What is the purpose of collimators in cancer therapy?

[lavster]

hi can anyone tell me what a broom beams and a pencil beams are. and their characteristics?
Im reading some papers on their use in radiotherapy but i do not fully understand what they are...

thanks

 

[Bob S]

Are you referring to charged particle beams (e.g., protons) or electron beams (e.g., X-rays?). In both radiotherapies, the particle beams are focused to a point (pencil) or a thin wide beam (broom). The objective is to focus the beam without excessively increasing the beam divergence. (The product of beam size times beam divergence in each transverse dimension is a constant.*). Usually with a broom beam, it has to be swept in one dimension, while for a pencil beam, it may have to be rastered in two dimensions. Furthermore, with proton beams, the beam energy can be varied to change the depth of penetration.

* Similar to depth of field in a camera lens; large lens aperture settings have smaller depths of field.

Bob S

 

[lavster]

thanks!

does that mean that broom beams are the type of beam that is produced by passive scattering? and it needs things such as a dynamic multileaf collimeter to focus it onto the tumour in cancer therapy? however, what does thin wide beam mean - i took from what you said that it starts off thin then gets wider? but isn't that undesirable and the opposite of being focused? so why would you use it? Finally, why do you split the two types of beams into charged particle beams and electron beams...an electron is a charged particle?

cheers, lav

 

[Bob S]

does that mean that broom beams are the type of beam that is produced by passive scattering? and it needs things such as a dynamic multileaf collimeter to focus it onto the tumour in cancer therapy?

Passive scattering (do you mean bremsstrahlung target?) increases beam divergence in both transverse dimensions. Collimators are then used to limit and shape the beam (protons) or bremsstrahlung (electrons) divergence.

however, what does thin wide beam mean - i took from what you said that it starts off thin then gets wider? but isn't that undesirable and the opposite of being focused? so why would you use it? Finally, why do you split the two types of beams into charged particle beams and electron beams...an electron is a charged particle?

Let me ask you this question: If you use an F-stop of 1.4 on a camera lens, how do you prevent the depth of field from getting paper thin? Electron beams are never used directly on patients. Only collimated X-rays or bremsstrahlung from electron beams are used on patients. See Fig, 4.8 on page 42 in

http://books.google.com/books?id=BaAJ4UFerxMC&pg=PA41&lpg=PA41&dq=Varian+electron+bremsstrahlung+radiation+therapy&source=bl&ots=lJCc4qhLA_&sig=hVZiJzl_tWzjxueNRkKq1JXGqVk&hl=en&ei=7anATLWgEYqisAO10siSDA&sa=X&oi=book_result&ct=result&resnum=2&ved=0CBgQ6AEwAQ#v=onepage&q=Varian%20electron%20bremsstrahlung%20radiation%20therapy&f=false

Bob S

 

[Bob S]

...and it needs things such as a dynamic multileaf collimeter to focus it onto the tumour in cancer therapy?

Collimators cannot focus anything. Collimators only block divergent radiation outside the tumour area. Only magnets can focus only charged particle beams. A dynamic collimator is made of leaves of tungsten, and the shape is dynamically changing as the accelerator pivots around the patient. On an electron machine (e.g., Varian), the patient can see the tungsten leaves inside the end window of the accelerator.

Bob S