Effect of Total and Hypoxic Tumor Volumes on the TCP in Hypofractionated Radiotherapy
A Chvetsov1*, R Stewart1, L Hanin2, J Zeng1, R Rengan1, S Lo1, (1) University of Washington, Seattle, WA, (2) Idaho State University,Pocatello, ID
Presentations
PO-GePV-T-443 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: Clinical studies in the hypofractionated stereotactic body radiotherapy (SBRT) have shown a reduction in the probability of local tumor control with increasing initial tumor volume. A plausible mechanism for these volume effects would be useful for the analysis of treatment outcomes and the design of dose escalation protocols.
Methods: In our earlier work, we obtained and tested an analytical dependence of the TCP (tumor control probability) on the total and hypoxic tumor volumes using conventional radiotherapy model. We used a two-level cell population tumor model for hypoxic tumors and the TCP equation derived from the Poisson probability distribution. Compared to radiotherapy with conventional fractionation schedules, simulations of hypofractionated radiotherapy may require different models for cell survival and the Oxygen Enhancement Ratio (OER). Our TCP simulations in hypofractionated radiotherapy are based on the Liner-Quadratic (LQ) model and the Universal Survival Curve (USC) developed for the high doses used in SBRT. The predicted trends in local control as a function of the initial tumor volume were evaluated in SBRT for non-small cell lung cancer.
Results: Both LQ and USC based models cannot describe the TCP reduction for larger tumor volumes if the tumor is completely oxygenated. The TCP calculations are in agreement with the clinical data if the subpopulation of radio-resistant hypoxic cells is considered with the volume that increases as initial tumor volume increases.
Conclusion: First, the extent of hypoxia is likely a primary reason of the TCP reduction with increasing the initial tumor volume in SBRT for non-small cell lung cancer. Second, the LQ model can be an acceptable approximation for the TCP calculations in hypofractionated radiotherapy if the tumor response is defined primarily by the hypoxic fraction. The larger value of OER in the hypofractionated radiotherapy extends the applicability of the LQ model to larger doses.
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