What is called as Asymmetric field in radiotherapy?

Asymmetric fields will used in radiation therapy for treating cancer patients.

All this means is that the field of radiation is not equal on both sides. Historically, radiation machines were only able to produce symmetric fields. That is, in height and width would be the same on either side of the center of the field. Example, a 10 x 10 cm field, the center would be right in the middle (5 cm on either side).

With newer machines we are able to use asymmetric fields where we can close off one side of the field so that the center of the beam is not necessarily in the middle of the field. Picture the 10 x 10 cm field shrinking to 10 x 8 cm, but only the right side moves in and the center point stays the same. This is an asymmetric field.

Dose calculations for asymmetric fields defined by independent collimators using symmetric field data

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