AbstractID: 8196 Title: The impact of daily patient setup error and tissue inhomogeneity on PTV coverage
and OAR avoidance using IMRT
IMRT can yield plans with very high degree of conformality and steep penumbra and fulfill both the
conformal tumor coverage and the organ at risk (OAR) avoidance requirements for most treatment sites.
However, in the process of doing so, highly modulated intensity fields are generated that often present with
steep intensity gradients in order to satisfy the optimization objectives.
In this work we investigated two effects that could alter the precision of the plan. Firstly, we explored the
effect of daily setup error in the dose distribution and secondly we investigated the effect of inclusion or
exclusion of tissue inhomogeneity in the dose calculation. Our investigation was done for patients with
brain and with lung tumors.
The Pinnacle IMRT planning system was used for the dose calculation which employs a convolution
superposition dose calculation engine, coupled with a pencil beam algorithm for the dose optimization
phase. The daily setup error was modeled as a shift of the isocenter in three orthogonal directions (x,y,z)
while the effect of tissue density inhomogeneity was studied by enabling and disabling respectively the CT
to density table. Consequently, the dose was computed based on the patient’s true density distribution when
the inhomogeneity was enabled, and with water density equivalence in the homogeneous case.
In our study we found that the daily positioning error with IMRT can have a strong impact on dose to OAR
while the effect of tissue inhomogeneity although reduced, probably due to the large number of beams
employed is still significant.