Quick question about electrons

[Tosh4.0]

Dumb question about something basic in rad onc. I know for certain skin cancers etc. or breast boosts we use electrons. I keep reading in textbooks that these require skin collimation. What are they referring to? Isn't the cerrobend block placed in the treatment head?

For that matter why can't you just block with MLCs like you do for a field edge in photons?

And what is this difference with orthovoltage? In orthovoltage, you have the cone that basically comes right next to the skin and you don't need further skin collimation?

Thanks.

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[Gfunk6]

1. Lower energies of electrons (6-9 MV) may not deliver sufficient dose to the surface of your target (skin). To account for this we can use either a tissue-equivalent "bolus" or metal "chainmail" to build up the dose before it hits the target. In addition, you may not want the electrons to go too deep especially if you are treating hear the eye or mouth. In those cases, a lead shield is placed in front of the deep structure (e.g. under the upper lip) to stop electrons from penetrating. Finally, you typically use a metal cut out to shape the beam into the appropriate configuration before it hits. The latter is considered skin collimation.

2. Orthovoltage is very low energy beams which do not require bolus/chainmail.

3. You can't use MLCs to collimate electrons because (a) the leaves are in not fine enough to shape them appropriately and (b) if there is too much distance between collimation and target the electrons will scatter. This is why the mount holding the electron cut out is placed very close to the patient rather than at the treatment head.

 

[scarbrtj]

Dumb question about something basic in rad onc. I know for certain skin cancers etc. or breast boosts we use electrons. I keep reading in textbooks that these require skin collimation. What are they referring to? Isn't the cerrobend block placed in the treatment head?

For that matter why can't you just block with MLCs like you do for a field edge in photons?

And what is this difference with orthovoltage? In orthovoltage, you have the cone that basically comes right next to the skin and you don't need further skin collimation?

Thanks.

To discover the electron, they had to be discovered/observed in the absence of air in a vacuum sealed tube. Air and its atomic nuclei (*EDIT:and coulombic forces) being so massive in relation to the puny electron causes too much scattering. MLCs could shape electrons if we could treat patients in a vacuum. I have actually had some funny ideas about developing a vacuum device to go between the gantry head and the patient so the MLC could be used.
*EDIT: (you have to shape the electron "beam" close to the patient because air)

 

[Cancerdancer]

In those cases, a lead shield is placed in front of the deep structure (e.g. under the upper lip) to stop electrons from penetrating. Finally, you typically use a metal cut out to shape the beam into the appropriate configuration before it hits. The latter is considered skin collimation.

I think this is a little unclear. A metal cut out to shape the beam is usually the block placed 10 cm from the target in the path of the beam, aka "the block".
Despite this, sometimes if you're treating near the eye (for example) you'll actually still thereafter place a piece of lead directly over the eye (or other vital structure) to ensure no sig dose is spraying into that region. That's what I call skin collimation--lead directly on skin.