Document Type: Conference Proceedings
Department of Nuclear Engineering, Shiraz University, Faculty of Mechanical Engineering, Email: email@example.com Tel: +98 9385741949
1Department of Nuclear Engineering, Shiraz University, Faculty of Mechanical Engineering,
Department of Nuclear Engineering, Shiraz University, Faculty of Mechanical Engineering,
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.