KERMA and Dose:




    1. An important distinction to realize is the difference between KERMA and Dose.

    2. KERMA is the Kinetic Energy Released per unit MAss.  The key here is released, as it has nothing to do with energy being absorbed.

    3. Dose, on the other hand, is the amount of energy absorbed per unit mass.  The key here is absorbed.

    4. Both KERMA and dose have the same units (J/kg).

    5. When a megavoltage photon beam interacts with a medium it releases energy at a nearly constant rate, but due to the high energy of the secondary particles, they have a significant range and are not absorbed until much deeper leading to the buildup in dose.

    6. It is important to realize that the area under both the KERMA curve and the Dose curve must be identical for the conservation of energy to remain true.  Notice that while KERMA is initially higher, beyond dmax it is lower and these end up canceling out.

      1. Remember that this is only in an infinite volume as in a finite volume some of the KERMA energy could escape and not be absorbed.




    1. Penumbra is composed of three components: geometric penumbra, transmission penumbra and scatter.

    2. Penumbra is usually defined as the distance between the 80% and 20% isodose curves.

    3. Geometric penumbra:




    1. Geometric penumbra follows the following relationship:




      1. s - is the source size (increases with source size).

      2. SSD - is the source to skin distance (increases with SSD).

      3. d - is the depth in the patient where penumbra is of interest (increases with depth).

      4. SCD - is the source to collimator distance (decreases with SCD)

        1. putting blocks on the patient will yield the sharpest penumbra, but you will lose skin sparing.  You must keep the blocks about 20 cm away due to electron contamination ruining skin sparing.

  1. Transmission penumbra:




    1. Transmission penumbra is caused by differential attenuation through collimators or MLCs.

    2. One way to help with this is to use divergent collimators that match the beam divergence.


Linac Divergent Jaws


      1. Note also that some MLCs use a rounded end design (e.g. Varian) which yields a constant transmission penumbra over the entire course of movement

    1. Penumbra varies with energy, and, in general, it is best for lower energy beams 6/10 MV and worse for higher energy beams 18/20 MV due to an increase in transmission penumbra.

      1. Side scatter can also increase penumbra and as this decreases with increasing energy there is a competing effect.

      2. The best balance between scatter and transmission penumbra occurs around 10 MV.