The Potential Increase in Biological Effectiveness from Temporal Optimization for Radiation Therapy
Schmitt, Jonathan D.
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Stereotactic Radiation Body Therapy (SBRT) is a radiotherapy technique which uses high dose fractions with multiple co-planar and non-coplanar beams. Due to the large fractional doses, treatments are typically protracted and the number of fields are greater than a typical IMRT treatment. We demonstrate, using theoretical models, that temporal optimization methods could be applied to SBRT treatments to enhance biological effectiveness. Using the Lea-Catcheside protraction factor (G-value), we cycled through every field order permutation, of actual SBRT patient's fractions, and determine an optimal and sub-optimal field. For comparison, an actual field order shot in the clinic was included. To quantify the difference in survival fraction we utilized the Lethal Potential Lethal (LPL) model. To acquire the parameters needed by the LPL model we fit the data to three non-small cell lung cancers (NSCLC): H460, H660, and H157. The results are expressed as the ratios ( V :▵) N and ( C :▵) N , whre N is the number fractions in the SBRT protocol. Our results verified those of studies by previous authors, that maximization of cell kill is achieved by orienting the fields in a triangle pattern, where the fields with greatest dose are positioned in the center. Minimization of cell kill occurred when fields with smallest dose were positioned centrally and higher dose fields were placed in the beginning and end of the fraction. This orientation resembled a V shape. The survival fraction ratios calculated using the LPL showed that regardless of the cell type the ▵ shape had lowed survival fractions compared to both the clinical example (C) and the V arrangement. For H460, with T 1/2 = 0.25 h, an average ratio of ( C :▵) 5 = 10.8 , meaning a the ▵ pattern is approximately 10 times more effective than a clinical field order, after 5 fractions. Using theoretical models we have shown that rearrangement of the field orders for a SBRT treatment could optimize cell kill and potentially affect overall treatment outcome.