Cerrobend Blocks:

image032

 

  1. Cerrobend is another beam modifier that is popularly used in radiation oncology although its use as a photon block has largely been replaced by the use of MLCs.
  2. Cerrobend is a metal alloy (also known as Lipowitz metal) that has an especially low melting point (160 oF) making it easily worked within a clinic’s block room.
  3. The composition of Cerrobend is bismuth, lead, tin, and cadmium (respectively by percent-contribution, and a rather toxic combination).
  4. Photon applications include using these blocks for treatment such as TBI where the blocks are designed to cover the lungs after a certain dose has been reached in order to elevate the dose received by the rest of the body to ensure depletion of the immune system by a leukemia patient.  Also, the blocks can be used for simple brain patients where a small eye block is needed to protect the patient from cataract formation.
  5. The blocks themselves are placed in a plastic tray at the bottom of the treatment unit head.  Thus, the therapists can see, using the light field, a silhouette of the blocked region and align the patient appropriately.  However, in doing so, it is important to include a blocking tray transmission factor which will be small, but still applies to the entire field and needs to be taken into account.
  6. The HVL for cerrobend is approximately 1.5 cm for 6 MV photon beams and 1.8-2.0 cm for 18 MV photons.
  7. Also, in order to effectively use cerrobend as a block, we must attenuate a beam to approximately 3-5% (which is accomplished with about 5 HVLs).  Thus, 7.5 to 8.0 cm of cerrobend would be needed to block a 6 MV photon beam.
  8. The density of cerrobend is 9.4 g/cm3 which is approximately 80% that of lead.
  1. A simple rule-of-thumb can be applied with this knowledge to go from the required thickness of cerrobend to the required thickness of lead which would provide equivalent blocking.  We just need to look at a ratio of the densities:
  2.  image034
  1. We can further note that if we have a small cerrobend island block (such as an eye block), the amount of dose contribution behind the block will be a combination of the transmission through the block (3-5%) plus the scatter components from the surrounding field that are able to get under the block and the scatter components that are scattering from the patient to the point under the block.  The in-scatter from the patient at the point under the block will have the largest dosimetric impact. This is due to the use of a small block, and there’s a large fluence of secondary particles being created very proximal to this point.  This could result, depending on the size of the block and the depth of the point-of-interest, on a value of 10-20% of the dose outside of the block. Thus, the cumulative effect could be 20-30% as compared to if there were no block at all present but this is very dependant on block size.

 

The Treat-To:

  1. This, historically, has been the physician’s call for an isodose selection.
  2. If an area is displaying a non-uniformity of percent doses, we can pick the percentage to treat-to to ensure that the entire area gets at least that percentage.
  3. It basically takes the selected isodose line and delivers 100% of the prescription dose to that line.

 

Treat To

 

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