Room: Exhibit Hall
Purpose: Because of small size of the beamlets in IMRT, determination of physical penumbra is important and small changes in the penumbra will have a great impact on output. Physical penumbra definition is a one dimensional quantity indicative of the lateral distance between two specified isodose curves at a specified depth of the phantom. In this study the physical penumbra was evaluated and physical penumbra measurement in two directions ( X and Y axes) was performed. Factor assessed is impact of the location of leaves and field shape in two directions. Surface between two isodose curve (20% and 80%) in z=5 cm and X-Y plane was calculated. Two new dosimetric parameters proposed and calculated, those are intended to be suitable for MLC fields
Methods: Calculations were performed by using IAEA phase space data for Varian IX just above MLC and BEAMnrc/DOSXYZnrc for source to surface distance (SSD)=100 cm and in a homogenous water phantom.
Results: Physical penumbra width for each condition on right and left sides of isodose curve is not equal . The same phenomena are observed for the physical penumbra width in y direction .Physical penumbra width is depend on location of measurement.
Conclusion: The intended purpose from physical penumbra in its conventional definition is not satisfied for non-uniform MLC setups. For such cases, the newly defined surface penumbra and uniformity index are demonstrated to be more suitable and indicative of the field non-uniformity. CTV to PTV margin recipe is based on random and systematic errors, as well as the beam penumbra while defining the physical penumbra is incomplete and one dimensional parameter; the surface penumbra is proposed as a substitute for the physical penumbra to determine the PTV and CTV. It is also shown that for an idle treatment planning system, the uniformity index approaches unity.
Penumbra, Intensity Modulation