TBI & TSET

Question 1

What is the main patient cohort having TBI?

Answer 1

Leukaemia patients prior to bone marrow transplant

Question 2

What is the aim in TBI and the purpose?

Answer 2

Uniform dose to the whole body
Suppress bone marrow before transplant to reduce risk of host rejection of transplanted stem cells

Question 3

How is TBI carried out?

Answer 3

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

Question 4

Why is extended distance used in TBI?

Answer 4

To treat whole body
To ensure circulating cells don’t evade beam which could happen with matched field technique

Question 5

What are the main steps in the patient pathway?

Answer 5

CT Topogram (to identify lung geometry)
Test dose (to verify bolusing and check uniform dose)
Treatment

Question 6

What is identified at CT topogram?

Answer 6

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

Question 7

Why is bolus used in TBI?

Answer 7

TO produce lateral uniformity

Question 8

What do compensators compensate for?

Answer 8

Thinner separation of patient’s head/neck wrt widest part of body
Lung compensators compensate for air in lungs

Question 9

What other methods could be used for TBI?

Answer 9

Sitting position
Suspended method

Question 10

What is the rational for the use of high energy photons?

Answer 10

Higher dose to tissue in bone
Lower dose to lung
Greater homogeneity in bilateral irradiation

Question 11

What needs commissioning for TBI?

Answer 11

Uniformity (sup-inf and laterally)
Midline dose/ MU vs separation
Midline/surface ratios
Compensators
Verification on rando phantom

Question 12

How is sup/in uniformity measured in TBI?

Answer 12

Place ionisation chamber at different points along sup/inf in water equivalent phantom on TBI couch

Question 13

How is lateral uniformity commissioned for TBI?

Answer 13

Take measurements at different points in phantom, but must use phantoms of different widths (bc this is affected by separation)

Question 14

How is midline dose/ MU vs separation commissioned?

Answer 14

Take measurements of dose at midpoints of phantoms with different widths
Same MU each time
Plot separation vs Gy/MU

Question 15

How are compensator thicknesses commissioned?

Answer 15

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

Question 16

Why must calibration be done?

Answer 16

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

Question 17

How is verification with rando phantom done for TBI?

Answer 17

Place TLDs in different slices of rando phantom and measure delivered doses across body
Should be within 5%

Question 18

For which cohort is TSET used?

Answer 18

Patients with cutaneous group of lymphomas which primarily involve the skin
(mycosis fungoides and sezary syndrome)

Question 19

What is typical technique for delivery of TSET?

Answer 19

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

Question 20

What needs commissioning for TSET?

Answer 20

Lateral field uniformity
Sup/inf uniformity
Surface and midline doses per 1000 MU
PDDs at various sections with film

Question 21

How is lateral field uniformity measured?

Answer 21

Measure with IC with build up cap across beam laterally
Should be within 10%
Not affected by beam angle

Question 22

How is sup/inf uniformity measured TSET?

Answer 22

Take sup/inf measurements with gantry in different positions (pointing up and down) and sum the two

Question 23

Why is PDD different for six beams than single beam?

Answer 23

Dmax shifts towards the surface because of oblique fields, less electrons are travelling perpendicular to the skin surface

Question 24

How is in vivo used in TSET?

Answer 24

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

Question 25

Why do we not treat patients with abutting electron fields?

Answer 25

Not practical, uniformity would be poor at junctions and around curvature
Delivery would be slow