Intrafraction geometric uncertainties in frameless image-guided radiosurgery.

PURPOSE: Frameless radiosurgery allows the possibility of intrafraction patient movement. Because radiosurgery delivers the entire radiation dose during one or a few fractions, intrafraction misalignment can result in significant underdosage of the treatment site. This article compares alignment errors and their effect on target coverage for treatments that rely only on initial (fixed) alignment vs. those that make intrafraction corrections (dynamic alignment).

METHODS AND MATERIALS: This study analyzed 577 records of intrafraction patient movement observed during frameless cranial and spinal radiosurgery. For each fraction, the average misalignment per fraction was calculated. Then each fraction was divided into n minifractions and margin formulae developed for hyperfractionated radiotherapy were used to estimate the planning margin that would be necessary to preserve target coverage for the observed intrafraction movement.

RESULTS: Dynamic alignment reduced the number of fractions with a mean misalignment greater than 2 mm from approximately 20% to nearly zero. For fixed alignment, the estimated margins for optimal target coverage were 3.6-4.5 mm for the various treatment sites. For dynamic alignment, the optimal margins were 1.2-1.6 mm.

CONCLUSIONS: The estimated margins show the large influence of systematic intrafraction shifts and the capacity of dynamic alignment to correct for them. For dynamic alignment, the margin approximately equates with the traditional precision tolerances for radiosurgery, whereas for fixed alignment, the margin is three times greater. Although these margins may not be directly applicable to radiosurgery planning, they expose the effects of intrafraction motion on target coverage.