Field-in-Field:

  1. This is a technique within treatment planning which attempts to modulate hotspots for conventional external beam plans.
  2. The technique works by first associating the traditional fields for the plan (e.g. lateral and medial breast fields).
  3. When unacceptable hot spots are discovered, identical fields are created with the MLCs transitioned over the hot spots and smaller ratios of MUs (relative to the original fields) are delivered such that the hot spots can be managed.
  4. Modern treatment planning systems now possess the ability to take the combination of fields and their respective subfields and combine them into one treatment field that is delivered with a transition in the MLCs at the appropriate point during delivery to result in a quick and automated treatment of what would have been two separate fields.

 

Stereotactic Radiosurgery:

  1. The advantages of radiosurgery:
  1. Less clinic time required for the patient.
  2. Less recovery time needed for the patient.
  3. Less risk to critical structures.
  4. Generally, six or fewer treatments (compared to 30, or so, fractions for conventional external beam therapy).
  5. Margins are exceedingly small, so less risk to normal tissue.
  1. Requirements of radiosurgery:
  1. Accurate knowledge of the target volume.
  2. We have to be able to superimpose a 3D dose distribution over patient anatomy accurately.
  3. We have to be able to accurately deliver dose to the target volume both numerically and spatially.
  4. We have to be able to provide sharp dose gradients immediately outside the target volume.
  1. Gamma Knife versus Cyberknife:

 

image036

 

  1. With Gamma Knife, we use a helmet-like apparatus that the patient’s head is placed into.  The target volume is established to be the isocenter of the machine which is equipped with 201 channels each containing a Co-60 seed.  We may change the field size of the setup (in a very limited fashion) by attaching another helmet to the patient that has different hole sizes.  The advantage of the Gamma Knife is in precision the targeting accuracy of the system is around 0.3 mm.  However, it is noted that the Gamma Knife is limited to smaller field sizes or using multiple isocenters for a single lesion.
  1. There is a new Gamma Knife model called the Perfection that utilizes 192 Co-60 sources and has 3 collimators (16, 8, 4mm) built into the unit (no switching helmets) which can greatly speed up treatment times.
  2. There is another Gamma Knife model available called the Icon whose major novel abilities include the ability to use CBCT positioning and IR monitoring of the patient allowing frameless (mask based) radiosurgery. It is also capable of treating some limited extracranial sites.
  1. The CyberKnife is, in effect, a LINAC mounted on a high-precision robot arm.  The machine is capable of providing many fields to a treatment volume from a huge variety of directions (the machine has 6 degrees of freedom).  The machines are equipped with orthogonal x-ray systems that allow precise anatomical positioning on-the-fly.  Unlike the Gamma Knife, the CyberKnife can treat extracranially.  The machine operates at about 600 MU/min.

 

image037

 

  1. These are compared to linac-based radiosurgery in which a linear accelerator has been equipped with on-board imaging techniques and has micro-multileaf collimators.  The machine operates at about 1000 MU/min.  The method of radiosurgery with linear accelerators is to use many arcs and table angles to form non-coplanar arcs.  The advantage of the linac-based radiosurgery device in comparison to both Gamma Knife and CyberKnife is that the linac can be used for conventional treatment options as well (therefore, the machine can provide revenue without treatment radiosurgery patients exclusively).

Anonymous
2013-12-03, 22:42
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