Scoring Quantities

Several quantities can be scored inside the defined regions, e.g. dose, LETd. Present implementation of scores in FRED is restricted to voxel-based scorers defined on the same grid of the referenced region. For instance, when scoring the dose on the phantom region, FRED accumulates the energy deposited in each voxel of the phantom during particle tracking. At the end of the run, the deposited energy is divided by the mass of the voxel and the dose map is saved to disk in mhd format.

Scorers are actived within the definition of each region, as reported here.

An example of requiring dose and LETd scoring on a CTscan phantom is here below:

region: phantom ; CTscan = MrHead ; score = [dose,LETd]

There are two main ways of scoring quantities at the voxel level: accumulator and influence matrix.

In accumulator mode, the scorer accumulates in the voxel all scoring events for a given simulation, and at the end of the run FRED outputs in the out/score folder the final map. Typical example is the total dose delivered by all pencilbeams defined in a given treatment plan. The mode is activated using the score field in region definition.

In influence matrix mode, FRED computes the scored quantities for each pencilbeam separately. Typical example is the dose influence matrix, also called the Dij matrix, which can be used to perform inverse-planning in a TPS code. The mode is activated using the scoreij field in region definition. In order to save space,all pencilbeam maps are zero-suppressed and stored in a single binary file. The output is saved in the out/scoreij folder.

The following example shows how to score the total dose and total LETd, and also producing the Dij matrix:

region: phantom ; CTscan = MrHead ; score = [dose,LETd]; scoreij = [dose]

This will generate the file layout

out
├── score
│   ├── Phantom.Dose.mhd
│   └── Phantom.LETd.mhd
└── scoreij
    └── Phantom.Dose.bin

Available scorers

Edep

total energy deposited in a voxel

\[E_{dep} = \sum_{events} \Delta E\]

for each energy deposition event the voxel scorer is incremented by the \(\Delta E\) energy released in the voxel. This energy is typically due to the collisional stopping power \(\frac{dE}{dx}\) of the particle in the medium. It also can have contributions from heavy fragments produced in nuclear interactions which, due to very short range, are deposited locally at the production site.

Scoring units = MeV

dose

total energy deposited in a voxel divided by the mass of the voxel

\[\mathrm{Dose} = D = \frac{E_{dep}}{m} = \frac{E_{dep}}{\rho\;V}\]

This is equivalent to dose-to-medium in dosimetry jargon.

Scoring units = Gy

dose-to-water

equivalent dose to water obtained from stopping power conversion

\[D_{w} = D \cdot \frac{\rho}{\rho_w} \cdot \frac{\left(\frac{dE}{dx}\right)_w}{\frac{dE}{dx}} = D \cdot \frac{\left(\frac{dE}{ds}\right)_w}{\frac{dE}{ds}} = D / RMSP\]

where the dose-to-medium \(D\) is converted into dose-to-water \(D_{w}\) using the density ratio and the stopping power ratio. It is equivalent to divide the dose \(D\) by the relative mass stopping power RMSP.

Scoring units = Gy

LETd

dose-averaged LET according to Eq. 14 in Polster et al, PMB 2015

\[\mathrm{LET}_d = \frac{\sum_{events} \frac{dE}{dx} \frac{1}{\rho}\;\Delta E} {\sum_{events} \Delta E} = \frac{\sum_{events} \frac{dE}{ds} \;\Delta E} {\sum_{events} \Delta E}\]

Scoring units = \(\mathrm{\frac{MeV\,cm^2}{g}}\)

activation

positron emitting activation of patient tissue. See Beta+ activation section for details.

spectra: spectra of a given quantity for each voxel. See spectra scorer section for details.

Description of specific scorers