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The effects of target size and tissue density on the minimum margin required for random errors
Author(s) -
Witte Marnix G.,
van der Geer Joris,
Schneider Christoph,
Lebesque Joos V.,
van Herk Marcel
Publication year - 2004
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.1809991
Subject(s) - margin (machine learning) , standard deviation , dosimetry , mathematics , radiation treatment planning , nuclear medicine , statistics , radiation therapy , physics , materials science , biomedical engineering , medicine , computer science , radiology , machine learning
The minimum margins required to compensate for random geometric uncertainties in the delivery of radiotherapy treatment were determined for a spherical Clinical Target Volume, using an analytic model for the cumulative dose. Margins were calculated such that the minimum dose in the target would be no less than 95% of the prescribed dose for 90% of the patients. The dose distribution model incorporated two Gaussians, and could accurately represent realistic dose profiles for various target sizes in lung and water. It was found that variations in target size and tissue density lead to significant changes in the minimum margin required for random errors. The random error margin increased with tissue density, and decreased with target size. The required margins were similar for dose distributions of spherical and cylindrical symmetry. Significant dose outside the spherical high dose region, as could result from multiple incident beams, lead to an increased margin for the larger targets. We could confirm that the previously proposed margin of 0.7 times the standard deviation of the random errors is safe for standard deviations up to 5 mm , except for very small targets in dense material.