Defining treatment beams will be one of the most important tasks within the Astroid planning system. Defining appropriate beams will require users to use their knowledge and experience to properly select many of the parameters that define a treatment beam. These parameters include the target, geometry (isocenter, gantry and couch angles), beamline devices, air gap, and spot placement options. The Beam task utilizes a series of blocks to organize the beam creation process into a common step-by-step sequence. Several blocks are optional as not all beams will use all features. Additionally, it is important to point out that the treatment room & default spot placement parameters are set outside of the individual beam creation tasks as these apply to all beams (however, spot placement parameters can be overridden within each beam if desired). An example of constructing a lateral beam, with the isocenter at the centroid of the PTV is given below to illustrate the features available when defining a beam.

1. From within the Plan Overview select the Beams block
2. Select the treatment room from the drop down then click the blue OK button
3. Next select the Spot Placement Parameters. Edit these parameters to define the spot placement grid for each beam in the plan, noting that individual beams can override these values during beam creation if you so desire
4. The next step is to create the beam by clicking the Create New PBS Beam button
5. Now we will proceed step-by-step through the various “blocks” to create a complete beam as shown below:
   1. In the General block select the intended Geometric Target from the dropdown. You may choose an existing target or create a new structure. For this example we chose the PTV_7920 as the geometric target. The geometric target will be used to define the extents of the aperture (if used) and will be linked to the isocenter position (if target centroid is selected in the approach block). Also specified here is the Spot Target, which will be used to define the extent of the pbs spot placements for the beam. The spot target can either match the geometric target, or the user can choose to use the target for the fraction group in which the beam is used (this is a useful option as it allows the same geometric beam to be used in multiple fraction groups by simply recomputing the spot positions based on the fraction group target).
   2. In the Approach block specify the desired isocenter from the dropdown. You may choose to use the centroid of the Geometric Target (as shown below) or you can select or create a new point to define the location for the isocenter. The gantry angle and couch angles are also entered here as well. Editing these values can be done by typing directly in the provided fields or by using the sliders. The patient in this example is feet first so we will use 90 for the Gantry angle and 0 for the Couch Angle to create a left lateral beam.
   3. The next block is the Snout block. A list of snouts associated with the specified treatment room will be available to choose from. The 12cm snout was chosen here.
   4. If desired, add an aperture in the Aperture block. (No Aperture was chosen in this example)
      1. Refer to Creating a New Aperture for detailed instruction
   5. If desired, select the range Shifter to use based on the ones available for the selected snout. If no Shifter is needed as in the example given, the user may go to the next step
   6. Once the beam line devices are defined, we can move to specify the Air Gap distance. The valid air gap range will be listed based on the selected snout. The user may choose any value in this range. 30mm was chosen in the below example.
   7. With the beam positioned and any beamline devices put in place, the user is ready view the PBS Spots and adjust the Spot Placement values if needed. The Spot Placement box, if chosen, will allow the user to set new parameters, overriding the spot placement parameters for this one beam if desired. The example below illustrates the message shown when using the spot placement values from the plan level.
   8. The last block in the Create Beam task is the Proton DRR Options. These do not impact the beam and are used purely for visualization purposes so that you can set the DRR Options to levels that generate appropriate anatomy visualizations. An example DRR is shown below. Note that Astroid allows you to define 2 distinct DRRs and then blend them together using a simple weight factor to create a single DRR image on the screen. This gives users the freedom to create high contrast, high quality DRR visualizations. A single image was used in the example below as the second set of DRR options has the weight set to 0.