Increasing dose gradient and uniformity in small fields using modulation: Theory and prototypes for cone-based stereotactic radiosurgery

Authors


Abstract

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.

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