Investigation of physical penumbra definition in treatment planning

Due to the small size of the beamlets in IMRT (intensity modulated radiotherapy), physical penumbra is one of the important dosimetric parameters and small changes in the penumbra have a notable impact on the results. The physical penumbra width is defined as the lateral distance between two specified isodose curves at a specified depth of phantom. In this study, after demonstrating the inconsistency of conventional physical penumbra definition for non-uniform MLC (multileaf collimator) fields, two new dosimetric parameters proposed that are suitable for MLC fields. Physical penumbra evaluation was obtained using IAEA (international atomic energy agency) phase space data for a Varian IX and EGS (electron gamma shower)nrc Monte Carlo code package.  These measurements have been performed in water phantom at an SSD of 100 cm and a depth of 5 cm. With conventional definition, physical penumbra width for all MLC setups on right and left sides of isodose curves (parallel to X and Y axes) are not equal.  With a novel beam penumbra parametrization, the effect of MLC setup is taken into consideration. The conventional definition of physical penumbra for non-uniform MLC setups is not comprehensive. For such cases, surface penumbra and uniformity index parameters are demonstrated to be more suitable and indicative of the field non-uniformity. It is also shown that for an idle treatment planning system, the uniformity index approaches unity.