Increasing dose gradient and uniformity in small fields using modulation: theory and prototypes for cone-based
1Department of Human Oncology, School of Medicine and Public Health, University of Wisconsin-Madison, 600 Highland Avenue, Madison, Wisconsin 53792.
Purpose:
To investigate the theoretical limits to the tradeoff between dose gradient and uniformity when modulation is used in the context of cone based SRS, and to design a prototype collimation device that allows for steeper dose gradients and/or higher target uniformity as compared to a standard circular collimator.
Methods:
An inverse planning optimization is performed in the context of idealized phantom geometry to determine the ideal fluence pattern that best approximates a "rect function" dose distribution. Ideal fluence patterns were approximated in a prototype device and radiochromic film dosimetry was utilized to compare the prototype device to a standard circular collimator.
Results:
For choices of prescription isodose lines above approximately 50%, utilizing modulation allows for an improved tradeoff between dose gradient index and dose heterogeneity index. Compensators placed within the circular collimator can achieve the necessary modulation.
Conclusions:
Using modulation with features on a submillimeter distance scale, it is possible to increase the dose gradient and/or uniformity in small fields.
