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Early conventional irradiation megavoltage devices as cobalt unit and old linear accelerators have no independent movable collimator jaws, and they can determine only symmetric fields. If we want to block of a part of the field without changing the position of the isocenter, we have to use different kind of shielding.
The aims of shielding are:
1. To protect critical organs
2. Avoid unnecessary irradiation to surrounding normal tissue
3. Matching adjacent fields

Disadvantages of shielding
1. The production of low-melting blocks is time consuming. The alloys very often include toxic materials, while during the casting toxic gases are released.
2. Before the treatment the blocks have to be mounted on the block trays and inserted in a special slot of the treatment head. Be careful because the block is too weighty, not to drop it onto the patient.
3. The lifting of blocks may cause significant physical strain for RTTs.
4. The blocks need place for storage and the block trays periodically have to be changed for radiation damage.
The shielding blocks can be of two types: positive blocks, where the central area is blocked, and the negative blocks, where the peripheral area is blocked.
During the treatment always have to be used diverging blocks, what means that the block follows the geometric divergence of the beam.


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