20 chap 18 total body irradiation
TRANSCRIPT
1
Chapter 18 Total Body Irradiation
Used as part of the conditioning regimen for Bone Marrow Transplant (BMT):
• acute lymphoblastic leukemia (ALL)• acute myelogenous leukemia (AML)• chronic myelogenous leukemia (CML)• non Hodgkin’s lymphoma• aplastic anemia• multiple myeloma
2
18.1 Techniques and Equipment
AP/PA TBI: patient standing position
More homogeneous dose distribution, may need partial transmission lung blocks
Patient fatigue
Some protocols require low dose rate ( <15 cGy/min ).
Bilateral TBI: patient sitting or laying down on a couch
Patient comfort
Less homogeneous dose distribution due to variable body thickness, needs compensating blocks.
3
18.1 A – Beam Energy
15% dose spread
Adult AP separation Adult lateral separation
Choice of beam energy depends on patient thickness and degree of dose uniformity.
Dpeak
Dmid
4
18.1 B – Initial Dose Build-up
~ 400 cm
To ensure sufficient skin dose (no skin sparing required for TBI), a 1-2 cm plastic screen is usually used as a beam spoiler.
source
Large distance to provide total body coverage, more homogeneous dose and to reduce dose rate to <15 cGy/min
5
18.1 C – Patient Support/Positioning Devices
Patient support and positioning devices are designed to implement a given treatment technique.
Important criteria include:
• Patient comfort
• Stability
• Reproducibility of set-up and treatment geometry that allows accurate calculation and delivery of dose in accordance with the TBI protocol.
6
18.1 C – Patient Support/Positioning DevicesBilateral Total Body Irradiation
Arms shadow the lung to provide protection
Sagittal laser or floor marks to define setup distance
Compensators used to compensate body parts of small thickness such as head/neck, legs, etc.
7
18.1 C – Patient Support/Positioning DevicesAP/PA Total Body Irradiation
50% transmission block to protect the lungs, chestwall boost by electron beams
Standing position for adults
Laying position for children
8
18.1 D – Dosimetry DataDirect output calibration, table of (output/MU vs d)
Source to TBI treatment distance
Patient midline depth, d
chamber
Large phantom 40x40x40
Maximum collimator setting, collimator angle = 45°
9
18.1 D – Dosimetry Data Calculation formalism
TFdOARffrdTMRrSrSkMUD pppcc )('),()()(/ 2
Dose rate under standard calibration conditions, e.g. 1cGy/MU
Collimator setting at isocenter distance
Field size equivalent to patient field size
Prescription depth, typically patient midline depth at the umbilicus
Source to chamber distance under standard calibration conditions
Source to body axis distance for TBI treatment setup
Off-axis ratio for the prescription point at treatment depth d
Transmission factor for the tray, spoiler screen
10
18.1 E – Compensator Design
The thickness of compensator required along a ray-line depends on:• the tissue deficit compared to the reference depth at the prescription point, • material of the compensator (e.g., its density),• distance of the compensator from the patient, • depth of the point of dose compensation, • field size, • and beam energy
A good approximation for the compensator thickness is given by:
cc TDt
Where
tc = compensator thickness,
TD = tissue deficit,= thickness ratio ~0.7
c = compensator physical density
11
18.1 E – Compensator Design
Alternatively:
dRReff
tdRR
g
OARdAT
dATt
edAT
OARdAT
I
I eff
),(
),(ln
1
),(
),(
where Ig, and I are the doses administered before and after the compensator is added, T(AR,dR) and T(A,d) are the tissue-phantom ratios or TMRs for the reference body section and the section to be compensated for equivalent fields AR and A at midline depths dR and d, OARd is the off-axis ratio at depth d relative to the prescription point, and eff is the effective linear attenuation coefficient for the compensator material measured under TBI conditions.
12
Simple 1D compensators for variable body thicknesses
Simple one-dimensional compensator used for lateral field irradiation technique. The compensator corrects for tissue variations along one line only. The numbers shown in this figure are direct dose measurements. The numbers in parenthesis are calculated from the entrance and exit surface measurements. AAPM report no.17 (1986)
13
18.1 F – In-vivo Patient Dosimetry
In vivo patient dose can be measured with TLD, diodes with suitable buildup bolus.
Expected doses are calculated taking into account thickness, and off-axis ratio.
Measured and expected doses should agree to within ±5%.
Dose uniformity on the patient should be within ±10%.
14
18.1 G – TBI Program Implementation
The use of TBI in conjunction with bone marrow transplantation involves numerous protocols, specifying many different regimens:
single fraction with low dose rate, single fraction with high dose rate, fractionated TBI,hyperfractionated TBI,AP/PA technique,bilateral technique,use of compensators or no compensators, blocking of critical organs or no blocking
Implementation of a TBI program:patient measurements (AP or lateral separation), patient set-up,dosimetry, quality assurance procedures,worksheets specifically designed for TBI.