- Field shaping in radiotherapy aims to minimize dose to healthy tissues while irradiating the target volume. Techniques for field shaping have evolved from lead shielding blocks to modern multileaf collimators. - When adjacent fields are used, divergence can cause hot and cold spots at the junction. Various techniques like double junctions, dynamic junctions, gaps, and divergent beam adjustment are used to match fields and achieve uniform dose. - Asymmetric collimators allow irregular field shapes without moving the isocenter, and the half-beam method uses blocking to remove beam divergence at the junction of split fields. Continuous improvement of techniques and technology helps maximize sparing of healthy tissues.

2. MSc. Phy. Özgehan ONAY
Cerrahpasa Medical Faculty
8-9 May 2015, İSTANBUL
SIDE EFFECTS Nasopharyngeal Carcinoma
in Radiotherapy and Patient Care
Issues in Field Shaping and
Field Matching

3. Goal of Field Shaping
• Basic goal of radiotherapy treatment is the irradiation of a
target volume while minimizing the amount of radiation
absorbed in healthy tissue.
• Shaping the beam is an important way of minimizing the
absorbed dose in healthy tissue and critical structures.

4. Field Shaping in NSF
• 1950 conformal radiotheraphy -shielding blocks
• 1960-1969 pair of lead cylinders suspended by gravity
defined a different field aperture at each orientation.
• 1959 patent for a Multileaf Collimator (MLC)
• 1960 custom blocks
• 1985 MLC required features of such a device
• 1988 Inverse Planning
• 1992 "micro-size" MLC with 40 non-focused leaves
projecting to a width of just 1.7 mm at isocentre has also
been engineered for stereotactic linac-based radiotherapy
• 1993 Studies have shown that the MLC can be an
effective block replacement and also save time

5. Field Blocking and Shaping devices
Shielding blocks.
Custom blocks.
Asymmetric Collimator.
Multileaf Collimators.

6. Shielding Blocks
• Common use
• Provide by concern
• Determine shape and size before theratment
• Density 11,36 g/cm3, melting point 327 0C
• Geometrical and dosimetrical useless
• Divergent problem

7. Shielding Blocks
• As far as possible eliminate the radiation dose at some special
parts of zone at which beam is directed.
• Shielding is achieved more easily with high energy radiation
than low energy due to low scattering in high energies

8. Shielding Blocks
Beam Quality
• 1.0 mm Al HVL
• 2.0 mm Al HVL
• 3.0 mm Al HVL
• 1.0 mm Cu HVL
• 2.0 mm Cu HVL
• 3.0 mm Cu HVL
• 137 Cs
• 60Co
• 4 MV
• 6 MV
• 10 MV
• 25 MV
Pb Thickness
• 0.2 cm
• 0.3 cm
• 0.4 cm
• 1 cm
• 2 cm
• 2.5 cm
• 3 cm
• 5 cm
• 6 cm
• 6.5 mm
• 7 cm
• 7 cm
A primary beam transmission of 5% through the block is considered
acceptable

9. Custom Blocks
• LIPOWITZ metal
• Commercial name Cerroband
(% 50 Bismuth, % 26,7 Lead, % 13,3 Stannic,
% 10 Cadmium)
• Density 9.4 gr/cm3
• It melts at 70 c and can be easily cast to any
shape
• At room temperature it is harder than lead
• In Megavoltage range of photon beams the
most commonly used thickness is 7.5 cm
• Divergent adjustment
Bi
Pb
Sn
Cd

10. Custom Blocks
Cm Cm

11. MLC
• Made of Tungsten
• Density 17-18.5 gr/cm3
• Leaf transmission %2,
• Interleaf transmission %3,
• Jaw transmission %1

14. MLC
• Reducing productive effort
• Easy to use
• Reliability
• Financial gain
• Time consuming
• Data transfer
• Methodical and safe work space
• Aplicability of high level treatment
techniques
• Tangent to the field edge

15. Adjacent Fields
• X-ray beam is DIVERGENT
• Adjacent beams will meet at a region because of divergent
• Divergence causes hot spots (overdose) at depths and cold areas
(underdose) near the surface
• Dose HETROGENIETY
• A field-matching methods must be verified by actual isodose distributions
before it is adopted for general clinical use.
• For NSF treatment you must choose the more sophisticated and accurate
methods.
• Management of the clinical case.

16. Adjacent Fields
• Double junction
• Dynamic junction
• GAP
• Divergent adjustment method
• Asimmetric collimator- half beam
method

17. Double Junction

18. Dynamic Junction

19. • Fields on the same surface.
Fields separated at the skin surface. The junction point is at
a depth where dose is uniform across the junction
• Fields on orthogonal surfaces
Two principal methods:
1- Split field method (Half-beam block).
2- Collimator and table rotation.
Adjacent Fields - Divergent Adjustment Method

20. Adjacent Fields - Divergent Adjustment Method

21. Collimator Angle Couch Rotation
Adjacent Fields - Divergent Adjustment Method

22. Adjacent Fields - GAP
• An arrangement in which the central axes of the
adjacent fields are orthogonal
• A geometrical method of field separation with
S=1/2 L ( d/SSD)
d is the depth of field junction

23. Adjacent Fields - GAP
• Electron- photon combination field.
• 3 phase
• Sparing ms

24. Asymmetric Collimator
• Conventional collimator jaws are used for shaping a rectangular treatment field;
but, as usually treatment volume is not rectangular, additional shaping is required.
• Asymmetric fields are sometimes used to block off a part of the field without changing
the position of the isocenter.
• Although blocking is often used to generate irregular field shapes, rectangular
blocking can be easily done by independently movable collimators, or jaws. This
feature is very convenient when matching fields or beam splitting.
• The beam is blocked off at the central axis to remove divergence. Whereas half-beam
blocks have been used as beam splitters in the past, this can now be done simply by
moving in the independent jaws.
• Most modern machines are equipped with independently movable jaws. Some
machines have one independent jaw, others have two independent pairs, and some
have all four jaws as independent.
• Operationally, the independent jaw option is interlocked to avoid errors in the setting
of symmetric fields, in which case the opposite jaws open or close symmetrically

25. Adjacent Fields - Half Beam Method
Half-beam blocking gives rise to tilting of the isodose curves toward the
blocked edge. This effect is due to missing electron and photon scatter
from the blocked part of the field into the open part of the field

26. Adjacent Fields – Asymmetric Collimators

27. Adjacent Fields - Asymmetric Jaw

28. Adjacent Fields - Asymmetric Jaw

29. Conclusion
• What we have what we can
• Clinic acceptance
• Sparing OAR is the most important key
• Improving knowledge
• Keep up with technology
• İnnovative

30. THANK YOU