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Technical Planning Aspects:
- Sometimes it is necessary to treat using lead shields to protect tissue behind the shield. This occurs, for instance, in eyelid treatments or in treating the lower lip.
- The thickness of shielding required can be quickly approximated using the practical range in water (MeV/2) and then taking the ratio of the density of the shielding material to water.
- For instance, lead has a density of 11.34 g/cm3. Therefore, you require about 1/11th the amount of lead as a water equivalent shield.
- So, for a 12 MeV beam with a practical range of 6 cm in water we need a lead shield of about 5 mm.
- The high Z material will stop the electrons but will also produce a lot of backscatter to the overlying tissue within a few millimeters (see above image).
- To prevent this, high Z materials can be wrapped in low Z materials such as wax to minimize this effect.
- Sometimes it may be necessary to abut an electron field with either another electron field or a photon field. This is always problematic and will inevitably result in either hot spots or cold spots.
- The problem is that, often, the low dose isodose lines bulge and the high dose isodose lines pinch in. You cannot join them together without overlap or underlap in the isodose lines (see above figure).
Electron Arc Therapy:
- A technique that is not frequently encountered but of interest is electron arc therapy. It may be used to treat curved surfaces such as the scalp or the chest wall.
- A couple of important facts:
- The %DD appears to increase during arc therapy due to the velocity effect:
- The velocity effect occurs when a deeper point in the patient is exposed to the beam longer than a shallower point due to a differential in angular velocities.
- This results in an apparent increase in penetration.
- The isocenter should be placed such that it is equidistant from all surfaces.
- In order to produce sharp field edges near the ends of the arc, lead collimators should be placed over the patient to define the field aperture.
Total Skin Electron Therapy (TSE):
- TSE treatments are given to patients with systemic skin diseases such as Mycosis Fungoides.
- Treatments are usually delivered at extended SSDs using a degrader and 2 fields.
- A degrader is placed in the beam to scatter the electrons and reduce their energy.
- Each field is usually separated by about 20 degrees and scatter in the air and degrader yields a homogeneous dose distribution on the patient within +/-8%.
- For more information, see the Special Techniques summary.