20 chap 18 total body irradiation


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20 chap 18 total body irradiation

  1. 1. Chapter 18 Total Body IrradiationUsed as part of the conditioning regimenfor Bone Marrow Transplant (BMT): • acute lymphoblastic leukemia (ALL) • acute myelogenous leukemia (AML) • chronic myelogenous leukemia (CML) • non Hodgkin’s lymphoma • aplastic anemia • multiple myeloma 1
  2. 2. 18.1 Techniques and EquipmentAP/PA TBI: patient standing position Bilateral TBI: patient sitting or laying down on a couchMore homogeneous dosedistribution, may need partial Patient comforttransmission lung blocksPatient fatigue Less homogeneous dose distribution due to variable body thickness, needs compensating blocks. Some protocols require low dose rate ( <15 cGy/min ). 2
  3. 3. 18.1 A – Beam Energy Choice of beam energy depends on patient thickness and degree of dose uniformity. Dpeak Dmid 15% dose spread Adult AP separation Adult lateral separation 3
  4. 4. 18.1 B – Initial Dose Build-up To ensure sufficient skin dose (no skin sparing required for TBI), a 1-2 cm plastic screen is usually used as a beam spoiler. sourceLarge distance toprovide total bodycoverage, more ~ 400 cmhomogeneous doseand to reduce doserate to <15 cGy/min 4
  5. 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. 5
  6. 6. Bilateral Total18.1 C – Patient Support/Positioning Devices Body Irradiation Sagittal laser or floor marks to define setup distance Compensators used to compensate body parts of small thickness such as Arms shadow the head/neck, legs, lung to provide etc. protection 6
  7. 7. AP/PA Total Body18.1 C – Patient Support/Positioning Devices Irradiation Standing position for adults Laying position for children 50% transmission block to protect the lungs, chestwall boost by electron beams 7
  8. 8. Direct output calibration,18.1 D – Dosimetry Data table of (output/MU vs d) Patient midline depth, dMaximum collimatorsetting, collimator angle =45° chamberSource to TBI treatment distance Large phantom 40x40x40 8
  9. 9. 18.1 D – Dosimetry Data Calculation formalism Dose rate under standard calibration conditions, e.g. 1cGy/MU Collimator setting at isocenter distance Field size equivalent to patient field sizeD / MU = k • S c (rc ) • S p (rp ) • TMR (d , rp ) • ( f f ) 2 • OAR(d ) • TF Prescription depth, typically patient midline depth at the umbilicusSource to chamber distance under standardcalibration conditionsSource 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 9
  10. 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 energyA good approximation for the compensator thickness is given by: t c = TD • (τ ρ c ) Where tc = compensator thickness, TD = tissue deficit, τ= thickness ratio ~0.7 ρc = compensator physical density 10
  11. 11. 18.1 E – Compensator Design Alternatively: I T ( AR , d R ) • OARd −µ t = = e eff Ig T ( A, d ) 1  T ( A, d )  t= ln  µ eff  T ( AR , d R ) • OARd   where Ig, and I are the doses administered before and after the compensatoris added, T(AR,dR) and T(A,d) are the tissue-phantom ratios or TMRs for thereference body section and the section to be compensated for equivalentfields AR and A at midline depths dR and d, OARd is the off-axis ratio at depthd relative to the prescription point, and µeff is the effective linear attenuationcoefficient for the compensator material measured under TBI conditions. 11
  12. 12. Simple 1D compensators for variable body thicknessesSimple one-dimensional compensatorused for lateral field irradiation technique.The compensator corrects for tissuevariations along one line only. Thenumbers shown in this figure are directdose measurements. The numbers inparenthesis are calculated from theentrance and exit surface measurements. AAPM report no.17 (1986) 12
  13. 13. 18.1 F – In-vivo Patient DosimetryIn vivo patient dose can be measured with TLD, diodes with suitablebuildup bolus.Expected doses are calculated taking into account thickness, and off-axisratio.Measured and expected doses should agree to within ±5%.Dose uniformity on the patient should be within ±10%. 13
  14. 14. 18.1 G – TBI Program ImplementationThe use of TBI in conjunction with bone marrow transplantation involvesnumerous 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 blockingImplementation of a TBI program: patient measurements (AP or lateral separation), patient set-up, dosimetry, quality assurance procedures, worksheets specifically designed for TBI. 14