Physical Science & Biophysics Journal (PSBJ)
ISSN: 2641-9165
Research Article
Increasing Number of Fractions in the Case of Non-Uniform Dose and Fixed Nominal BED Leads to Increased Cell Killing in the Treatment Target
Abstract
Purpose: To evaluate how heterogeneity of the target dose affects cell survival in the target and biologically effective dose (BED) depending on the number of fractions (Nf ) .
Methods: Effect of dose non-uniformity on the probability of cell survival in the target volume is studied by using the linear-quadratic model. In this work we compare cell killing for different fractionation schedules under the assumption that the nominal biologically effective dose (BEDnom) nom is fixed.
Results: It is theoretically shown that in the case BEDnom= const the probability of cell survival in the target decreases with increasing Nf for an arbitrary ratio σ/ D̅ where σ and D̅ denote variance and mean of the target dose, respectively. This result is valid for an arbitrary distribution of the target dose. To demonstrate dependence of BED on Nf and σ/ D̅, we computed BEDs by using DVHs for 57 clinical cases of early-stage, non-small cell lung cancer. The computed BEDs demonstrate potential increase in cell kill for the considered cases when Nf is increased from 5 to 20 for a fixed BEDnom.
Conclusion: Small variations in the target dose (i.e., σ / D ≤ 0.1) can significantly reduce BED in Stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS). The magnitude of decrease in BED can be reduced by increasing Nf. The obtained results indicate that moderate hypofractionation with Nf = 15−20 can yield higher BED as compared to the frequently used SBRT schedules with five or fewer fractions.
Keywords: Heterogeneous Target Dose; BED; Hypofractionation; SBRT
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