Low energy mammogram

  1. Question:
    Explain why it is necessary to use relatively low energy X-rays to produce an image of the breast.

    My answer:
    Currently x-rays energy levels used during mammograms have approximately a one in a million chance of inducing cancer, this means that approximately for every two-hundred cancers found one is caused. This risk level is generally seen as being acceptable. Increasing the energy of these x-rays used would increase the risk of inducing cancer, so low energy levels are used to avoid a threat to the patient.

    Does this answer seem OK to everyone? If you think there's anything i have missed or should add let me know :)
     
  2. jcsd
  3. Evo

    Staff: Mentor

    I don't know what the point of your thread is, but here is a recent study,

    http://www.ncbi.nlm.nih.gov/pubmed/19454801
     
  4. Choppy

    Choppy 3,125
    Science Advisor
    Education Advisor

    I think you're missing the point of the question.

    In mammography contrast is key. You're trying to detect microcalcifications which are indicators of cancer, and these are very small in dimension.

    Contrast arises from differences in linear attenuation coefficients between materials, which in turn arises from differences in the interaction cross sections. The photoelectric cross section varies with the inverse cube of photon energy (approximately), and directly with the cube of the material's effective atomic number. This is the dominant process at most x-ray imaging energies.

    By using a relatively low energy for mammography the very small microcalcifications can be detected because they have differences in their photoelectric cross sections. As energy increases, these differences diminish and you get less contrast.

    You can't go too low in energy, of course, because everything would be attenuated too much and you would loose too much signal for the noise produced.

    Cancer induction is indeed a concern for screening programs. The RBE of ~4 cited in the study above is actually rather high in my experience although this depends strongly on its definition and the end-point used (I've done some research in this area), but the fact of the matter is that the probability of inducing cancer for a given dose will increase as energy goes down. So I think you're off track with your current answer.
     
  5. russ_watters

    Staff: Mentor

    That is surprising. Can you explain why that is?
     
  6. Choppy

    Choppy 3,125
    Science Advisor
    Education Advisor

    Short version: LET increases with decreasing energy.

    Longer version:
    We're comparing equivalent doses - so the same amount of energy per unit mass is deposited. What changes then are patterns of energy deposition in relation to a target - usually assumed to be the cell's DNA.

    As electrons slow down in media they tend to deposit the majority of their energy towards the end of the track - leading to the "Bragg peak." (Note that I'm talking in terms of track length - electrons scatter all over the place so you don't see a Bragg peak in a depth-dose curve from an electron beam as you might from other heavier ion radiation.) LET - linear energy transfer - refers to the energy that's deposited locally in the immediate vicinity (about one micron in water) of the electron's track. As an electron slows, the energy deposited locally increases. So you get ionization events that are closer together.

    DNA is a "double helix" molecule. Cells are generally pretty good at repairing single strand breaks in the DNA, because when the other side is intact it serves as a template for the broken one. But when both strands are broken, the repair is a lot more difficult. In that case the repair process can lead to deletions or other errors in the chain. Those can then go on to cause cancer down the road.

    So the idea is that the double strand breaks that lead to cancer are more likely when you have an increased density of ionization events along a single track of radiation.
     
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