Field Size and Shape

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Field size and its shape are the crucial in the treatment of conventional, 3DCRT, IMRT etc that can be achieved by using the Asymmetric jaws, customized blocks, Multileaf Collimators (MLC), and micro MLC’s. Let’s discuss in brief below.


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. In the latter case, 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

Multi Leaf Collimators (MLC):

The MLC has movable leaves, which can block some fractions of the radiation beam. Typical MLCs have 40 to 120 leaves, arranged in pairs (Figure a). By moving and controlling a large number of narrow, closely abutting individual leaves, one can generate almost any desired field shape.

Figure (a) shows the irregular shape of MLC.

The advantages of MLCs are simple and less time consuming preparation, use without needing to enter the treatment room, and simple change or correction of field shape. The therapy expenses are lower because individual shielding blocks are not needed, thus eliminating the need to handle the Wood’s alloy, which is toxic. With MLC, we shorten the therapy time, and thus also the period during which patient must remain in still position. Other advantages are constant control and continuous adjusting of the field shape during irradiation in advanced conformal radiotherapy. MLC has also some disadvantages, which include a stepping edge effect, radiation leakage between leaves, wider penumbra, and problems with generating some complex field shapes.


1. Disadvantage of MLC’s is

a) Leakage between leaves

b) Easy to shape

c) Thickness of MLC

d) All

2. Advantage of MLC’s is 

a) Easy to make irregular shapes

b) Sparing of critical structures

c) Reduces the man work

d) All


1. a) Leakage between Leaves

2. d) All


1. The Physics of Radiation Therapy by F. M. Khan



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