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- On a LINAC when delivering electron fields, the jaw size is fixed at a constant value. This is due to the huge variation in output with jaw size especially at lower energies (see left figure above) which can vary as much as 200%.
- Output is much more stable when the cone size is varied, however (see right figure above) and varies only about 15%.
- This large difference is due to the huge amount of scattering surfaces in the LINAC head as compared to the cone.
- Electron isodose curves show the following trends:
- At low energies, the low dose isodose lines (20,30%) bulge out, but the high dose isodose lines (90,100%) are fairly straight.
- At high energies, the low dose isodose line tends to remain straighter but the high dose isodose lines pinch in.
- When extended SSDs are used to treat a patient there is more penumbra.
- Uniformity Index:
- The uniformity index is a way to quantify the amount of penumbra in an electron beam.
- It is defined as the ratio of the area of 90% isodose curve to that of the 50% isodose curve.
- Values greater than 0.7 are considered acceptable for fields larger than 10x10 cm2.
- Inhomogeneities have a strong effect on electron fields. Consider the above image (this can be generalized to any low density/ high density environment such as bone and water):
- The water increases lateral scatter to the surrounding air and leads to hot spots adjacent to it.
- The air attenuates the field much less and the isodose curves extend deeper.
- A loss of scatter from the air causes a significant pinching in of the isodose curves traversing the water.
- The most important thing to remember is that Monte Carlo must be used to calculate around inhomogeneities as the pencil beam algorithms are inadequate.