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The need for rotational margins in intensity-modulated radiotherapy and a new method for planning target volume
Mark Peter Langer1, Lech Papiez, Siarhei Spirydovich
1Department of Radiation Oncology, Indiana University School of Medicine, Indianapolis, IN 46202, USA. mlanger@iupui.edu
Purpose:
The effect of rotational errors on the coverage of clinical target volumes (CTVs) is examined. A new planning target volume (PTV) construction that considers the individual paths traced by movements of the target boundary points is developed.
Methods And Materials:
A standard uniform margin expansion was compared with a PTV constructed from the space swept out by a concave moving target. A new method formed the PTV by aggregating the separate convex hulls taken of the positions of the individual target boundary points in a sampling of CTV displacements.
Results:
A 0.5-cm uniform margin adequate for translations was inadequate given CTV rotation about a fixed off-center axis. A PTV formed of the target's swept-out area was 22% smaller than needed for coverage by a uniform margin, but computationally is not readily extended to translations combined with rotations about a shifting axis. Forming instead the union of convex hulls of the boundary points in a sampling of CTV displacements represented these movements in the PTV design and retained the target's concave shape.
Conclusions:
Planning target volumes should accommodate target rotation. The union of convex hulls of the boundary point positions in a sampling of displacements can effectively represent multiple sources of deviations while preserving target concavities.
