The document discusses various considerations for magna field irradiation (TBI) including: 1) Biological factors like repair, repopulation and fractionation that are important for TBI. Fractionation increases lung tolerance up to 175%. 2) Physical factors that must be addressed like dose calibration, scatter corrections, and ensuring dose uniformity throughout the body. 3) Methods for calculating and prescribing dose including using average or single point doses. Most protocols prescribe to the umbilicus midpoint. 4) Techniques used to achieve homogeneity like bolus, compensators and beam energies. Homogeneity of ±10% is typically achieved. 5) Determining lung dose accurately through methods like CT-based calculations or

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. LD50/30 (Dose with 95% confidence limits) is a good indication of the
survival level of bone marrow stem cells. As the dose rate decreases,
the LD50/30 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.
o Central Axis dose data (Solid phatom) o Dose calibrations cGy/MU
o Beam Profiles (Solid phantoms) o Buildup Characteristics (after applying
Lucite sheet, how buildup regions changes
that need to check)
o Inverse Square Law data (To check scatter
effect)
o 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

9. 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.

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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)

16. Homogeneity and Methods used to achieve
Institution
Beam
Energy
Patient
Position
Bolus Compensators Homogeneity
University of California 1.25MeV AP/PA
- -
89 – 121%
Johns Hospital 1.25MeV AP/PA
- -
80 – 110%
Fred Hutchinson
cancer Research center
1.25MeV 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%

17. 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

18. 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 cm2 cobalt -60 fields using
different calculation methods

19. 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

20. 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)
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

21. 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

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

23. THANK YOU…