Total body irradiation

Total body irradiation

1. TBI- Total BODY IRRADIATION DR Kiran

2. TBI Total body irradiation (TBI) with MV photon beams is most commonly used as part of the conditioning regimen for bone marrow transplantation It is used in the treatment of a variety of diseases such as Leukemia aplastic anemia Lymphoma multiple myeloma autoim­mune diseases

3. Role of TBI The role of TBI is to destroy the recipient's bone marrow and tumor cells, and to immunosuppress the patient sufficiently to avoid rejection of the donor bone marrow transplant.

4. Categories of TBI High dose TBI Low dose TBI Half Body Irradiation Total Nodal Irradiation

5. High Dose TBI TBI with dose delivery in a single session of dose 750 to 900 cGy or in up to six fractions of 200 cGy each in 3 days (total dose: 1200 cGy ). Use adjuvant therapy in treatment of Ewing’s sarcoma non-Hodgkin’s lymphoma.

6. Low Dose TBI TBI with dose delivery in 10 to 15 fractions of 10 cGy to 15 cGy each. It is used in the treatment of: lymphocitic leukemia Lymphoma neuroblastoma

7. Half Body Irradiation Half body irradiation with a dose of 8 Gy delivered to the upper or lower half body in a single session.

8. Total Nodal Irradiation Total nodal irradiation with typical total nodal dose of 40 Gy. It is used as adjuvant treatment of autoimmune diseases.

9. Techniques and Equipment Numerous techniques have been used to deliver TBI The choice of a par­ticular technique depends available equipment photon beam energy maximum possible field size treatment distance dose rate patient dimensions selectively shield certain body structures

10. Contemporary TBI techniques use megavoltage photon beams that are produced either by cobalt-60 machines or linear accelerator. The beams used for TBI are: Stationary, with field sizes of the order of 70x200 cm2 to encompass the whole patient. Moving, with smaller field sizes, in some sort of translational or rotational motion to cover whole patient with the radiation beam.

11. Dedicated Irradiators Treatment machines specially designed for total body irradiation.

12. Cobalt-60 machine dedicated for TBI. Machine collimator has been removed to obtain a large field for TBI irradiation at SSD of ~200 cm.

13. Two linear accelerators mounted in such a way that they produce two parallel opposed beams simultaneously.

14. Modified Conventional MV Radiotherapy Equipment Treatment at extended SSD. Treatment with a translational beam. Treatment with a sweeping beam.

15. Treatment at extended SSD

16. Treatment with translational Beam

17. Treatment with swiping beam

18. Beam Energy The choice of photon beam energy is dictated by patient thickness and the specification of dose homogeneity. If the patient thickness is <35cm, 6MV photon atleast 300cm SSD can be used for parallel opposed beam. If the patient thickness is >35cm, higher energies (>6 MV) should be used to minimize tissue lateral effect.

19. As the patient thickness increases or the beam energy decreases the central axis maximum dose near the surface increases relative to the midpoint dose.

20. Initial Dose Buildup Surface or skin dose in MV beams is less than the dose at the point of D max , The dose build-up char­acteristics depend on Energy field size SSD Dose build-up data obtained at isocenter do not apply at TBI distances However, most TBI do not require skin sparing. Instead, a large spoiler screen of 1 to 2cm thick acrylic is sufficient to meet these requirements The screen is placed as close as possible to the patient surface to bring the surface dose to at least 90% of the prescribed TBI dose

21. Patient Positioning Devices Designed to implement a given treatment technique. Criteria include patient comfort Stability reproducibility of setup

22. Treatment at extended SSD AP/PA Technique Bilateral Technique Lying Position Seated Position

23. Seated Position Technique Technique involving left and right lateral opposing fields with the patient seated on a couch in a semi fetal position

24. A special TBI couch allows the patient to be seated comfortably with the back supported & legs semi collapsed. The arms are positioned laterally to follow the body contour and placed in contact with the body at the mid-AP thickness level. Patient setup is recorded in terms of distances measured between external bony landmarks

25. Lying Position Technique

26. Lateral body thickness along the patient axis varies considerably in the bilateral TBI technique. To achieve dose uniformity within approximately ±10% along the sagittal axis of the body, compensators are designed for head and neck, lungs and legs.

