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3D-CRT vs. IMRT in Radiotherapy

  1. Jun 18, 2015 #1
    I'm having some trouble truly understanding the difference between these two.

    As far as I know, the 3D-CRT is just beams from different angles, where you can modulate the beam intensity by lead blocks custom made for each patient, cut out to fit the outline of a tumour.

    IMRT is when the Multileaf Collimator (MLC) is introduced. So you have a step-and-shoot option, where the MLC is static during treatment, and then the sliding window technique where you can move the collimators real time during treatment.

    Is this correctly understood ?

    I think my main concern is, that I maybe thought/think that MLCs are also used in 3D-CRT. But maybe it turns into IMRT when MLCs are used, or...?

    Thanks in advance
  2. jcsd
  3. Jun 19, 2015 #2
    You are basically correct. MLC's are used in IMRT. 3D-CRT uses beam modifying devices as wedge filters and compensators to produce a more uniform dose in 3 dimensions using 3D treatment planning software to assess the uniformity in 3 dimensions. 3D-CRT doesn't do as good a job as IMRT in achieving uniformity and concentrating the radiation dose to the targeted volume.
  4. Jun 20, 2015 #3
    Hmmm, I actually thought, that MLCs were used in 3D-CRT, just as a static procedure, like it was done with lead blocks years back. But that is not the case ? MLCs are IMRT only ?
  5. Jun 20, 2015 #4
    Initially IMRT started with the step and shoot technique and changed into the sliding window scheme as the technology evolved. 3D-CRT was instituted with the development of real 3D treatment planning systems. You certainly can use MLC in 3D-CRT but initially field shapes where configured with manually made blocks base on 3D imagining techniques as CT or MRI. 3D-CRT plans are optimized manually where IMRT plans are optimized by computer based on boundary conditions set up by the radiation oncologist.including such things as max and min doses to the treatment volume and max doses to sensitive structure whose doses must be limited.
  6. Jun 20, 2015 #5


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    Yes multileaf collimators are used in 3D conformal plans. As Gleem said, they used to use blocks to shape the fields to the target, but since just about all modern machines are equipped with an MLC these days it's a lot faster to design a plan with an MLC shape that will be moded up to the linac when you want to deliver the plan. The therapist doesn't have to go into the room, there's less chance that the wrong blocks will be put in place and you will have a record of the positions the leaves were in during delivery. And because it's faster, there's less chance that your patient (or the stuff inside the patient) will move on the couch after initial setup.

    The potential downside to using and MLC for 3D conformal therapy (or IMRT for that matter) is leakage between the leaves, but the general consensus seems to be that this is a minor concern when weighed against the advantages.
  7. Jun 20, 2015 #6
    Ahhh, okay... So MLCs can actually be used in both cases, but it is actually the treatment planning system that is the main difference between 3D-CRT and IMRT ?
  8. Jun 20, 2015 #7


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    By strict definition the difference is that with 3DCRT the MLC (or blocks) will be used as an aperture that conforms to a projection of the target volume (plus a margin) in the beam's eye view for each field. The intensity projected at the target is uniform across the field. The intensity that reaches the target may not be uniform because tissues of varying density and composition will be in the way.

    With IMRT, the key work is "modulated." So rather than delivering a uniform field to the projected target, you modulate the intensity by either adding up a series of smaller fields (step and shoot) or dynamically driving the MLC leaves across the field while the beam is on. It also allows you more degrees of freedom in limiting the dose to the sensitive structures you're trying to avoid and delivering a uniform dose to your target.

    You're right in that the main difference comes down to the treatment planning system. 3D-CRT is generally forward-planned. The dosimetrist will play with beam angles and adjust the MLC to conform to the target and play with beam weightings and then the system will calculate a dose bases on the settings that are chosen. IMRT is planning inversely. As Gleem said, you start with a set of dose goals and constraints, and then run an optimization algorithm that figures out what fluence pattern is necessary to meet those goals and then you run another algorithm that translates that into the dymanic MLC patterns or the set of smaller fields you want.
  9. Jun 21, 2015 #8
    Thank you very much for the great explanation :)
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