What does the 'first treatment fraction' refer to in radiotherapy?

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In radiotherapy, a treatment fraction is one daily dose delivered in a single session. The first treatment fraction is the first day on which the patient actually receives radiation after planning and setup. This initial delivery typically follows immobilization and image-guided verification to ensure precise alignment with the target, and it marks the starting point of the treatment course. Fractionation—delivering the total prescribed dose in multiple fractions—helps maximize tumor control while allowing normal tissues time to repair between sessions.

The other ideas aren’t what this term refers to: planning work such as creating an initial plan or finalizing a plan happens before treatment delivery, and a hypothetical dose scenario is part of planning rather than the actual first day of treatment.