Parallel Opposed Fields:

 

Photon Parallel Opposed PDD Original Version

 

    1. In general, megavoltage isodose curves are very similar except for the following:

      1. The depth of dmax increases with energy.

      2. Skin sparing increases with increasing energy.

      3. For treatment planning the hot spot decreases with energy.

      4. For treatment planning uniformity increases with energy.

    2. For kilovoltage beams, isodose curves display a large penumbra due to increased side scattering in the medium.

    3. A parallel opposed field plan will look like a classic hourglass shape with hotspots near the surface.

 

image037

 

 

Inhomogeneities and Their Effects:

 

    1. Air/lung:

      1. Air, or more commonly encountered in a patient - lung, is one of the strongest perturbations commonly encountered by a radiation beam.

      2. In general, these effects occur when a radiation beam is incident on a slab of lung.

        1. Immediately in front of the lung, the dose will drop due to a lack of backscatter.

        2. In the lung, the dose drops compared to water due to a lack of scatter.

        3. If there is a solid tumor in the lung, then the beam must undergo build up similar to at the surface.

          1. This is why it can be advantageous to use 6X during lung treatments for a smaller buildup length in a tumor.

          2. Another reason 6X beams are commonly employed in lung treatments is that the lateral electronic equilibrium is better modeled by the algorithms that aren’t Monte Carlo based.

        4. Beyond the lung, the dose will increase relative to water since there was little attenuation in the lung.

        5. Lung will increase the penumbra of a beam due to the ability of scattered electrons to travel farther beyond the collimated field size. This effect worsens at higher energies, which is another reason for using 6X in lung treatments.

      3. The density of the lung is about 0.33 that of water.  This can be used to calculate the amount of increased attenuation.

        1. For example, if we have 5 cm of lung and want to know the difference in attenuation relative to a homogeneous water phantom.

          1. 5 cm * 0.33 yields an effective path length of 1.65 cm.

          2. For a 6x beam attenuation in water is 3% per cm and for 18x it is 2% per cm.

          3. So, for a 6x beam the expected attenuation in water would be 5 cm * 3% = 15%, but with the lung in the beam, the attenuation is only 1.65 cm * 3% =  5%.

          4. For a 6x beam, 5 cm of lung leads to about 10% of a heterogeneity correction (for 18x this would only be about 6.5%).

        2. Important point: the higher the energy of the beam the less heterogeneity affects it.

    2. Bone:

 

Photons Inhomogeneous Bone Dose

 

      1. Bone has the following effects:

        1. Increased backscatter near the anterior interface leads to about an 8% increase in dose immediately adjacent to the bone in MV fields.

          1. kV beams have as much as a 200% increase in dose due to the photoelectric effect.

        2. Inside the bone (for MV beams), the dose to soft tissue, such as marrow, is expected to be higher than in water, but the dose to bone mineral (hard bone) is expected to be about 4% less.

          1. The reduction in dose to the bone mineral is due to a lack of hydrogen content, this causes the electron density relative to mass to decrease resulting in a lower dose.

          2. The graph above demonstrates the expected dose to the soft tissues inside bone.

        3. There is also a decrease in dose far from the bone due to increased attenuation.

        4. In high energy photons beams (18/24 MV), increased pair production produces even higher doses around and inside the bone than seen above in the 6 MV image.

        5. The density of bone is about 1.8 that of water, and this can be used to calculate a rough effect for bone attenuation in a beam similar to the discussion in lung.

    1. Metal (prostheses):

      1. Metal prostheses such as hip replacements have the following effects:

        1. Increased backscatter near the anterior interface can increase the dose by about 50%.

        2. At the posterior interface, above 10 MV increased pair production leads to an increase in dose of about 50% (but there is a decrease for 6 MV).

        3. The dose far from the implant for a single field through the implant is expected to drop by about 20-30% due to increased attenuation.

      2. For more information see the summary on TG-63 Hip Prostheses (available with full membership).

 

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