Increasing dose gradient and uniformity in small fields using modulation: theory and prototypes for cone-based stereotactic radiosurgery.
OBJECTIVE
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.
Department of Human Oncology, School of Medicine and Public Health, University of Wisconsin-Madison, 600 Highland Avenue, Madison, Wisconsin 53792.
Edward T
ET
Bender
D000465:Algorithms; D004867:Equipment Design; D005240:Feasibility Studies; D005373:Film Dosimetry; D008954:Models, Biological; D009339:Needles; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*