Percentage Depth Dose (PDD) in Radiotherapy
In the field of radiotherapy, precision is paramount to ensuring that radiation beams deliver the correct amount of energy to a target while sparing surrounding healthy tissue. One of the fundamental tools used by medical physicists to plan these treatments is the Percentage Depth Dose (PDD) curve. This curve describes how the absorbed dose deposited by a radiation beam changes as it penetrates deeper into a medium along the beam's central axis.
To calculate PDD, the dose measured at a specific depth is divided by the maximum dose, known as d max. The resulting value is expressed as a percentage of that maximum dose. Because human tissue behaves similarly to water in terms of radiation scattering and absorption, these measurements are typically conducted in water or water-equivalent plastics using an ionization chamber—a specialized instrument used to measure the intensity of ionizing radiation.
[ไม่มีภาพประกอบ]Key Facts
- PDD Definition: The ratio of the absorbed dose at a specific depth to the maximum dose (d max), expressed as a percentage.
- Measurement Medium: Water or water-equivalent plastic is used to simulate human tissue.
- Primary Factors: PDD is influenced by beam energy, field size, source to surface distance (SSD), and the depth of interest.
- General Trend: PDD typically refers to depths beyond the d max point and generally decreases as depth increases.
Factors Influencing Percentage Depth Dose
The behavior of a radiation beam as it travels through a medium is not static; it is affected by several physical variables. Understanding these relationships is critical for accurate dose distribution.
Depth and Attenuation
As the radiation beam penetrates deeper into the medium, the PDD decreases. This reduction occurs for two primary reasons: the inverse square law (where intensity decreases as the distance from the source increases) and the attenuation of the beam, which is the loss of intensity as photons are absorbed or scattered by the medium.
Radiation Field Size
The size of the radiation field directly impacts the PDD. An increase in field size leads to an increase in PDD. This happens because a larger field generates a greater number of primary photons and increases the amount of scattered photons returning to the central axis from the irradiated medium.
Source to Surface Distance (SSD)
The Source to Surface Distance (SSD) is the distance from the radiation source to the surface of the patient or phantom. PDD increases as the SSD increases. This is because the relative variations caused by the inverse square law over a fixed distance interval are smaller when the total distance from the source is larger.
| Variable | Change in Variable | Effect on PDD | Primary Reason |
|---|---|---|---|
| Depth | Increase | Decrease | Inverse square law and attenuation |
| Field Size | Increase | Increase | More primary and scattered photons |
| SSD | Increase | Increase | Reduced inverse square variation |
Frequently Asked Questions
What is d max in radiotherapy?
d max refers to the depth at which the radiation beam deposits its maximum absorbed dose.
Why is water used to measure PDD?
Water is used because its properties regarding radiation scattering and absorption are very similar to those of human soft tissue.
How does the inverse square law affect PDD?
The inverse square law causes the intensity of the radiation beam to decrease as the distance from the source increases, contributing to the overall decrease in PDD as depth increases.
Why does a larger field size increase the PDD?
A larger field size increases the volume of the irradiated medium, which results in a higher number of scattered photons contributing to the dose along the central axis.
What is the relationship between SSD and PDD?
PDD increases as the Source to Surface Distance (SSD) increases because the relative change in dose due to the inverse square law is less significant over a fixed depth when the source is further away.