Non-uniform spatiotemporal fractionation schemes in photon radiotherapy

Abstract

Fractionation decisions in radiotherapy face the tradeoff between increasing the number of fractions to spare normal tissues, and increasing the total dose to achieve the same level of tumor control. In that regard, the ideal treatment would fractionate in normal tissues while simultaneously hypofractionating in the tumor. Interestingly, this is possible to a limited degree by delivering distinct dose distributions in different fractions. The dose distributions have to be designed such that similar doses are delivered to normal tissues while delivering high single fraction doses to parts of the tumor. In this paper, proof-of-concept is provided that this concept may lead to an improved therapeutic ratio for rotation therapy treatments using conventional photon beams. An idealized model of a large tumor treated in two fractions is considered. Fractionation effects are modeled via the biologically equivalent dose model. It is shown that the optimal treatment delivers the dose to the center of the tumor in a single fraction, while the second fraction delivers dose only to the rim of the tumor. The approach may potentially be interesting for large tumors embedded in a dose-limiting organ treated with stereotactic regimens.