Investigation of physical penumbra definition in IMRT applications

Introduction: Because of small size of the beamlets in IMRT, physical penumbra is one of the important dosimetric parameters and small changes in the penumbra will have a great 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 fields, two new dosimetric parameters proposed, those are intended to be suitable for MLC fields   Materials and Methods: Physical penumbra evaluation was obtained using IAEA phase space data for a Varian IX and EGSnrc Monte Carlo code package. The homogenous water phantom is located at SSD=100 cm. Results: With conventional definition, physical penumbra widths for all MLC setups on right and left sides of isodose curves (parallel to X and Y axes) are not equal. With the proposed dosimetric parameters, the effect of MLC setup is taken into consideration. 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. It is also shown that for an idle treatment planning system, the uniformity index approaches unity.