An RT Treatment Session is a collection of RT treatment events that are performed in a contiguous manner without any break in-between (other than time needed for required preparations) during a single Visit. It is bound by the time period between the patient entering the treatment room and leaving the treatment room. In a treatment session one or more RT Radiation Sets (RSet in Figure 7.14-2) may be treated, each one instructed by an RT Radiation Set Delivery Instruction. An RT Treatment Session may also include imaging. A group of radiation deliveries that are separated by an intentional delay to accommodate radiobiological recovery effects are considered separate Treatment Sessions.

The Treatment Session UID (300A,0700), if present, uniquely identifies a RT Treatment Session.

Each treatment of an RT Radiation Set is labeled as an RT Treatment Fraction (often abbreviated as Fx) with a fraction number starting with 1 at the first RT Treatment Session in which the RT Radiation Set is delivered, incremented by 1 at each subsequent treatment session.

An RT Treatment Fraction is the delivery of a portion of the total dose (whose delivery is defined by an RT Radiation Set) which has been divided equally into smaller doses to be delivered over a period of time (e.g. daily for 4-6 weeks). In radiotherapy, this division of dose over a period of time is known as dose fractionation.

Partial treatments annotate RT Treatment Fractions, that are not completely performed for any reason (e.g. patient sickness, delivery device breakdown). The remainder of the RT Treatment Fraction is usually delivered at a later time. This remaining portion has the same fraction number as the one of the Partial Treatment Fraction. Further treatments will start a new RT Treatment Fraction with an incremented fraction number.

In Figure 7.14-3, the shaded areas of each Radiation Set represent the portion where dose is actually delivered. Partially shaded Radiation Sets therefore represents a partial treatment.