Spine SRS

Tartalomjegyzék

Plan Analysis Table

SRS Prescription (Controlled Inhomogeneity) “On, Off”



The software supports two modes of prescribing dose. They differ in the intended PTV dose homogeneity.

  1. SRS Prescription Mode “Off”
    • Defined coverage, e.g. Dose (99% Vol.) = 16 Gy
    • (optional) Maximum dose constraint, e.g. Dose (1% Vol.) ≤ 20 Gy

    The first mode (SRS Prescription (controlled inhomogeneity) “Off”) is used if a certain coverage of the PTV is desired (e.g., Dose (99% Vol.) = 16 Gy) but the prescription does not contain a well-defined maximum dose that shall be received in the treatment plan to be created. There might be a maximum dose constraint that shall not be surpassed, however, the optimizer has the freedom to choose a maximum dose that is less than the given constraint. This mode is used to generate rather homogeneous PTV dose distributions used in fractionated radiotherapy. This is the only mode supported in version 1.0 of the software.

  2. SRS Prescription Mode “On”
    • Defined coverage, e.g. Dose (99% Vol.) = 16 Gy
    • (mandatory) Desired dose constraint, e.g. Dose (1% Vol.) = 20 Gy

    The second mode (SRS Prescription (controlled inhomogeneity) “On”) is used when two constraints shall be fulfilled simultaneously, a certain PTV coverage and a well-defined maximum dose that shall be achieved in the treatment plan. In this scenario, the maximum dose is not seen as an upper limitation but as a desired dose (note: this has nothing to do with the desired dose of iPlan RT Dose). This mode should therefore typically be applied to generate rather inhomogeneous PTV dose distributions used in Stereotactic Radiosurgery (SRS, hence the name of the mode).

    As an alternative to specifying the coverage dose D1 and the maximum dose D2, it is possible to specify the inhomogeneity in terms of an isodose line percentage (% IDL). This is done by entering a coverage dose D1 (e.g., 25 Gy) and the %IDL value P < 100% (e.g., 83.3%). This automatically sets the maximum dose D2 to D1 / P (e.g., 30 Gy).

    The edit field for doing this is only available in Advanced Editing mode (see below).

Sliders



The VMAT sliders are found in the Plan Analysis Table on the left-hand side of the Overview and Gallery View layouts.

Normal Tissue Sparing Slider

Controls the importance of sparing the normal tissue surrounding the PTV. This includes the OARs but also extends into all other areas outside the PTV.

Internally, the optimization creates a so called “ring object” enveloping (but not including) the PTV. It has a thickness of 35 mm and is completely hidden from the user. The range is from Low to High. At all selectable positions, the optimizer tries to reduce the dose to the normal tissue, however, the reduction of the normal tissue dose is different for each position. More normal tissue sparing produces better (i.e., lower) CI and GI values. All sliders are discretized to allow for a better reproducibility of the optimization results.

Modulation Slider

The Modulation slider controls the amount of modulation found in the treatment plan. Modulation is determined by the following:

  • The number of monitor units

  • The amount of variation between the monitor units of control points of all arcs
  • The amount of leaf movement between control points

  • The amount of leaf “movement” within one segment

  • The size of the segments (smaller field sizes correspond to an increase in modulation)

The optimization tries to reduce the amount of modulation no matter which slider position is used, however, the slider sets the importance. The range is from Low to High modulation.

Off, Smart and Strict Constraint Settings

The Off, Smart and Strict constraint settings control the importance of each individual DVH constraint.

SettingDescription

Off

Setting the slider to Off means the constraint will not be taken into account during the optimization. Thus, if all constraints of a given OAR are set to Off, the OAR will effectively be ignored.

Smart

  • Setting the slider Smart means that the OAR is taken into account in a normal way.

  • The optimizer sees the constraint and penalizes any violation. However, since there are often other optimization goals working against the fulfillment of the constraint, the final treatment plan might still violate one or more constraints. To give a higher importance to the OAR (and therefore to the constraints belonging to it), use the Weighting slider for the Most Important OAR, which is preselected in the clinical protocol or the Guardian slider for all OARs except the Most Important OAR (see below).

Strict

  • Setting the slider to Strict means the constraint is given very high importance.

  • If the constraint is violated during optimization, the optimizer moves the objective behind the constraint to a lower dose value. For example, if a constraint is set to Strict for 12 Gy and the constraint is violated, the optimizer will lower its objective to a lower value (e.g., 10Gy), while leaving the actual constraint at 12 Gy. This procedure increases the term that penalizes the violation of this constraint and thus effectively increases its importance. Therefore, the optimizer tries hard to fulfill the constraint. However, there are still cases where other parts of the objective function win.

