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Imaging of Tumor Response to Radiotherapy: New Results and Radiobiological Principles

J Bayouth1*, J Bissonnette2*, J Deasy3*, (1) University of Wisconsin, Madison, WI, (2) The Princess Margaret Cancer Centre - UHN, Toronto, ON, (3) Memorial Sloan Kettering Cancer Center, New York, NY


(Monday, 7/15/2019) 4:30 PM - 6:00 PM

Room: 221AB

New imaging technologies provide practical means for conducting serial imaging studies of tumor response during a course of radiotherapy. These technologies include repeated MRI studies using standard devices, repeate PET-CT studies, as well as newer whole-body/high-sensitivity PET imaging systems, in-room MRI, and daily in-room cone-beam CT’s. Understanding mechanisms behind the variability of response of individual tumors to radiotherapy is likely to inform treatment individualization and optimization. Each of these modalities offers different advantages as a probe of tumor biology and radiobiological response. In this session, we will cover basic principles of tumor radiobiology and the evolution of tumor response to radiotherapy over courses of hypo-fractionated radiotherapy or prolonged, standard fractionation (i.e., 2 Gy/weekday, typically for 6 weeks.) In particular, we will discuss serial MRI, serial PET-CT, and serial cone-beam-CT tumor response images, with an emphasis on what we are learning about response variability. Recent tumor radiobiological modeling (i.e., Jeong et al. "Modeling the cellular response of lung cancer to radiation therapy for a broad range of fractionation schedules," Clin Ca Res., 2017), provides a basis for understanding tumor response variability. Tumor regression depends critically on the fact that only a subset of cells can respond quickly to radiation damage, as quantified by the growth fraction, the fraction of tumor cells receiving enough oxygen and glucose to proceed through mitosis, with most other viable cells being more radioresistant due to a lack of oxygen. Hypoxic cells are likely to give rise to any surviving cellular clones, however. Slowly shrinking tumors have a smaller growth fraction and a larger hypoxia-resistant fraction of cells. The measured variability in response is then a probe of the underlying tumor microenvironment.

Learning Objectives:
1. Understand the basic driving mechanisms of tumor radiobiological response to a course of radiotherapy, including regression-rate and ultimate local control.
2. Understand the basic practical issues associated with serial PET-CT scanning during a course of radiotherapy, and the characteristics of the data for non-small cell lung cancer.
3. Understand the basic characteristics of serial in-room MRI imaging for characterizing tumor response.



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