Magna field irradiation

Total Body Irradiation Introduction

1. MAGNA FIELD IRRADIATION SABARI KUMAR P M.Sc. RADIATION PHYSICS

2. Introduction Magna field radiotherapy (TBI/HBI/TNI) is becoming increasingly prominent and involves Dosimetric problems that are much more pronounced than they are for conventional field sizes. In this review, Biological considerations in TBI Physical considerations in TBI calculation & prescription of dose for TBI Techniques in TBI Special considerations to lung dose delivery and reduction of dose (due to low density tissues and low tolerance to irradiation) are outlined.

3. In principle, it difficult to obtain the dose delivery information of magna field radiation for variety of reasons. Dose delivery (ranges from 5Gy in single fraction using dose rate 50cGy/min to 14Gy in multiple fractions over a no. of days) Dose prescription points may vary from institute to institute Overall lack of Clinical evaluation All this points raise the question: with what accuracy must the dose be delivered? ICRU has recommended an overall accuracy in dose delivery of 5% But recent data indicates that 5% change in dose to lung could result in a 20% change in the incidence of radiation pneumonitis If the prescribed dose is well below the radiation pneumonitis and it is sufficient for adequate tumor control, then the guideline of 5% accuracy can be relaxed to 10 % or even 15%

4. Radiobiological Considerations In the context of TBI as applied to Bone Marrow Transplantation, Repair and Repopulation are the most significant of the four R’s From the pioneer work of Elkind & Sutton, during fractionated Radiotherapy regimen (or) continuous low dose rate exposure, both repair of sub lethal damage and repopulation may occur between fractions. In fact, the data indicate that when the dose is given in multiple fractions rather than single increases the lung tolerance up to 175% Radio biologically three measurable factors got importance in TBI such as Fractionation Dose, Dose Rate and Total Dose The increase in Dose fractionation results increase in therapeutic ratio and this effect occurs optimally at dose fractions of order 2 Gy

5. LD 50/30 (Dose with 95% confidence limits) is a good indication of the survival level of bone marrow stem cells. As the dose rate decreases, the LD 50/30 increases In addition to fractionated dose and dose rate, one has to consider total dose, uniformity of the dose throughout the bone marrow and body. This is majorly depends on age of the patient, differences in conditioning chemotherapy regimens and the delay in the actual transplant. Based on the all the above factors, fractionation method become more practice in most of the hospitals instead of Single Fractionation

6. Physical Considerations J. Van Dyk , M.Sc., F.C.C.P.M had discussed about the physical considerations in Magna Field Irradiation Technique and suggested to consider the no. of factors before initiating a large field radiotherapy. Irradiation Methods: In the first instance, a method must be devised to produce radiation fields large enough to cover the entire target volume adequately This is majorly depends on type of equipment available Basic Dosimetry: Large field treatments are usually performed under unusual geometric conditions and hence experimental data should be determined specifically for that geometry. Central Axis dose data (Solid phatom ) Dose calibrations cGy /MU Beam Profiles (Solid phantoms) Buildup Characteristics (after applying Lucite sheet, how buildup regions changes that need to check) Inverse Square Law data (To check scatter effect) Output Factors Sp and Sc need to measure)

7. Patient Dosimetry: Once the basic parameters have been determined, factors specially related to the patient must considered before dose delivery Dose Prescription : the dose prescribed to a single point at the patient’s midline at the levels of the pelvis Patient Contour : to make dose uniformity through out patient’s shape, use the tissue equivalent bolus on skin surface and use the compensators in the beam remote from the patient surface Dose distribution : most magna-field radiotherapy procedures are performed with AP-PA or Lateral Opposed fields. When comes to Co-60 radiation, Lateral beams contains larger dose variation as compared to AP-PA. In Higher Energy radiation, this dose variation is very less in Lateral beams

8. Prescription & Calculation of dose for TBI The use of large Total Body fields creates a unique set of problems that stress the accuracy of technique routinely used for dose calculation Difficulty results from the complexity of the dose distribution due to wide variations in the dose from point to point For this reason, it is difficult to describe the resulting dose distribution clearly (or) to state the prescribed dose accurately Different approaches are found to calculate and prescribe the dose to TBI patients. First Method: One approach is that use the integral dose for entire body to calculate the average dose for all points But it fails to define the dose to specific areas such as the lungs or other sensitive structures.

9. Second Method: Another approach is that averaging of limited number of points The averaging technique is aimed at modifying the delivered dose downward when high dose areas occur and upward if low doses are found This mechanism guarantees that critical areas do not receive either excessively high or low doses of radiation This approach has some appeal such as it can be used to guard against the large dose variations which can result when the irradiation technique is changed i.e. bi lateral irradiation fields has been shown considerably dose variations from the AP/PA fields

10. Third method: CCSG protocol prescribe the dose by using a single value corresponding to a single point in the body i.e. the mid point at the level of the umbilicus (intersecting point of the mid planes AP/PA and Lateral fields) Because of this mid point selection, this approach is independent of the treatment technique used Overall dose distribution is controlled within the stated limits at least for the point specified The main advantages by selecting Umbilicus as a dose prescription point are: The point is equal to half height of the patient so that 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 no need of any inhomogeneity corrections

11. Although, the prescription point has advantages, important problems remain such as: Inverse square correction Collimator size correction factor Estimation of scatter volume Accuracy of TPR Attenuation in air column Scatter from air column Back scatter from wall Output changes as a function of distance and field size. To reduce this effect of inverse square corrections & collimator size corrections, for all patients standard distance and standard field size are to be used

