The cst cell
The constraints of all defined volumes of interest (VOIs) are stored inside the cst-cell. It is structured as follows:
<a name="cell" /a>
Screenshot of the cst-cell:
| Column | Content | Description |
|---|---|---|
| 1 | VOI index | number to identify the VOI |
| 2 | VOI name | string describing the VOI |
| 3 | VOI type | Specification whether the VOI is an organ at risk (OAR), a target volume or should be ignored |
| 4 | Voxel indices | Vector containing the indices of all voxels of the CT that are covered by the VOI |
| 5 | Tissue parameters | Structure containing information about the tissue of the VOI and it's overlap priority |
| 6 | Dose objectives | Structure containing information about the constraints used to calculate the objective function value |
All defined VOIs are enumerated starting with 0.
The VOI name is a string containing an organ name or a short description of the volume (e.g. 'BODY', 'Liver', 'GTV',...).
The VOI type specifies how the volume is considered during treatment planning:
| VOI type | handling during treatment planning |
|---|---|
| TARGET | The VOI will be covered with spot positions (protons / carbon ions) and bixels (photons) as defined in the stf struct. During the fluence optimization it will be considered according to the defined dose objectives. |
| OAR | The VOI will not be covered with spot positions or bixels. During the fluence optimization it will be considered according to the defined dose objectives. |
| IGNORED | The VOI will not be considered during the treatment planning. |
The indices of all voxels (of the CT-cube) that are covered by the VOI are stored in a vector (see cst-cell) within a cell array. I.e. we store the segmetation for the VOI as a binary mask, the polygon contour data is not part of matRad's standard data sets. As the same voxel can be covered by more than one VOI, an overlap priority (see tissue parameters) is defined to handle potential discrepancies when calculating the objective function value and generating the stf struct.
| Field | Description |
|---|---|
| Tissue class | The baseData-struct contains depth-dependent α- and β- values, which are different depending on the tissue class. This parameter defines the tissue class of the VOI. |
| αX | α is the linear parameter of the linear-quadratic model, here the constant value for a photon irradiation is given. It is used to check the consistency of the baseData-struct and the cst settings, and to calculate the reference effect in the biological objective function. |
| βX | β is the quadratic parameter of the linear-quadratic model, here the constant value for a photon irradiation is given. It is used to check the consistency of the baseData-struct and the cst settings, and to calculate the reference effect in the biological objective function. |
| Priority | The overlap priority defines to which VOI a voxel belongs, if there are several VOIs covering the same voxel. If the overlap priority is the same for two VOIs, "overlapping" voxels are considered twice (once for each VOI) during the calculation of the objective function value. |
Note that the tissue class, α, and β are only considered during biological optimization for carbon ion therapy.
Within this struct, matRad stores the objectives and constraints defined for inverse planning. matRad supports inverse planning based on the minimization of a weighted sum of objectives subject to non-linear yet differentiable hard constraints. The following kind of individual objectives, are currently supported:
| Type | Effect on the objective function |
|---|---|
| square underdosing | Only dose values lower than the threshold dose for this VOI are considered for the objective function. The deviations are squared, multiplied with the penalty factor and added to the objective function value. The penalty is normalized to the number of voxels per VOI. |
| square overdosing | Only dose values larger than the threshold dose for this VOI are considered for the objective function. The deviations are squared, multiplied with the penalty factor and added to the objective function value. The penalty is normalized to the number of voxels per VOI. |
| square deviation | All deviation from a reference dose for this VOI are considered for the objective function. All deviations are squared, multiplied with the penalty factor and added to the objective function value. The penalty is normalized to the number of voxels per VOI. |
| mean | All dose values inside this VOI are weighted with the specified, and normalized (see above), penalty factor and added to the objective function value. |
| EUD | EUD is the abbreviation for equivalent uniform dose. For this method a weighting factor and an exponent a have to be defined. For the calculation of the objective function value the dose in each voxel is taken to the power of a. Then the sum of all these values is taken (∑ Dia ) and divided by the number of voxels. The a-th root of this value is then multiplied with the weighting factor and added to the objective function value. |
| Min/max DVH objective | Only deviations from the reference dose over/under a reference volume are considered according to Wu & Mohan (2000 Medical Physics). |
The definition of constraints on the minimum and maximum dose, on the minimum and maximum mean dose, on dose volume points or the EUD within an organ can be done in a similar way via the same interface. Just select the according constraints in the column obj. / const. and fill in the required parameters.
matRad distinguishes between different objectives according to the column obj. / const.. The relative weighting of the objective within the overall objective function corresponds to the column penalty; potential dose reference parameters are to be specified in the column dose (e.g. a reference dose for the squared deviation dose). The exponent for the EUD objective/constraint goes into the column EUD, and the volume parameter for DVH objectives/constraints can be specified in the column volume. The column robustness is only available in a special research version.
The patient data contained within matRad (BOXPHANTOM, HEAD_AND_NECK, LIVER, PROSTATE and TG119) have default values defined within the cst-cell.
These values are chosen to produce a reasonable treatment plan, when using coplanar and equidistant photon beams. They can be used as a reference point for more sophisticated treatment plans.
