WIth IMRT plans the variety of possible dose distributions is quite large. Typically if a physician does not like an IMRT plan they will request a plan to be re-run. This requires the planner to input new constraints and objectives and a new plan to be run from the beginning of the optimization process. This is a time consuming process. Astroid eliminates this cycle using a Multi Criteria Optimization (MCO) approach that allows planners and physicians to visualize the tradeoff of target volume coverage vs reduced dose to the OAR's in real time. MCO treatment planning is based on a set of Pareto optimized plans, where a plan is considered Pareto optimal if it satisfies all the constraints and none of the objectives can be improved without worsening at least one of the other objectives. So instead of creating just one plan, Astroid creates a set of optimal plans that satisfies the treatment plan constraints and puts an interactive exploration of dosimetric objectives at the planners and physicians fingertips via a unique, highly intuitive, Pareto surface navigation slider bar system.

Constraints play an important role in the optimization process, as they bound the solution space and ensure your navigation process is focused only on plans that meet your non-negotiable, highest priority dosimetric needs. It should be noted that if the constraints are too tight, there may be no feasible plans. However, if the constraints are too loose, too many solutions will exist and the navigation will be too broad to provide adequate resolution over the truly clinically useful plans. Therefore care should be taken to ensure appropriate constraints are set, which is facilitated using the Astroid feasibility check feature. So while constraints supply hard limits, objectives are the negotiable goals, they do not have a hard level that must be obtained, but “pushing” them harder does result in benefit to the patient. The number and type of objectives chosen should be such that all the relevant trade offs can be demonstrated and explored.

After the constraints have been entered, the user may start the Feasibility calculation by clicking calculate in the Feasibility block. The Feasibility calculation is based solely on the constraints and it should be used to ensure there is a feasible plan possible. The Feasibility calculation may be an iterative processes in order to get appropriate constraints established for a particular plan. In other words, the user may need to enter a constraint, check the feasibility, then progressively drop the constraint and check the feasibility until the plan is no longer feasible. It is recommended practice to start by obtaining a feasible plan utilizing only target constraints then add OAR constraints as desired. Remember, using a narrow range of constraints can improve the optimizer performance and improve the resolution of the Pareto surface navigation.

The user also needs to be aware of the impact of constraints being set on Fraction Group level versus the Plan level. For example, it is possible to have a constraint set in the Plan level so that the whole dose to an OAR is given on one day and none on the other day. This could happen when there are two Fraction Groups and the OAR dose is not split between the two by using Fraction Group level constraints.

The Objectives, as stated before are the negotiable goals where they may be no hard limit, but there is benefit to improving them. Astroid allows Objectives on both Targets and OAR's. Objectives can be placed on structures to either increase or decrease dose. The Objectives are the sole driving force guiding the MCO and it is important to recall from the discussion above that Astroid will only navigate to plans that are “optimal” in at least one objective (meaning again that this objective cannot be improved without another objective getting worse. Unlike Constraints, Objectives should be added all at once and there is no need to place them in any particular order (order is irrelevant). Additional information about Objectives can be found here. Since the MCO is finding a large set of optimal solutions the optimization can be a lengthy process. The following factors have the largest impact on the optimization run time:

• The number of points in the calculation grid (linear impact)
• The total number of spots from all beams (linear impact)
• The number of objectives (quadratic impact)

The number of calculation points and number spots will have a direct 1:1 (linear) impact on the calculation times; in other words, doubling one of these items will (roughly) double the MCO calculation time. The number of objectives scales quadratically, meaning that doubling the number of objectives creates a four-fold increase (2² = 4) in the number of MCO calculations that are required. For objectives, this does not always increase the overall wait time on the calculation however, thanks to the parallelization that be achieved using the Astroid cloud services backend. So for small numbers of objectives 1-3, you may not notice much (if any) increase in wait time by increasing up to 3-5 objectives (but this does depend on the availability and load on the Astroid cloud calculation servers).

Once all the desired Objectives are entered the MCO calculation is started just by clicking the calculate option in the Navigation block. It should be noted that the Feasibility will be re-checked if any of the Constraints have changed since the feasibility was last run. The MCO calculations will run in the cloud and the user can simply leave the Astroid application running and move on to other things while the calculations process. Please note that at this time the Astroid App should be left open in this state to ensure the calculations run to completion, however, users may open additional instances of Astroid and work on other plans while these calculations proceed (no performance issues should be encountered when using multiple instances since the “heavy” calculations are off-loaded to the cloud calculation servers).

Discuss how to check progress (put in later when progress widget done)

The user has many options for how the dose is displayed. The options for controlling the display of the dose are on the right hand side of the display under Dose Options. The Dose Options provide controls over the DVH type (relative or absolute volume), the colors and scaling of the display dose, and the type of dose display shown (colorwash, isolines, or isobands).

The planner has the option of viewing the dose for the DVH in relative volume (dose per percentage of the volume) or in absolute volume (dose per cc of the structure) using the Absolute DVH option. The user may also hover over any area of the DVH curve to obtain the dose and percentage of a given structure or click on a line to obtain start tracking the cursor value for one or more lines.

As in the DVH the user has multiple options for displaying the dose. The dose can be displayed in either relative (percentage) or absolute. This can be chosen by using the drop down menu under Levels. If the user chooses to view dose in relative mode they must then enter the 100% line dose- usually the prescription dose. The percentage isodose lines that the user wants to see must then be entered. If the user chooses to view absolute dose they need to choose absolute from the drop down and then enter the dose lines that they want to see.

The user also can choose to view the isodose as either isobands, isolines, colorwash or combinations of these. The user may use the sliders to set the opacity for each of these as well. They may also choose the line width and whether it is solid, dashed or dotted for isolines.

Add image and explanation of isobands Add image and explanation of isolines Add image and explanation of colorwash

Once the plan has been calculated the Navigation block will become active. This block contains all the active structures and their objectives. Under each objective is a slider bar. This slider bar allows the user to adjust an objective and to see how the change will effect DVH and other objectives.

On each slider there are two vertical bars. The thin blue bar denotes the dose the objective is at. The thick white bar is the dose limit the solution can go to. The objective will not go past this point. It can never get worse than this. The blue and white numbers to the upper right of each slider correlate to the dose the blue and white and white vertical bars are at. The numbers at the end of each slider bar denote the range that the user has to work in. This is the bar the user should grab to slide. Thick white bars that are on opposite ends of the slider means that the objectives are competing very little with each other. An example of this is in the diagram below. The CTVPO+3mm min max is competing very little with the CTVPO+3mm max mean. This means the user can make large adjustments to each of these without effecting the other. If both white slider bars are on the same side and an adjustment is made it will have a great impact on the other structures. The thick grey horizontal bar it the range or window that the user has to adjust the vertical bars. The user will notice as they drag the white bar to the left the window the user has to work in reduces. All of this allows the user to know the limits they have to work in. All of these adjustments are able to be done without running a new plan as would needed to be done in traditional treatment planning systems. This allows the user to look at many different solutions in a short amount of time.

If the user does not like the adjustments they have made to the structure objective slider they may hit the Reset button in the bottom left corner to reset all the objectives to their original state. It the user does like the adjustment that was made they may choose the Save button in the bottom right hand corner. This will save the objectives at their current position. The Cancel button will close the Navigation block in its current state.