27. Thickness of compensator depends on Tissue deficit compared to the reference depth at the prescription point, material of the compensator , distance of the compensator from the point of dose compensation, field size and beam energy. Compensator is designed to be dosimetrically equivalent to a bolus but placed at a distance from the skin surface. The required thickness of a tissue equivalent compensator that gives the same dose at the point of interest as would a bolus of thickness equal to the tissue deficit, is called the thickness ratio.

28. The thickness of a compensator ( tc ) at any point in the field is TD is the tissue deficit ρc is the density of the compensator material. τ is thickness ratio.

29. AP/PA Technique It generally provides a better dose uniformity along the longitudinal body axis but the patient positioning may pose problem. The patient is irradiated anterio -posteriorly by parallel opposed fields while positioned in a standing upright position at the TBI distance The principle of the technique is that the standing TBI allows shielding of certain critical organs from photons and boosting of superficial tissues in the shadow of the blocks with electrons.

30. Dose Prescription Point The TBI dose is prescribed to a point inside the body, usually at the midpoint at the level of the umbilicus. Prescribing the dose at the level of the umbilicus is convenient for two reasons. This position is approximately equal to the half-height of the patient when the legs are slightly bent so that the central axis of the photon beam can be made to correspond with this point Tissues in the vicinity of the umbilicus are close to unit density, so that inhomogeneity corrections are not needed. The TBI procedure must deliver the prescribed dose to the dose prescription point and should maintain the dose throughout the body within ±10% of the prescription point dose.

31. Dosimetry Dosimetry is recommended for TBI as it is the treatment at the extended SSD. A direct output calibration of the machine for TBI may be performed. 0.6cc Farmer Type Chamber Water phantom of dimension 40x40x40 cm3 Position of the chamber is fixed at the TBI distance. Collimator is opened to its maximum size chamber depth is varied by moving the chamber and the phantom while keeping the source to chamber distance constant.

32. Alternative to direct output factor measurement, a formalism based on the TMRs, Sc , Sp and the ISL factor can be use. The basic equation to calculate dose per MU (D/MU) is: D – Dose in Gy k – 1cGy/MU under reference calibration condition TMR – Tissue maximum ratio at depth ‘d’ and field size equivalent to the patient (r e ) Sc – Collimator scatter factor for the field size projected at isocenter Sp – Phantom Scatter factor for the patient equivalent field size f - Source to calibration point distance f’ – Source to patient axis distance at the prescription point OAR – Off – axis ratio at depth ‘d’ TF – Transmission Factor for the block tray , beam spoiler.

33. TMR data obtained under standard condition and ISL factor must be checked for its validity at the TBI distance. D/MU calculated by above equation should be compared with directly measured output factor (D/MU) at the TBI distance. If the difference is within ±2%, above equation may be used for TBI.

34. In Vivo Dosimetry Doses delivered during TBI are difficult to evaluate accurately because of: The very large fields employed so, the patient is positioned very close to the floor or to the walls of the treatment room, which may produce significant electron and photon backscattered components that reach the patient’s skin. The necessity to correct for tissue inhomogeneities and to monitor the dose to the organs at risk ( lungs,liver,etc .) The difficulty of achieving a reproducible setup The possibility of the patient movement during the treatment, especially for low dose-rate single fraction treatments.

35. In vivo dosimetry generally consists of entrance and exit dose measurements performed on different regions of the patient’s skin. The TLD capsule or diodes surrounded by suitable build up bolus placed on the patient’s skin at strategic location for dose measurement. An agreement of ±5% between calculated and measured dose is considered good.

36. Diodes Provide an immediate response, which enables the immediate correction. It is important to balance the diode measurement circuit accurately to minimize the offset current. Consider the effects of temperature on the diodes, which may affect offset current & sensitivity. Have directional asymmetry and it is important to consider this aspect when calibrating them for use in TBI. Type of diode must be selected according to the energy in use.

37. TLD TLDs allow a large number of sites to be measured at the same time and give accurate results because of their closeness to tissue equivalence.

38. Complications of TBI The late complications of TBI such as Sterility Secondary malignancies Cataracts Growth diseases

39. Thank You