Total Body Irradiation

Question 1

what is the reason for TBI

Answer 1

cytoreductive conditioning regime prior to bone marrow transfer
- immunosuppression
- reducing tumour burden
- eradicating diseased marrow
- deplete the bone marrow to allow physical spare for engraftment of healthy donor marrow
- eradication of cells with genetic disorders

Question 2

what is the dose to the body for TBI

Answer 2

+/- 10% of prescription dose

Question 3

how to produce a large enough beam

Answer 3

• homogeneity
• accuracy
• reproducibility
• ease of setup
• room size
• dedication of room
• specialist equipment
  -> timing is crucial
  -> breakdowns
  -> service etc.

Question 4

High dose

Answer 4

• 75-900cGy in one session
• or in up to 6 fractions of 200cGy each in 3 days

Question 5

what diseases is TBI used for

Answer 5

leukaemia, malignant lymphoma, aplastic anaemia

Question 6

how did TBI used to be delivered

Answer 6

• Co-60 machine dedicated for TBI. the machine collimator removed to obtain large field for TBI irradiation at an SSD of 230cm.
• modified conventional megavoltage radiotherapy equipment: treatment with a translational beam
• modified conventional megavoltage RT equipment: sweeping beam technique with a column mounted linac

Question 7

extended SSD

Answer 7

• conventional equipment
• maximum collimator setting (jaws opened to maximum field size (40cmx40cm))
• large SSD (about 4cm)
• gantry at 90 or 270deg
• beam divergence to produce large field
  -> horizontal beams (primary shielded wall) (bremsstrahlung produces x-rays in a diverged way. You get bremsstrahlung from electrons hitting the target. generally forward bias but it is still a beam that is diverging. width of beam divergence is defined by the primary collimators)
• field size is defined at the isocentre (40cm x 40xm field size at the isocentre means that the field size is larger at the jaws)

Question 8

what is the effect on homogeneity by increasing SSD

Answer 8

you are getting better homogeneity across the patient -> to some extent you are getting some filtering of low energy photons (small effect). Inverse square law. As you increase SSD, better homogeneity

Question 9

what is the effect on homogeneity by increasing energy

Answer 9

better homogeneity. penetration is higher. more of x-rays will go through the patient. energy profile has proportionally fewer low energy photons. there is less side and back scatter at higher energies. Forward scatter, which is electron dose will be forward scattered into the patient. Dmax is further into the patient - larger skin sparing effect, larger build-up region. This would be relatively inhomogeneous. To get dose into this build up region, you can introduce bolus to counteract skin sparing effect because it acts as tissue. This would mean that the build up region would be in the bolus. To get dose into the patients skin you could use electrons. Perspex sheet next to the patient will produce electrons and give dose close to the patients surface. This may be a way of negating the increased penetration
Ideally we should go to higher energies

Question 10

what is the effect on the homogeneity by increasing thickness

Answer 10

less thickness will give us better homogeneity. Lateral fields lose homogeneity because you are going through much more tissue thickness. Ant post is smallest thickness. Slope is smaller for ant-post for smaller tissue thicknesses in attenuation I=I0 e-ux. Turning patient on their side gives a lot less attenuation. This is about attenuation law.
ATTENUATION LAW

Question 11

Energy

Answer 11

• constant SSD and thickness
  -> homogeneity increases with increasing energy
  -> high energy recommended
• constant SSD and energy
  -> homogeneity decreases with increasing thickness. sometimes referred to as the “tissue lateral effect”
  -> ant and post is better than lateral (lower separations
• constant thickness and energy
  -> homogeneity improves with SSD
  -> increased SSD is beneficial

Question 12

dose build up

Answer 12

• TBI doesn’t want skin sparing effect
• extended SSD decreases the skin sparing effect anyway (SSE is driven by the energy of the photons. KERMA and the absorbed dose)
• a bolus or screen is used to bring the surface dose up to 90% of the prescription dose (sometimes bolus between legs is used)
• 1-2cm thick acrylic screen placed as close as possible to the patient (this produces electrons)

