Defining Fraction Groups is the first step in the SOBP Optimization process within Astroid. Most commonly, a fraction group is simply an arrangement of beams that will be used in a typical daily treatment fraction. The Fraction Group contains the Beam Set where each beam can be weighted and normalized based on the needs of each case. These are key concepts within Astroid that allow for high levels of control over the Astroid SOBP Optimization engine. Further details of these critical items are provided below.

• Color: Display color of the Fraction Group
• Description: Optional, user specified text describing the Fraction Group
• Prescription: Prescription that the Fraction Group implements; note that only targets containing dose statements from this Prescription will be available when selecting the Target for the Fraction Group
• Number of Fractions: The total number of fractions to be delivered for this Fraction Group; this is very important as it will determine the appropriate Monitor Units for the individual beams