A therapist verifies the light field during setup for an isocenter-based treatment with a 100 cm source-to-isocenter distance. The chart lists a nominal 10 x 10 cm field at the isocenter, the central-axis target depth is 10 cm, and a ruler on the patient's skin shows about 9 cm between the light-field edges. Which statement correctly explains the skin measurement?
Select an answer to reveal the explanation.
Short Explanation
Your 9 cm reading matches a 10 cm isocenter field reaching skin that sits 10 cm closer to the source. Nothing's off with the setup; the beam simply widens as it travels. That change in size with distance is plain divergence.
Full Explanation
The collimator defines the nominal field at the isocenter plane, 100 cm from the source in this scenario. The skin lies 90 cm from the source, so a 10 cm field at the isocenter projects to about 9 cm on the skin, which matches the ruler reading. Adjusting the jaws so the skin reads 10 cm treats a harmless projection difference as an error and leaves the field wider than planned at the isocenter. Moving the couch to force a 10 cm skin reading changes the source-to-surface distance and moves the isocenter away from the target, creating a setup error. Reading the chart value as a skin field reverses the field-size convention for this isocentric plan, and closing the jaws would make the field too small at the target. Compare the measured surface value with the charted isocenter value by accounting for the distance from the source before changing the jaws or couch.