Question 13

Patient positioning

Answer 13

normal extended SSD techniques have different options
- lateral beams
-> lying down
-> sitting/standing
-> arms can be positioned to shadow the lungs
-> inferior dose homogeneity

• AP/PA
  -> lying on their side - difficult to hold for long
  -> sitting/standing (even aided can be difficult)
  -> shielding organs can be difficult
  -> maintaining the position reproducibly can be difficult
• standing

Question 14

planning measurements

Answer 14

include the thickness of the patient, the distance from soles of the feet, and the distance from central axis at the following anatomic points: top of the head, forehead, chin, suprasternal notch, xiphoid, umbilicus, central axis, pelvis, thigh, knees, calves, ankles and toes

Question 15

compensators

Answer 15

• often used for H&N, lung and legs because of the irregularity of body contours
  -> heterogeneity of 10-20% without compensators

Question 16

imaging

Answer 16

• full CT scan
• 3mm slice thickness
• immobilisation devices (knees + arms)
• body, heart, lung, kidney, brain delineated
• the spoilers (electron producing screens) are generally not present at CT and are added later with HU = 0

Question 17

dose

Answer 17

• dose prescription point
  -> often midpoint at the level of the umbilicus
  -> sometimes lung (dose limiting organ) if unshielded
• radiation pneumonitis increased frequency >6Gy. most places limit lung dose to 10Gy
• other sites shield the lung
  -> blocks or compensators
  -> this can create inhomogeneities

recent trend is towards IMRT Boost (in standard isocentric conditions

Question 18

lung (the choices)

Answer 18

1. custom built compensators based on CT data (attached to accessory tray)
2. constant thickness attenuators on or near skin
3. full lung shields (50% transmission) with electron boost
   -> LMA such as cerrobend
4. lung shields for fraction of treatment time
   -> the patients arms can be used for lateral fields but may not be appropriate for children
5. MLCs/ large field IMRT/ Tomotherapy

Question 19

necessary steps

Answer 19

• dose measurement i.e. calibration
  -> against a water phantom is normal
  -> TPR and TMR data at extended SSD distances for calculating MU
• homogeneity measurement
  -> beam flatness and energy
• equipment
  -> ionisation chamber / diodes / TLDs

Question 20

issues

Answer 20

• phantom size is smaller than the field
  -> side scatter is unpredictable
  -> normal consideration is a dosim in a semi infinite slab
• inverse square law means that the ionisation chamber (not the one in the head but the measurement) is very low
  -> signal to noise ratio low (when compare to leakage current)
• ionisation chamber cable may be irradiated ( this can cause spurious (fake) signal )
• unreliability of monitor chambers for long periods of irradiation
• possible lack of diode sensitivity

Question 21

calibration

Answer 21

• small volume (<1cm^3) farmer type ionisation chamber
• in a large water phantom (40 x 40 x 40cm^3)
• chamber should be placed along the line that the axis of the body would be
• then the phantom is moved (while keeping the chamber fixed to give varying depths)
  -> OF (dose per MU) as a function of midline depth
• TMR is not very sensitive to field size at very large field sizes

Question 22

dose per monitor unit (D/MU)

Answer 22

D/MU = k . TMR(d, re) . Sc(rc) . Sp(rc) . (f/f’)^2 . OAR(d) . TF
- D is dose in cGy
- k is 1cGy/MU under reference calibration conditions
- TMR is the tissue-maximum ratio at depth d and field size equivalent to the patient (re)
- Sc is the collimator scatter factor for the field size projected at isocentre (rc)
- Sp is the phantom scatter factor for the patient-equivalent field size (re)
- f is the source to calibration point distance
- f’ is the source to patient axis distance at the prescription point
- OAR is the off-axis ratio at depth d
- TF is the transmission factor for the block tray, beam spoiler, or any other absorber placed between the machine diaphragm and the patient

Question 23

field setup

Answer 23

• lateral fields
• most of the MUs delivered in open field
  -> with MLC shielding for lung, kidney, abdo, brain
  -> boots as necessary
  -> tissue compensation between legs
• verification
  -> diodes taped onto patient at head, arm, abdo, legs - entrance and exit