#### Photon Percentage Depth Dose Curves (%DD):

1. Megavoltage photon %DD curves exhibit a low skin dose, a peak of maximum dose, a gradual fall off, and no discrete range.

1. 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.

2. Kilovoltage photon %DD curves exhibit a near 100% surface dose (as seen with the 3.0 mm Cu HVL curve).

3. 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).

1. 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.

1. 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.

2. %DD dependencies:

1. As the SSD increases, the %DD increases:

1. This is due to the inverse square law being built into the %DD relationship.

2. Important application: to decrease a hot spot on a treatment, you may increase the SSD.

1. As the field size increases the %DD increases.

1. This is due to increased scatter being present in the beam.

2. This will also increase the %DD at the surface (increases surface dose).

1. Equivalent Square fields:

1. An equivalent square field is defined as a square field having the same %DD curve as the field in question.

2. 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:

.

1. For example: for a 10x15 cm2 (A/P = 3) field, the equivalent square field would be a 12x12 cm2 field.

1. 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.

1. The F-factor attempts to correct for changes in the inverse square law relationship contained within the %DD values.

2. For more information on this see the Hand Calculations summary (available with full membership).

2. 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