Design, construction, and performance of an asymmetrical collimator for a cobalt unit

Besides four linear accelerators, our institute has a 100-cm SAD cobalt unit which has some specific and useful features for radiotherapy. The unit is equipped with a electronic portal imaging device (EPID) and an asymmetrical collimator to perform similar treatment techniques as on the linear accelerators, using the same shielding and half-beam techniques. The design, construction, and performance of the asymmetrical collimator are described.

The new design includes the doubling of the drive and read-outs for both X and Y jaws. Spare parts of the standard collimator provided by the manufacturer were used as much as possible. Special trimmers were designed to allow asymmetry to the center of the field. The trimmers are removable, but the design allows the use of trimmers, wedges, and shadow tray simultaneously.

Transmission through the trimmers is less than 5% of the dose at d(max). The use of the trimmers extends the source to diaphragm distance to 53 cm, resulting in a smaller penumbra as compared to the standard collimator. Without the trimmers, the maximum field size remains unchanged, i.e., 40 x 40 cm(2); with the trimmers, it is 38 x 36 cm(2). Percentage depth doses and output factors were similar to the standard collimator.

The modified collimator is compact and allows use of the same shielding blocks as the linear accelerators, providing the same treatment techniques and capabilities. This simplifies an exchange of patients between our treatment machines.