Field shaping, separation and matching

1. FIELD SHAPING, SEPARATION,
MATCHING AND SKIN DOSE
Dr. Sachin. S,
Junior Resident,
Department of Radiotherapy and Radiation Medicine,
SSH, IMS, BHU

2. OUTLINE
• Field shaping
• Custom blocks
• Independent Jaws
• Multileaf Collimators
• Skin dose
• Field separation
• Geometric
• Dosimetric

3. WHY TO SHAPE A BEAM?
• Collimators provide cuboid field shape
• Target can be
• regular
• Cylindrical
• Spherical
• Ovoid
• cuboid with corners trimmed
• irregular

4. FIELD SHAPING
• Dictated by:
• Complete coverage of tumour with prescription dose
• Including local and distal disease
• Minimal dose to normal tissue
• Vital organ tolerance
• Primary beam transmission of < 5% through blocked region

5. HALF VALUE LAYER
n is the number of HVLs

6. • High atomic number
• High density
• Easily available
• Less expensive
FIELD BLOCKS – IDEAL SHIELD

8. DIVERGENT BLOCKS
• To reduce transmission penumbra
• Uniform penumbra across the beam
• Better suited for Linac beams (those with
small geometric penumbra)

9. CUSTOM BLOCKS
PROPERTY LEAD CERROBEND
Density
(g/𝑐𝑚3)
11.3 9.4
(85% of lead)
Melting point 327℃ 70℃
(95℃ for cadmium less)
Thickness
(4MV)
6cm 7.5cm
(1.21 times of Lead)

10. CUSTOM BLOCKS

11. POSITIVE BLOCKS NEGATIVE BLOCKS

12. CUSTOM BLOCKS DISADVANTAGES
• Production is time consuming
• Cumbersome to use
• Increase in treatment and setup time
• Toxic fumes while fabricating
• Daily variation (manual placement)

13. INDEPENDENT JAWS
• Asymmetric fields (rectangular blocking) – “beam
splitting”
• Transmission of 1% (approx.)
• Blocks field without changing isocenter
• Shift of isodose curves towards blocked edge
(elimination of photon and electron scatter from
blocked part)

14. MULTILEAF COLLIMATORS (MLC)

15. MULTILEAF COLLIMATORS (MLC)
• Large number (60-80) of collimating leaves or blocks
• Driven automatically, move independently
• Generate a field of any shape
• <1cm wide, 6-7.5 cm thick, Tungsten alloy
• Transmission: Intra-leaf (<2%), interleaf (<3%)

16. MLC - ADVANTAGES
• Less time consuming (no need to enter treatment room often)
• Less setup and overall treatment time
• Automation of field shaping and beam intensity modulation
• Suited for treating multiple fields

17. MLC - DEMERITS
• Island blocking not possible
• Larger physical penumbras than Cerrobend
blocks and jaws
• Jagged boundaries – difficulty in field
matching
• Radiation leakage between the leaves

18. SKIN DOSE
• Skin sparing - desirable feature of MV
beams
• Reduced or lost – secondary electrons
contamination or backscattered radiation
• 2° electrons from beam shaping devices or
air

19. SKIN SPARING – FUNCTION OF PHOTON ENERGY

20. ABSORBER TO SKIN DISTANCE
• Block tray absorbs the 2° electrons produced
already but produces own 2° electrons
• Skin dose increases with decreased tray to
surface distance
• Point of maximum dose buildup also moves
close to the surface

21. EFFECT OF FIELD SIZE
• Larger field size – more 2° electrons
• Poor skin sparing for large field sizes

22. ELECTRON FILTERS
• Medium atomic number materials produce
better skin sparing
• Less forward scattering
• Used in large field size and block-tray distance
15-20cms
• Thickness equal to 2° electron range: 0.9mm
of tin for 𝐶𝑜60

23. OBLIQUE INCIDENCE
• As the angle of beam incidence
increases:
• Skin dose increases
• Depth of 𝑑𝑚𝑎𝑥 decreases

24. OBLIQUITY FACTOR
• Obliquity factor (OF) - defined as the dose at
a point in phantom on central axis of a beam
incident at angle θ, with respect to the
perpendicular to the surface, divided by the
dose at the same point and depth along
central axis with the beam incident at angle 0
degrees
• For tangential beam incidence:

25. FIELD SEPARATION
• Need for treatment with adjacent fields for large areas
• Hodgkin’s lymphoma
• Medulloblastoma
• Some head and neck fields
• Divergence of adjacent fields produce
• Hotspots at depths
• Cold spots at surfaces
• Large dosage errors across the junction

26. FIELD SEPARATION TECHNIQUES

27. FIELD SEPARATION TECHNIQUES

28. FIELD SEPARATION TECHNIQUES

29. METHODS OF FIELD SEPARATION
• Geometric
• Fields joined at 50% isodose lines
• 100% at the Junction point
• Dosimetric
• Composite isodose distribution
• Uniform at the desired depth

30. GEOMETRIC

31. IDEAL (NO OVERLAP) MATCHING

32. THREE FIELD OVERLAP

33. THREE FIELD OVERLAP
S1 + S2 + ∆𝑆′

34. ORTHOGONAL FIELD MATCHING

35. CRANIOSPINAL FIELDS

36. CRANIOSPINAL FIELDS

37. CRANIOSPINAL FIELDS

38. CRANIOSPINAL FIELDS
• More convenient method:
• Half beam block
• Independent jaws
• No couch rotation
• Feathering of junction to smear out
junctional dose distribution

39. FIELD MATCHING GUIDELINES
• The site of field matching should be chosen, in so far as possible, over an
area that does not contain tumor or a critically sensitive organ
• If the tumor is superficial at the junction site, the fields should not be
separated because a cold spot on the tumor will risk recurrence
• However, if the diverging fields abut on the skin surface, they will overlap
at depth

40. FIELD MATCHING GUIDELINES
• In some cases, this may be clinically acceptable, provided the excessive
dosage delivered to the underlying tissues does not exceed their tolerance
• In particular, the tolerances of critical structures such as the spinal cord
must not be exceeded
• In the case of a superficial tumor with a critical organ located at depth, one
may abut the fields at the surface but eliminate beam divergence using a
beam splitter or by tilting the beams

41. FIELD MATCHING GUIDELINES
• For deep-seated tumors, the fields may be separated on the skin surface so that the
junction point lies at the midline
• The line of field matching must be drawn at each treatment session on the basis of the
first field treated
• It is not necessary anatomically to reproduce this line every day because variation in its
location will only smear the junction point, which is desirable
• A field-matching technique must be verified by actual isodose distributions before it is
adopted for general clinical use

42. THANK YOU