L 9: Dose distribution and scatter Analysis Flashcards
Tissue equivalence
Tissue-equivalent materials or phantoms (with regard to photon beam attenuation and depth dose distribution) must have the same effective atomic number and the same electron density (number of electrons per cm3) as those of soft tissue.
Phantoms
Homogenous or Anthropomorphic
Homogenous
Water
Solid water
Water
- Water, polystyrene, and synthetic plastics such as solid water are examples of materials that are almost tissue equivalent.
- Density = 1
- Atomic number = 7.42
Anthropomorphic
Anthropomorphic phantoms such as Alderson Rando Phantom incorporate materials to simulate body tissues—muscle, bone, lung, and air cavities.
Percent Depth Dose
- PDD for photon beams in water (or soft tissue), beyond the depth of maximum dose (Dmax), **decreases **almost exponentially with depth.
- It increases with an increase in beam energy (greater penetration), field size (increased scatter), and SSD (inversesquare law effect).
- Mayneord F factor accounts for change in PDD with SSD but not for change in scatter (e.g., for large field sizes and large depths). In general, it overestimates the increase in PDD with increase in SSD
Side of an equivalent square of a rectangle field
- Rectangular, square, and circular fields of photon beams may be equated
approximately in terms of dose output and depth dose distribution by using published tables or by equating A/P (area over perimeter). For example, for a given rectangular field of area A and perimeter P - Side of equivalent square = 4 A/P
- Radius of equivalent circle = (4/square root pie) A/P
- Cannot use this method for irregular fields
- Irregular fields = Clarkson’s method
Mayneord Factor
Tissue-Air ratio
- Ratio of dose in the phantom at a given point to the dose in free space at the same point.
- TAR, like the PDD, depends on depth, beam energy, field size, and field shape but is almost independent of SSD.
- TARs have traditionally been used for dose calculation involving low-energy beams (e.g., cobalt-60) and isocentric beam geometry (e.g., rotation therapy or stationarySAD techniques).
- TARs for low-energy beams (up to cobalt-60) can be measured directly or calculated from PDD.
SAR represents the scatter component of TAR. It is a useful concept for the dosimetry of irregularly shaped fields (e.g., Clarkson technique). Like the TAR, this quantity may be used for cobalt-60 or lower-energy beams. A more universal quantity is the SPR (the scatter component of TPR) or the SMR (the scatter component of TMR).
Back scatter Factor (BSF)
- BSF = Dmax/Dfs = TAR
- BSF or PSF is the TAR at Dmax. It is a substantial factor for beams in the orthovoltage range (highest values are for beams of ~0.6 mm Cu half-value layer (HVL) and can be as much as 20% to 40%, depending on field size).
- BSF decreases to a few percent for cobalt-60 and approaches unity (0%) for higher-energy x-ray beams.
- BSF, like the TAR, is no longer used in dosimetry of megavoltage beams except for a few institutions where it is still used as a “dummy variable”
Phantom Scatter factor (Sp)
Tissue Max ratio (TMR)
- TMR is a special case of TPR in which the reference depth is a fixed reference dmax for all field sizes.
- The reference dmax is chosen to be for a small field size (e.g., 3 × 3 cm2) to minimize the influence of electron contamination.
Isocentric Technique MU Calculation
TMR
SAD factor
SSD Technique MU Calculation
Summary
- TARs and BSFs (or peak scatter factors (PSFs)) are OK to use for low-energy beams (up to cobalt-60) but they cannot be measured accurately for high-energy beams.
- They are superseded by TMRs (or TPRs) and the related output factors Sc and Sp, which have no limitations of energy.
- Dosimetric quantities for the calculation of dose/MU include percent depth dose (PDD), TMR (or TPR), Sc, Sp, and distance factors pertaining to whether the beam bears an SSD calibration or SAD calibration. Assuming SAD = 100 cm, the SSD calibration has the phantom surface at 100 cm, in which case the point of calibration is at (100 + dmax). In the SAD calibration, the point of calibration is at 100 cm, while the phantom surface is at (100 – dmax). The depth dmax in all cases is the reference dmax.
- Sc and Sp, respectively, pertain to the collimator-defined field and the field actually irradiating the phantom.
- TMR is a special case of TPR in which the reference depth is a fixed reference dmax for all field sizes. The reference dmax is chosen to be for a small field size (e.g., 3 × 3 cm2) to minimize the influence of electron contamination.
- Whereas PDDs depend on SSD, TMRs and TPRs are almost independent of SSD.
- TMRs and TPRs can be directly measured in a water phantom or calculated from measured PDDs.
- SMRs and SPRs represent the scatter part of TMRs and TPRs, respectively, and can be used to calculate scattered dose in an irregularly shaped field using Clarkson’s technique.
- Calculation of dose at an off-axis point or in an asymmetric field requires primary off- axis ratio (POAR, also called off-center ratio) at the point of calculation.
SSD Factor
