Specicalist Superficical Techniques Flashcards
When is TBI delivered and what is a brief outline of the patient pathway?
Delivered to patients before a bone marrow transplant.
- The patient will come for a CT tomograph of lungs to determine the compensators that are needed.
- Test dose is delivered to sample the uniformity (~8 cGy)
- Treatment delivery.
How is the compensator selected?
The positions of the lungs are marked using a tattoo at the inferior border of the lungs at full inspiration.
Compensator is selected basedon patient geometry - lung length, thickness and patient separation (typically measured at the shoulders, but whereever the patient is thickest).
What is the beam spoiler used for in TBIs?
To introduce scatter and reduce the build-up effect.
Why use the highest possible energy for TBIs?
- Higher dose to tissue within bone.
- lower dose to lungs.
Greater homogeneity in bilateral irradation.
In summary, the dose is more homogeneous overall.
How to commission TBI?
- Measure uniformity.
- Measure midline dose/MU vs separation.
- Midline/surface ratios.
- use of compensation
- verification on rando phantom.
How to measure uniformity in the sup-inf direction for TBI?
Use IC to measure ionisations at a number of positions along the phantom.
How to measure lateral uniformity for TBI?
Use IC at different positions and with different thicknesses of phantom.
Note that it has been found that the uniformity decreases with thicker phantoms. Therefore, we want as little bolus as is possible.
How to measure Dose/MU versus separation for TBI?
Measure the midline dose for a given number of MU (1000 MU or 500 MU, etc) and vary the separation to see effect on dose.
How to determine thickness of compensation required for TBIs?
Measure with IC at the midline thickness. Then measure with IC at the head thickness. The compensation is that required to bring these to measurements to equality. Repeat for a number of head thicknesses.
Same process for lungs, except the lung thickness is varied as well as the separation.
Then verify that the results are consistent when using both at once.
How to calibrate the IC measurements against TLD for TBI?
Position the IC and TLD 1 cm lateral from one another.
Positon where the beam is known to be flat, and generate a factor accounting for the different in the two measurements.
It cannot be assumed that the dose/MU as calibrated for higher doses is valid since the test doses will use very low doses. Therefore, need to calibrate using measurements for a low number of MU.
Account for daily variation in MU output if necessary.
Account for difference between test dose response and delivered dose response.
What is the recommended dose delivery method for TSET?
6 field Stanford method. 6 patient positions, and two differnt beams. This is designed to ensure that a majority of the patient’s surface receives a uniform dose.
What tests need to be done to commission a TSET?
- Lateral uniformity
- Sup - inf uniformity; designed to determine the pairing of gantry angles that will give the most uniform dose.
- Rando phantom surface measurements at a number of gantry angles.
- Film measurement in rando mid-section.
- TLD measurement at the centre of rando to assess mid-body X-ray dose.
- Standard output and energy at the time of commissioning.
