SAD
Also written as: SAD — Source-to-Axis Distance
Source-to-Axis Distance, the fixed distance from the radiation source to the machine's isocenter, typically 100 cm for linear accelerators.
Full Definition
Source-to-Axis Distance (SAD) is a fundamental geometric parameter in radiation therapy equipment design, representing the fixed distance from the radiation source (X-ray target) to the machine's isocenter (the point around which the gantry rotates). For most modern linear accelerators, this distance is standardized at 100 cm. SAD technique involves positioning the patient so that the treatment depth is at the machine isocenter, which simplifies dose calculations and allows for consistent beam characteristics regardless of field size.
Usage
Usage note: Often confused with SSD (Source-to-Surface Distance); SAD is fixed while SSD varies with patient thickness.
In Context
- "The treatment was planned using SAD technique with the isocenter placed at tumor center." — Treatment planning report
- "SAD setup provides more consistent dose rates across different field sizes." — Physics protocol document