12. In this approach, the scatter volume is estimated from the top of the shoulders to the bottom of the pelvis and has lateral & AP dimensions at the level of umbilicus. To calculate the total scatter volume, CCSG followed two procedures such as The entire scatter volume is considered as unit density the volume is exactly centered around the central axis of the field

13. After measured with Modular Plastic Phantom, CCSG had concluded that Change in the amount of scatter material behind the chamber doesn’t produce a significant change in detector reading Low density tissues at distances greater than about 15cm from the chamber do not change the measured dose relative to the condition where a scattering volume is made up entirely of unit density material CCSG found that correction methods used for extending the PDD’s to other SSD are sufficiently rigorous for the TBI situation CCSG protocol recommends to determine the PDD’s, TAR’s and TPR’s using a phantom that more closely correspond to the actual irradiation conditions Although TAR & TPR are independent of distance, the problems associated with the finite size of the scattering volume must be addressed Finally, CCSG recommends to calibrate the treatment unit at standard extended distance using maximum field

14. Techniques of Magna Irradiation: TBI technique is depends on a lot of variables such as Machine type and energy Dose prescription parameters (dose, no. of fractions, dose/fraction and dose rate) Patient position Therapy room constraints ( distance & Field size) Beam modifiers (Bolus and Compensators) Brenda Shank, M.D, PhD ., had surveyed about TBI treatment techniques in seven representative institutes and found that remarkable dose homogeneity throughout patient including at the Skin surface within 10% by using Bolus and Compensators for energies ranging from 1.25MeV (Tele cobalt Machine) to 10MV (Linear Accelerator)

15. Homogeneity and Methods used to achieve Institution Beam Energy Patient Position Bolus Compensators Homogeneity University of California 1.25 MeV AP/PA - - 89 – 121% Johns Hospital 1.25 MeV AP/PA - - 80 – 110% Fred Hutchinson cancer Research center 1.25 MeV AP/PA - - 87 – 111% Children Hospital 6MV AP/PA - - - University of Minnesota 10MV Laterals 0.95cm Lucite ( to increase skin dose) Al ; all except abdomen and pelvis 94 – 102% City of Hope Hospital 10MV AP/PA Face : 0.6cm Lucite Rest : 0.8cm tissue eq. blanket Pb ; calf, foot and neck 96 – 103% Memorial Sloan- kettering cancer center 10MV AP/PA 1 cm Lexan - 89 – 115%

16. Lung Dose Determination One of the major complications of large field radiotherapy is radiation Pneumonitis. (infection in Lung) It is imperative that the dose to lung be precisely controlled to ensure that the probability of minimal occurrence The below steps are followed to calculate lung dose and to reduce radiation Pneumonitis First step : Determination of inhomogeneity correction by using any of below methods Linear Attenuation Method Effective Attenuation method (ICRU) Generalized Batho method Equivalent TAR method

17. Considerations : Depth from surface – 12.5cm Lung depth – 9.0cm Method of Calculation Dose correction factors Linear attenuation method (3.5% /cm) 1.32 1.31 1.04 1.17 Effective attenuation method (ICRU) Generalized Batho Equivalent TAR 12% Variation in lung dose calculations for 50 x 60 cm 2 cobalt -60 fields using different calculation methods

18. Second step : Obtaining patient specific density & geometry of lung and dose determination by using any of below methods CT data and Pixel based dose calculations (accurate  3% ) CT data for contours and average density data (accuracy 5%) Transmission measurements Lateral radiographs Nomo graph relating dose correction factor and patient thickness (large errors occurs in diseased lung) In a study evaluating response of lung to radiation absorbed dose, CT scans were performed on 23 patients for dose calculation

19. When the dose correction factors for the middle of lung were plotted as a function of patient thickness, 80% of the data points fell within 1.5% of a straight line. The maximum dose deviation for normal lungs was 3.5% ; diseased lungs showed much larger deviations Higher energy data were derived by converting those dose corrections to an equivalent t depth and then determining the dose correction factor for higher energy. Why it is increasing with patient thickness increase ( inhomogeneity of patient thickness) Energy increase but dose correction factor decrease (as energy increases, Compton effect predominant and it is independence of Z) Patient Thickness (cm) Lung dose correction factor Co – 60 6 MV 25 MV 12 1.04 1.04 1.02 16 1.09 1.09 1.06 20 1.14 1.13 1.09 24 1.19 1.17 1.12 28 1.24 1.21 1.14

20. Third step : Reduction of lung dose to avoid probability of lung complications By use of lung compensators (causes under dose) Constant thickness lung attenuators (dose variation throughout lung volume occurs) Lung blocks for part of treatment (dose variation throughout lung volume occurs) Brenda Shank, M.D, PhD ., suggested multiple ways to decrease lung damage which include: Lowering TBI dose Patient Positioning ( by using Arms as absorber) Partial lung blocking Using higher beam energies Increasing fractionation Using a low dose rate

21. References: Symposium on Magna-Field Irradiation: Rationale, Technique, Results ; Giulio J. D’ Angio , M.D Radiobiological considerations in Magna Field Irradiation ; Richard G. Evans, PhD., M.D Magna Field Irradiation ; Physical Considerations ; J. Van Dyk , M.Sc., F.C.C.P.M Calculation & Prescription of Dose for Total Body Irradiation ; J. M. Glavin , D.Sc Techniques of Magna Field Irradiation ; Brenda Shank, M.D., PhD.,

22. THANK YOU…