AbstractID: 9403 Title: Penumbra and Edge of Field Measurements
Penumbra and Edge of Field Measurements
This work presents measured data of beam penumbra, defined as the 80% to 20% distance, and distance to the edge of the
treatment field, defined as the 90% to 50% distance, using a 4.0 mm inner wall diameter ion chamber. The measurements were made
in water for 6, 15, and 18 MV beams. The beam edge parameters were defined by Radionics, Siemens and Varian multi-leaf
collimators and by Siemens and Varian adjustable collimators. Measurements were made at depths of maximum dose, 10 cm, and 30
cm for field sizes of 10 cm x 10 cm and 30 cm x 30 cm. Measurements were made in the same beam plane with different beam edges
being defined by rotating the collimator.
The measured penumbra widths for 6 MV x-rays ranged from 5 mm for beam edge defined by the mMLC for a 10 cm x 10
cm field at dmax to 20 mm for 30 cm depth and a 30 cm x 30 cm field. The distances from the 90% to the 50% dose value were one to
two millimeters smaller than the penumbra values. The higher energy beams had slightly higher values for both parameters. It is the
90% to 50% values, which should be considered when defining conformal treatment fields. The data indicate that depth is a more
important consideration than the position or design of the collimator edge when defining conformal field apertures.
AbstractID: 9403 Title: Penumbra and Edge of Field Measurements