Constraints can be specified at various levels (Plan, Fraction Group, Target/Beam Set) with Astroid and they will affect different groups of beams depending on their level. Constraints at the Plan level are applied to the total dose resulting from all beams. Constraints at the Fraction Group level are applied to the total dose resulting from only the beams in the current Fraction Group. Constraints at the Target/Beam Set level are split evenly and applied individually to each Beam Set. In other words, the Constraint dose is divided by the number of Beam Sets in the Target, and this dose is then applied as a constraint to each Beam Set, so that either SFO and IMPT can be achieved (see Fraction Groups). The section below will provide a walk through of the different levels and how constraints are applied at each one.

It should be noted that all constraints are considered “hard limits”- values that must be achieved. Constraints drive the feasibility calculation- whether the plan is achievable and should be used to ensure certain minimal clinical parameters are met.

The following constraint types are available. Note certain constraints are available only for Target type structures.

• Min: The minimum dose the structure must receive
• Max: The maximum dose the structure may receive
• Min Mean: The minimum mean dose a structure must receive
  • This will drive the dose up across the structure
• Max Mean: The maximum mean dose a structure may receive
  • This will limit the mean dose across the structure

The user can choose to apply one or multiple of these constraints to any number of structure.

Constraints at the Fraction Group level are applied to the total dose resulting from only those beams in the current Fraction Group. Constraints at the Target/Beam Set level are equally split among the Beam Sets within the Target and are applied to the total dose resulting from the beams in each of the Beam Sets. The following steps are a brief walkthrough for creating a max constraint of 73 Gy(RBE) to the PTV for the whole Fraction Group, and then creating two SFO beams that each provide a minimum dose of 35 Gy(RBE).

1. Select the Fraction Group if it has been created or create a new one by clicking Create New Fraction Group
2. Choose the phase and number of fractions to be treated with this Fraction Group
3. Under Group Constraints one can add any desired structures to which the Fraction Group level constraints will be applied
   1. Select the PTV from the drop down of structures
   2. Define what constraint(s) should be applied to each structure by choosing the constraint and entering the dose
      1. In the field beside Max, enter the value 73
4. Once the constraints are set in the Fraction Group the user will add Targets to the Target List and assign additional Constraints if desired
5. The assigned constraint doses at this level will be divided evenly among the Beam Sets in the Target
6. Add a new Target, then create two Beam Sets, each with a single beam
   1. Under Constraints select the PTV from the drop down of structures
      1. Now set the Min field to 70 (this 70 Gy(RBE) dose will be split 35 per beam set, creating the desired two SFO beams

Constraints at the Plan level are applied to the total dose across all beams.

1. Open the Constraint sub block contained in the Plan Constraints/Objectives block and choose the Edit button.
2. Choose from the drop down the structure or structures to which constraints should be added
3. Define what constraint(s) should be applied to each structure by choosing the constraint and entering the dose
4. Follow this and enter the constraints for all applicable structures.
5. When finished click the OK button.
6. Once all the Constraints have been set the user can either start the Feasibility or move on to defining the Objectives