Photon Percentage Depth Dose Curves (%DD):
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Megavoltage photon %DD curves exhibit a low skin dose, a peak of maximum dose, a gradual fall off, and no discrete range.
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In megavoltage beams, the low surface dose has been labeled the skin-sparing effect and was one of the major benefits of Co-60 when it was introduced.
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Kilovoltage photon %DD curves exhibit a near 100% surface dose (as seen with the 3.0 mm Cu HVL curve).
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When dose is building up to dmax what is occurring is photons are releasing electrons in the medium which release more electrons causing a buildup to a maximum dose where attenuation and the inverse square law take over (see figure below).
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At the depth of dmax we establish charged particle equilibrium (CPE). Beyond that, on the descending portion of the curve, we have established transient charged particle equilibrium.
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This has important implications in measuring the absorbed dose in the medium and allows us to relate exposure in an ionization chamber back to dose through a calibration factor.
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%DD dependencies:
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As the SSD increases, the %DD increases:
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This is due to the inverse square law being built into the %DD relationship.
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Important application: to decrease a hot spot on a treatment, you may increase the SSD.
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As the field size increases the %DD increases.
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This is due to increased scatter being present in the beam.
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This will also increase the %DD at the surface (increases surface dose).
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Equivalent Square fields:
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An equivalent square field is defined as a square field having the same %DD curve as the field in question.
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One semi-empirical way of determining a square field is using the ratio of the area (A) to the perimeter (P) to find the side of a square field (a) with an equivalent %DD:
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For example: for a 10x15 cm2 (A/P = 3) field, the equivalent square field would be a 12x12 cm2 field.
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In order to apply a measured %DD that was acquired at a specific SSD (usually 100 cm) to another SSD, the Mayneord F-factor must be utilized.
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The F-factor attempts to correct for changes in the inverse square law relationship contained within the %DD values.
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For more information on this see the Hand Calculations summary (available with full membership).
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Another important note is that when measuring a %DD curve in a water tank with an ion chamber you are measuring the percentage ionization curve (%I), but as the ratio of the mass-energy attenuation coefficients does not vary much with depth you effectively measure %DD
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