The need for rotational margins in intensity-modulated radiotherapy and a new method for planning target volume design

• Published in: International journal of radiation oncology, biology, physics Vol. 63; no. 5; pp. 1592 - 1603
• Main Authors: Langer, Mark Peter, Papiez, Lech, Spirydovich, Siarhei, Thai, Van
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.12.2005
• Subjects: Concavities; IMRT; Margins; Radiotherapy Planning, Computer-Assisted - methods; Radiotherapy, Intensity-Modulated - methods; Rotation; Target design; Margins; Concavities; IMRT; Target design
• ISSN: 0360-3016; 1879-355X
• DOI: 10.1016/j.ijrobp.2005.08.021

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.