What is defined as 'Fraction' in radiation therapy?

In radiation therapy, 'Fraction' specifically refers to one radiation treatment applied to a particular target volume. This concept is essential in treatment planning, as it helps to manage the total dose of radiation that a patient will receive over a treatment course. Each fraction of radiation is delivered at specified intervals, allowing healthy tissues to repair between treatments while maximizing the damage to cancerous cells.

The correct understanding of fractionation is crucial because it influences the effectiveness of the treatment, the side effects experienced, and the overall therapeutic outcome. The aim is to optimize the dose delivered to the tumor while minimizing exposure to surrounding healthy tissues.

Other choices relate to different concepts in radiation therapy. A single treatment session does not capture the full scope of a fraction, as a fraction specifically denotes the context of the total treatment plan. Total cumulative doses consider the sum of all fractions but do not define what a fraction is in isolation. Treatment modalities refer to the different types of treatment approaches rather than the breakdown of doses, which is not synonymous with the term 'Fraction' in this specific context.