Flatness and Symmetry:

1. 1. Flatness:

1. 1. After electrons strike the target the resulting bremsstrahlung distribution exhibits a pointed isodose curve, and in order to flatten the isodose curves, a flattening filter is used.

   2. Flatness is usually defined at 10 cm depth, and in order to achieve this, the beam must exhibit horns near the surface.

      1. Horns are required due to differential hardening in the flattening filter.

      2. In the center, the beam is hardened and therefore penetrates more resulting in more fluence at 10 cm depth.

      3. Near the periphery, the beam is not hardened as much and therefore is attenuated more than in the center.  Therefore, it must have more fluence near the surface to achieve a uniform fluence at 10 cm depth.

   3. Flatness is defined inside of 80% of the field size at a depth of 10 cm.

      1. TG-142 now specifies the tolerance in terms of baseline values as it is important that it matches your treatment planning system.

      2. You should also contact the manufacturer of your LINAC to see what are their acceptance criteria for flatness.

         1. Different manufacturers calculate flatness differently.

         2. Their tolerance values may also differ.  For instance, for photons, Varian has an acceptance criteria of +/-2.5%.

      3. NCRP report 69 recommends an absolute tolerance of +/-3%.

2. 2. Symmetry:

      1. Symmetry refers to the degree of similarity between the isodose curve on one side of the central axis versus the other.

      2. Ideally, you want the beam to be symmetric about the central axis when compared point-to-point.

      3. Symmetry is defined over 80% of the field size at a depth of 10 cm.

         1. TG-142 specifies this in terms of baseline values as it is important that it matches your treatment planning system.

         2. You should also contact the manufacturer of your LINAC to see what are their acceptance criteria for flatness.

            1. Different manufacturers may calculate symmetry differently.

            2. Their tolerance values may also differ.  For instance, for photons, Varian has an acceptance criteria of +/-2%.

         3. AAPM TG-40 recommends that symmetry is within +/-3%.

3. 3. Symmetry and flatness are affected by beam steering.

      1. When symmetry or flatness is out (it is usually both at the same time), the beam is steered using solenoids so that it strikes the flattening filter in the middle yielding a nice uniform beam.