Elongation effects on the Therac 6 linear accelerator

The effects of field elongation on the central‐axis output at d max and on the depth‐dose curves for the 6‐MV x rays from the Therac 6 linear accelerator are considered. These parameters are independent of the collimator angle for both square and elongated fields. The exchange of collimator pairs results in significant output differences but has a negligible effect on the depth‐dose curves. Both the central‐axis output at d max and the depth‐dose curves agree satisfactorily with the predictions of the equivalent‐square technique. An alternative method of greater accuracy is also indicated for the prediction of central‐axis outputs at d max for elongated fields.