When measuring isodose distributions using a water phantom, how many centimeters must the phantom extend laterally beyond the field?

When measuring isodose distributions using a water phantom, it is essential for the phantom to extend laterally beyond the field to adequately capture the dose distribution and ensure accuracy in measurements. Extending the water phantom 5 cm laterally beyond the field is considered a standard practice because this distance helps to encompass the penumbral region where the dose gradient can be steep.

The penumbra is the transition area between the fully irradiated and the fully non-irradiated regions, and having sufficient lateral extension allows for a complete representation of this gradient. If the phantom does not extend far enough, it can lead to inaccurate assessments of the dose distribution, particularly the edges of the treatment field. This 5 cm extension ensures that the measurement does not truncate critical data and allows for reliable planning and verification of radiation treatments.