TBI & TSET Flashcards
What is the main patient cohort having TBI?
Leukaemia patients prior to bone marrow transplant
What is the aim in TBI and the purpose?
Uniform dose to the whole body
Suppress bone marrow before transplant to reduce risk of host rejection of transplanted stem cells
How is TBI carried out?
Patient not on normal couch
At extended distance
Treated with opposed lateral fields for uniformity: turn couch 180 degrees
Bolus achieves uniform thickness
Head/neck and lung compensators used
Perspex side panel increases superficial dose
Why is extended distance used in TBI?
To treat whole body
To ensure circulating cells don’t evade beam which could happen with matched field technique
What are the main steps in the patient pathway?
CT Topogram (to identify lung geometry)
Test dose (to verify bolusing and check uniform dose)
Treatment
What is identified at CT topogram?
Lower border of right lung
Tattoo made on patient marking inferior border of lung at full inspiration
Compensator then made based on lung length/thickness/patient width
Why is bolus used in TBI?
TO produce lateral uniformity
What do compensators compensate for?
Thinner separation of patient’s head/neck wrt widest part of body
Lung compensators compensate for air in lungs
What other methods could be used for TBI?
Sitting position
Suspended method
What is the rational for the use of high energy photons?
Higher dose to tissue in bone
Lower dose to lung
Greater homogeneity in bilateral irradiation
What needs commissioning for TBI?
Uniformity (sup-inf and laterally)
Midline dose/ MU vs separation
Midline/surface ratios
Compensators
Verification on rando phantom
How is sup/in uniformity measured in TBI?
Place ionisation chamber at different points along sup/inf in water equivalent phantom on TBI couch
How is lateral uniformity commissioned for TBI?
Take measurements at different points in phantom, but must use phantoms of different widths (bc this is affected by separation)
How is midline dose/ MU vs separation commissioned?
Take measurements of dose at midpoints of phantoms with different widths
Same MU each time
Plot separation vs Gy/MU
How are compensator thicknesses commissioned?
Change compensator thickness until dose to midline = dose between lungs or dose to head
Take these measurements for a range of different sizes and then extrapolate
Why must calibration be done?
dose/MU is not linear down to very small doses delivered at test dose, can’t assume that we can measure this correctly with TLDs
Need to correct for output when necessary, when output is low need to scale it
How is verification with rando phantom done for TBI?
Place TLDs in different slices of rando phantom and measure delivered doses across body
Should be within 5%
For which cohort is TSET used?
Patients with cutaneous group of lymphomas which primarily involve the skin
(mycosis fungoides and sezary syndrome)
What is typical technique for delivery of TSET?
6 field stanford technique where patient stands in different positions and rotates 60 degrees each time
Two fields delivered per psoition
HDRE beam used to keep treatment time reasonable
What needs commissioning for TSET?
Lateral field uniformity
Sup/inf uniformity
Surface and midline doses per 1000 MU
PDDs at various sections with film
How is lateral field uniformity measured?
Measure with IC with build up cap across beam laterally
Should be within 10%
Not affected by beam angle
How is sup/inf uniformity measured TSET?
Take sup/inf measurements with gantry in different positions (pointing up and down) and sum the two
Why is PDD different for six beams than single beam?
Dmax shifts towards the surface because of oblique fields, less electrons are travelling perpendicular to the skin surface
How is in vivo used in TSET?
TLDs in place for at least two full fractions
Use more than one TLD at each position to improve the SNR
Can change the dose in later fractions based on measurements
Why do we not treat patients with abutting electron fields?
Not practical, uniformity would be poor at junctions and around curvature
Delivery would be slow
