Photon Treatment: Factors Affecting Photon Dose Deposition Flashcards
How and why is the percent depth dose (PDD) at 10 cm affected by increases in field size, energy, source to skin distance (SSD), and physical wedges?
Field Size PDD: Increases, Why: Increased scatter
Energy PDD: Increases, Why: Increased penetration due to increased energy
SSD PDD: Increases, Why: Inverse square law (Mayneord factor)
Physical wedge PDD: Increases, Why: Beam Hardening
Explain the shape of a megavoltage central-axis percent depth dose (PDD) curve. Graph the PDD curve of a 6 MV beam. Label the curve at 0, 1.5, and 10 cm depth. Label the approximate slope of the beam between 1.5 and 10 cm?
The PDD curve reports dose deposition as a function of depth for a radiation beam. The PDD on the surface for megavoltage energies is below 100% as the incident photons produce high-energy electrons which travel a distance before coming to rest and depositing energy. This effect explains the “skin sparing” properties of megavoltage therapy. The peak in the curve known as Dmax, occurs deeper. The curve then declines, or attenuates, due to a combination of the inverse-square law, absorption, and scatter.
What is the percent change in the percent depth dose (PDD) per centimeter for cobalt-60 (Co-60), 6 and 10 MV photons under standard conditions?
Co-60 attenuates at approximately 4% per cm, 6 MV beams attenuate at approximately 3.5% per cm, and 10 MV attenuates at approximately 3.3% per cm. Note that these numbers are approximation, as the values change with depth.
Percent depth dose (PDD) values are generally tabulated for source to skin distance (SSD) of 100 cm. How can we estimate PDDs for SSDs other than 100 cm? What is the limitation of this technique?
The Mayneord F factor is used to estimate PDDs for different SSDs. The F factor is an estimation of PDD changes based solely on the inverse-square law. It assumes that the amount of scatter is constant as SSD changes, which is not quite correct and limits the accuracy of this method. For low energies, tissue-to-air ratio (TAR) correction to Mayneord F factor method can be used if TAR tables are available.
By convention, where is field size defined?
Field size is typically defined at machine isocenter, 100 cm from the source. This would then be on the patient surface for source to skin distance setup, or at isocenter for source to axis distance (SAD) setup.
Why are high-energy photons (>10 MV) generally not recommended in the treatment of lung cancer?
The difference in electron density between lung and the soft tissue/bone of the chest wall presents special problems in the management of lung cancer. High-energy photons can contribute to increased scatter dosing in the lung. Also, the larger buildup region of high-energy photons may compromise tumor coverage at its periphery.
What is the “buildup region” and why does it occur? How does the buildup region depend on the energy of a photon?
The buildup region is the initial region on the percent depth dose (PDD) curve between the surface of the patient and dmax. Remember that dose deposition of photons is dependent on secondary electrons. While the photons are steadily attenuated exponentially as a function of the inverse-square law, high-energy photons require some depth in tissue to generate secondary electrons, thus explaining why the deposited dose is low at the surface of the irradiated tissue.
How does depth of maximum dose (dmax) vary as a function of field size? Why?
The depth of dmax slightly decreases as a function of field size. For example, a dmax of 1.5 cm is estimated for a 6 MV photon and a 10 cm × 10 cm field, whereas a 20 cm × 20 cm field would expect to show a dmax of 1.4 cm. This is due to increased scatter contributing to superficial dose deposition, bringing dmax closer to the surface.
Why does percent depth dose (PDD) increase with field size?
PDD increases as field size increases due to scatter from the off-axis tissue. Note that both primary dose (photons emitted directly from the linac head) and secondary dose (scatter from surrounding tissue) contribute to PDD. Although both the dose at depth (Dd) and the dose at dmax rise, the Dd rises more than the dose at dmax (Dmax), therefore PDD increases.
How does the percent depth dose (PDD) field size dependence change with energy?
The increase in PDD with field size is larger for low energies. With higher energies, scatter is mostly in the direction of the photon, thus lateral scatter is less.
Explain the difference between and advantages of monitor unit (MU) calculation using tissue maximum ratio (TMR; source to axis distance [SAD] technique) compared to a percent depth dose (PDD; source to skin distance [SSD] technique).
PDD calculations are convenient when the SSD does not change and SSD is at the standard calibration distance of 100 cm. However, for a multifield treatment SSDs will typically vary or the patient will have to be moved for each beam. The TMR method is independent of SSD and has an advantage in this situation.
Draw a diagram defining the tissue-to-air ratio (TAR). Explain the effect field size and beam energy has on the TAR?
The TAR is defined as the dose at a point in tissue divided by the dose in air at the same distance from the source. Because the distance from the source is in both the numerator and denominator, the TAR does not vary with source to skin distance (SSD), giving a key advantage for calculations with a variable SSD; multifield source to axis distance (SAD) setup. If the field enlarges (with a constant depth), there is more scatter in the tissue (numerator). However, there is no scatter in air and the denominator remains constant, therefore the TAR increases. Since higher energies are more penetrating in tissue, the TAR increases with increasing energy.
What is the relationship between the peak scatter factor (PSF) and the backscatter factor (BSF)?
The BSF is the special case of PSF when low-energy photons with a dmax of zero are considered. If dmax is close to zero, the only scatter in tissue contributing to the dose to dmax is backward, hence the name “backscatter.”
What is the approximate peak scatter factor (PSF) for high-energy (≥6 MV) photon beams?
Since high-energy photons tend to scatter forward the PSF decreases to only a few percent for higher energy beams (PSF between 1.05 and 1.1). The PSF (or more precisely, the backscatter factor) is greater for low-energy photons due to increased lateral scatter in tissue (could be as high as 40% to 50% for diagnostic X-rays).
Both the definition of percent depth dose (PDD) and tissue maximum ratio (TMR) could be written as
Dd /Ddmax. Why are they different?
PDD is calculated using the same source to skin distance (SSD) and different depths, TMR is calculated using the same source to axis distance (SAD) and therefore is independent of SSD.
