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

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.

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

Field Blocking and Shaping devices

Shielding blocks.

Custom blocks.

Asymmetric Collimator.

Multileaf Collimators.

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

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

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

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

Custom Blocks

Cm Cm

MLC

• Made of Tungsten

• Density 17-18.5 gr/cm3

• Leaf transmission %2,

• Interleaf transmission %3,

• Jaw transmission %1

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

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.

Adjacent Fields

• Double junction

• Dynamic junction

• GAP

• Divergent adjustment method

• Asimmetric collimator- half beam method

Double Junction

Dynamic Junction

• 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

Adjacent Fields - Divergent Adjustment Method

Collimator Angle Couch Rotation

Adjacent Fields - Divergent Adjustment Method

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

Adjacent Fields - GAP

• Electron- photon combination field. • 3 phase • Sparing ms

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

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

Adjacent Fields – Asymmetric Collimators

Adjacent Fields - Asymmetric Jaw

Adjacent Fields - Asymmetric Jaw

Conclusion

• What we have what we can

• Clinic acceptance

• Sparing OAR is the most important key

• Improving knowledge

• Keep up with technology

• İnnovative

THANK YOU