The thickness used depends upon the energy of the radiation.
For practical purposes, the shielding material which reduces beam transmission to 5% of its original is considered acceptable.
The term half value-layer is a convenient expression for the attenuation produced by any material. Half-value layer is defined as the thickness of material, which will reduce the intensity of the primary beam by 50%.
The number of HVL (n) required is given by the following expression:
1/2 n = 5% or 0.05
Thus, 2 n = 1/0.05 = 20
OR, n log 2 = log 20 .
n = 4.32
The relationship holds true, only for mono energetic x-ray beams.
Practically thickness of lead between 4.5 - 5 half-value layers results in 5% or less of primary beam transmission.
Shielding 250 KV 4 MV Lesser amount of scattered radiation with megavoltage radiation means that the attenuation produced by shielding is also more. The higher scatter contribution to the overall dose results in lower dosage adjacent to the shielded area in kilovoltage radiation . 100% 50% 5.0 cm Co 60 (1.25 MeV) 7.0 cm 10 MV 6.5 cm 6 MV 6.0 cm 4 MV Required lead thickness Beam energy
Material used for custom locking is known as the Lipowitz metal or Cerrobend .
Melting point 70°C.
Density 9.4 g /cm 3 at 20°C ( 83% of lead).
1.21 times thicker blocks necessary to produce the same attenuation.
Most commonly thickness of 7.5 cms used.
Custom blocks Outline of the treatment field being traced on radiograph using a Styrofoam cutting device. Electrically heated wire pivoting around a point (simulating the source) cutting the styrofoam block Cavities in the styrofoam block being used to cast the Cerrobend blocks.
Wedge filters - Design This angle is the wedge angle. % DD at each point is calculated and tabulated A B C D E 105 76 43 39 35 Wedge isodose 62 68 67 55 40 Non wedge isodose E D C B A 1.7 1.1 .79 .71 .87 Ratio (W: NW) 1 .66 .42 .38 - Transmission ratio 0 6.5 13.6 15.2 - mm Pb
Wedged fields are generally used for relatively superficial tumors.
Beams are usually directed from the same side of the patient.
The broad edges of the wedges should be aligned together.
The wedge angle choosen depends on the angle between the central rays of the two beams also called the “ hinge angle ”( φ ).
Reduce the hot spots at the surface
Rapid dose falloff beyond the region of overlap.
The overlap region is also called the “plateau region”.
Thus the 2 factors on which the wedge angle is choosen are:
The hinge angle.
The wedge separation.
The wedge angle that will make the isodose curves parallel to each other and the hinge angle bisector is obtained using the equation.
φ Θ = 90 – φ / 2
Wedge filters Compensators may be used to overcome the deficit or a different wedge angle can be used, so that part acts as a compensator. The body contour can be more curved as a result, the isodose curves are not obtained in the manner desired. A small range of hinge angles can be covered by a given wedge angle without producing significant variation(±5%). A different value of wedge angle is required for different beam angles Solution Problem