Guardian

Sets the importance of the OAR with respect to all other OARs and targets.

This slider does not exist for the Most Important OAR because its functionality is contained in the Weighting slider (see below), which does the same for the Most Important OAR (and more).

Weighting

Controls the compromise between the PTV and the OAR by adjusting the following:

  • The importance of the targets (PTV and boost) versus all OARs. The leftmost (Target) slider position means more weight on the targets (PTV and boost).
  • The Guardian of the Most Important OAR. The rightmost (OAR) slider position means more weight on the Most Important OAR.
  • SRS prescription mode “off” only:
    • The importance of fulfilling the Maximum Dose Constraint of the targets (left slider position means more weight on the Maximum Dose Constraint)
    • The importance of having a homogenous dose in the targets (left slider position means more weight on a homogeneous dose)
  • PTV coverage volume reduction in case a constraint of the Most Important OAR is violated. The rightmost slider position means a more relaxed PTV coverage volume, i.e., a higher deviation from the desired coverage volume if it is required to fulfill any strict constraint on the Most Important OAR. Refer to the next section about the “Halo” for an explanation on how the PTV coverage volume reduction mechanism is used for finding the best compromise between PTV coverage and OAR sparing.
The leftmost (Target) slider position is special in that it:

  • Disables the possibility for any strict constraint on the Most Important OAR

  • Disables the PTV coverage volume reduction altogether. Thus, the desired coverage volume

    is guaranteed to be fulfilled.

Color Indicators

PTV

  • Coverage:
    • Volume at “Coverage Dose” above “Desired Volume”: green
    • Volume at “Coverage Dose” below “Desired Volume”, but above “Tolerated Volume”: yellow
    • Volume at “Coverage Dose” below “Desired Volume”, and below “Tolerated Volume”: red
  • Maximum Dose Constraint in SRS Prescription mode “off”:
    • Dose below “Maximum Dose Constraint Dose”: green
    • Dose above “Maximum Dose Constraint Dose”, relative dose deviation below “Tolerated Dose Deviation of Maximum Dose Constraint”: yellow
    • Dose above “Maximum Dose Constraint Dose”, relative dose deviation above “Tolerated Dose Deviation of Maximum Dose Constraint”: red
  • Desired Dose Constraint in SRS Prescription mode “on”:
    • Relative dose deviation below “Tolerated Dose Deviation of Desired Dose Constraint”: green
    • Relative dose deviation above “Tolerated Dose Deviation of Desired Dose Constraint”: red

OAR

  • Constraint:
    • Dose below “Constraint Dose”: green
    • Dose above “Constraint Dose”, relative dose deviation below “Tolerated Dose Deviation”: yellow
    • Dose above “Constraint Dose”, relative dose deviation above “Tolerated Dose Deviation”: red

Advanced Editing “On”, “Off”



By default Advanced Editing is disabled (“off”). In this mode, you have the possibility to select a protocol and an arc setup and modify all slider positions. This is usually sufficient to create high quality treatment plans for most patients.

If you require more control over the optimization result, Advanced Editing can be enabled (“on”). In this mode, it is possible to additionally change the following parameters:

  • Arc setup (table angle, gantry start, gantry stop angle) in Beam’s Eye View
  • PTV Coverage: Desired Volume, Tolerated Volume, Dose
  • PTV Maximum/Desired Dose Constraint: Volume and Dose
  • SRS Prescription mode “on” only: %IDL value (see above)
  • User defined OAR objectives. The software differentiates between constraints and objectives. Constraints are clinical values and are used to determine if an OAR receives too much dose or not (affecting the color of the violation indicator). They are specified in the Protocol template. In contrast to this, objectives have no direct clinical meaning and are only used to steer the optimizer into the right direction. By default each constraint entered in the Protocol automatically creates an objective (at the same dose and volume). In Advanced Editing mode, the user can change the dose value of the objective used during optimization. The color of the constraint’s violation indicator is still given by the original constraint (since this has a clinical meaning), however, the optimizer will see the objective only and act accordingly (stricter or less strict than with the original constraint, depending on the modified dose value).

  • Normalize to constraint or objective. It is possible to click on the actual dose value in the plan analysis table in order to scale a treatment plan such that the corresponding constraint is fulfilled (for both PTVs and OARs).
  • Scale number of monitor units directly. Clicking on the amount of monitor units in the plan analysis table allows you to enter a scaling factor (in percent) that can be applied to a treatment plan.
Art-No. 60919-40EN

A kibocsájtás napja: 2019-05-22