Comparison of the penumbra between focused and nondivergent blocks--implications for multileaf collimators.

The penumbra from a 10-MV x-ray beam has been measured using various field-shaping blocks located in the normal blocking tray position and compared to the penumbra of the adjustable photon jaws of the machine. The results showed that the penumbra was substantially degraded by these blocks only for field sizes greater than 15 x 15 cm2 and at the depth of maximum dose, dmax. At depths of 11 cm or greater, there was a significant difference only for the same range of field sizes in the 80% to 50% dose region.

Remotely controlled positioning lasers.

Positioning lasers used in radiotherapy generally require little adjustment. When required, this is easy to do for sidelights, but can be difficult and time consuming for ceiling mounted lasers. For this purpose, we have modified a commercial positioning laser by incorporating a device for rotating the alignment adjustment screws.