduce OAR dose
Maintain reasonable treatment time (beam on)
Head nd neck plans - 5-7min
TNI/CNS - 14MIN
Field width
Defined by primary Colli jaws
Three setting - 1, 2.5 and 5 cm
Field width is selected by planner according to length of treatment volume and degree of variation in target contour sup inf
Smaller field widths improve dose modulation in longitudinal distance but will increase treatment time
Dynamic jaws
Reduced penumbra
Tighter dose gradients
Faster treatment
Pitch
Distance couch travels per rotation of the gantry divided by field width
Tighter pitch - increased number of rotations-> more beamlets
Pitch <1 means the sup and inf edges of primary beam will overlap
Threading
Pitch = 0.86/integer
Result of helical beam junctinioning causing dose variation
Modulation factor
Refers to the limit of the range of leaf intensity values allowed ie: the
amount your MLCs leaves can remain open for
Increasing the modulation factor =
• better coverage
• Longer treatment time (slower gantry rotation)
• Potentially higher hotspot (leaves open more/longer)
Calculated as:
Max open time
Average open time
Modulation factor of 1.00 = equal
intensity values for all beamlets
Values set between 1.8-3.5
Blocks - directional
Primary beams may pass through a directional blocked structure if they pass through a target structure first - used for shoulders in H+N planning
Blocks - complete
Primary beams cannot pass through the structure
Can be used for eyes in complex h+n and brain planning
What cannot be changed after planning is started
Cannot change field width, pitch, contours, overlap priority
Gantry period
Indicates quality of the plan/gantry rotation speed
Changing modulation factor changes gantry period
Optimal gantry period = 20sec
Final calculation
Tomo will dump non-treatable beamlets and there will be a difference between optimised plan and final calculation
DQA
Dosimetry quality assurance
DQA process
- Tomotherapy plan is calculated on Mr Cheese phantom
- Ion Chamber measurement in homogenous dose region (≤ 0.5Gy
variation) - Film evaluates geometric conformity
- Ion chamber and film are positioned in phantom
- Phantom receives a fraction of the treatment
- Optical density of film is measured
- Failed DQA can mean total replan – reasons for DQA fail not always
clear
Plan adaptive
If significant change is observed - all fractions can be recalculated - takes 24 hours
Daily imaging -> evaluation -> significant change observed -> planning notified and patient enrolled -> plan adaptive -> possible RO review -> continue with treatment/replan
Synchrony
Roll cannot be changed during treatment
1cm field width - 2cm motion correction in sup/inf
2.5cm field width - 1.25cm
Allows real time tracking
Sup inf movement control = jaw
Ant-post control = gantry
Left -right control = mlc
Synchrony imaging
Perpendicular arrangment of kV imager and panel
Allows for imaging independently from treatment delivery
KV imaging up to 6 images/gantry rotation
No issues with FOV related to treatment field width
Synchrony tracking modes
Fiducial tracking prostate
Respiratory tracking lung
Respiratory and fiducials abdomen
Synchrony respiratory requirements
Regular breathing cycle
During a fraction, wont line back to sim
Tumour requirements - respiratory tracking
Above diaphragm, surrounded by lung tissue
Larger than 1.5cm but less than 8cm
Fiducial tracking
Reactive positioning
Assumes no motion between images
Respiratory tracking
Proactive positioning - both changing in every imaging and also surface guidance
Uses predictive model - verified with imaging
Daily kVCT versus MVCT
MVCT - 2.5cGY per scan
KVCT - 1 cGy
Variable slice thickness available
FOV limited for imaging - 40cm
Loss of distinction between soft tissue and bone in MVCT
Image matching process
- Sagittal: check for pitch across treatment volume, correct ant/post and sup/inf
- Coronal: check for yaw, check positioning left and right
- Transverse: check for roll, make final adjustments in left-right and ant-post
Synchrony summary
- allows monitoring and reporting on positioning during treatment delivery
- image verification frequent and at regular intervals
- AI/predictive algorithms adjust field placement
- RTs review all imaging verification in real time
- very clear criteria of what patients would be appropriate for synchrony treatment
- fall-back, non-synchrony plan created and approved simultaneously for use in case of failed synchrony treatment
