Room: AAPM ePoster Library
Purpose: To present a novel method for generating asymmetric lesion-specific margins that are robust against rotational uncertainties for targets remote from isocenter when compared to a uniform margin expansion.
Methods: Asymmetric rotational margins were generated through a brute-force method that samples a large number of 3D rotations within a given range of rotational uncertainty. 3D meshes were derived from target contours to preserve spatial relationships and rotated using a sampled set of Euler angles corresponding to each axis of rotation. Rotated meshes were sampled along image planes using a marching-cubes algorithm to produce a rotated contour. The union of 150 simulated rotations results in a rotational envelope. Envelope structures were then generated for spherical targets with diameters of 5, 10, and 15mm at distances between 1-15cm from isocenter and rotational uncertainties of 0.5 and 1.0° around each axis. Mean absolute volume differences between uniform expansions of 1mm and rotational envelopes are reported along with distance thresholds at which rotational envelopes exceed the uniform boundary.
Results: On average, 0.5° rotational envelopes were 0.14cc (68.6%), 0.40cc (37.6%), and 0.88cc (30.4%) smaller than 1mm expansions of 5mm, 10mm, and 15mm targets respectively. For 1° of uncertainty, envelopes were on average 0.07cc (34%), 0.10cc (10.2%), and 0.28cc (9.6%) smaller respectively. Envelopes corresponding to 0.5° of rotational uncertainty were completely contained by the 1mm expansion for all distances. 1.0° of uncertainty resulted in distance-from-isocenter thresholds of 9, 7, and 7cm at which rotational-envelopes protruded outside the uniform expansion for 5, 10, and 15mm spherical-targets respectively.
Conclusion: Lesion-specific rotational margins may be generated using a brute-force method to sample potential rotations for a given level of uncertainty. Asymmetric envelopes reduced treatment volumes when compared to a uniform-expansion approach. For larger rotational uncertainties and distances from isocenter, targets may deviate outside 1mm uniform margins.