Characteristics of induced activity from medical linear accelerators.

A study of the induced activity in a medical linear accelerator (linac) room was carried out on several linac installations. Higher beam energy, higher dose rate, and larger field size generally result in higher activation levels at a given point of interest, while the use of multileaf collimators (MLC) can also increase the activation level at the isocenter. Both theoretical and experimental studies reveal that the activation level in the morning before any clinical work increases from Monday to Saturday and then decreases during the weekend. This weekly activation picture keeps stable from one week to another during standard clinical operation of the linac. An effective half-life for a given point in the treatment room can be determined from the measured or calculated activity decay curves. The effective half-life for points inside the treatment field is longer than that for points outside of the field in the patient plane, while a larger field and longer irradiation time can also make the effective half-life longer. The activation level reaches its practical saturation value after a 30 min continuous irradiation, corresponding to 12 000 MU at a "dose rate" of 400 MU/min. A "dose" of 300 MU was given 20 times in 15 min intervals to determine the trends in the activation level in a typical clinical mode. As well, a long-term (85 h over a long weekend) decay curve was measured to evaluate the long-term decay of room activation after a typical day of clinical linac use. A mathematical model for the activation level at the isocenter has been established and shown to be useful in explaining and predicting the induced activity levels for typical clinical and experimental conditions. The activation level for a 22 MeV electron beam was also measured and the result shows it is essentially negligible.