51st AAPM Annual Meeting
Daniel Low, Washington University, St. Louis, MO, 63110, US
For more information about the American Association of Physicists in Medicine, visit aapm.org/

AbstractID: 12005 Title: Margins in Radiation Therapy
Conformal radiation therapy has historically used margins to account for geometrical
uncertainties in the position of a target volume. The margins, in their simplest form, are
simply expansions to the diameter of a treatment beam, to ensure that dosimetric planning
criteria are met in the presence of inter- and intra-fraction setup variations. Historically,
the size of the margin in a given treatment site was difficult to determine due to the
available technology and the time and effort required to obtaining accurate data.
Consequently, margins were estimated to accommodate a population of patients. As new
technologies have emerged, target volume position errors have become easier to measure,
their accuracy has increased, and the measurements can be made much more frequently.
As the quantity and quality of data has increased for patient populations, and even for
individual patients, the conceptual basis of employing margins has evolved. Strategies
for ensuring dosimetric coverage may now be individualized to a specific patient’s
geometric uncertainty characteristics with customized margins, or they may incorporate
geometric correction based on predefined action levels. Other strategies may incorporate
continuous monitoring to gate or modify the beam. And finally, other strategies seek to
eliminate margins by including the estimated geometrical uncertainty into the
development of the dose distribution.
Educational Objectives:
1. To provide an educational review of the technologies and methods of measuring
target volume positioning errors.
2. To review methods of determining population margins from measured data.
3. To review corrective and intervention strategies for patients with individualized
4. To review planning strategies which seek to eliminate margins.

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