Abstract 3611: An investigation of thermal dose as a parameter to model the thermal effects of high intensity focused ultrasound in cancer therapy
Authors: Mouratidis Petros , ter Haar Gail 
Published: August 2015

Abstract
Objectives: High Intensity Focused Ultrasound (HIFU) is a non-invasive technique for treating solid tumours by exposing target tissue to ablative temperatures, resulting in necrotic cell death. Progression of HIFU as a mainstream treatment option is hampered by lack of measurable dosimetric standards. Thermal dose (TD) is a parameter that can be used to describe HIFU treatments in the clinic and suggests that, above a reference temperature, for every 1°C increase in temperature, treatment time should be halved to achieve the same thermal damage. In our study we investigated whether TD can be used to describe the biological effect for a range of different thermal exposures. The activation of distinct programmed cell death processes in response to heat was determined. Methods: Two human colon cancer cell lines, HCT116 and HT29, were exposed to heat using a thermal cycler for long duration treatments, or using a pre-heated water bath for short duration treatments. Temperature was recorded using thermocouples. Cell viability was assessed using the MTT assay. Induction of apoptosis was estimated using an ELISA assay to detect cleaved cytoplasmic nucleosomes and using annexin V / propidium iodide (PI) staining to determine phosphatidylserine exposure to the outer side of the plasma membrane. Induction of autophagy was determined using immunoblotting with an antibody against LC3II, and using a fluorescent cationic amphiphillic tracer to detect autophagosomes. Necroptosis and basic heat shock response was determined by western blotting using antibodies against RIPK3 and Hsp70 respectively. The percentage of cells at different stages of the cell cycle was estimated by staining DNA with PI. Results: Exposure of cancer cells to TDs of 60 to 240 minutes resulted in decreased viability that depended on the TD applied and on time elapsed after treatment. This effect was associated with induction of apoptosis, autophagy and necroptosis. Autophagy induction was correlated with live cells. The percentage of cells undergoing each form of cell death was estimated at different time points up to 48 hours after treatment. A transient increase in the percentage of cells in the G1 phase of the cell cycle and activation of heat shock protein expression were associated with the response of cells to heat. Reduction in cell viability observed after exposure of cancer cells to TDs between 300 minutes and 1000 minutes depended on the method and duration of thermal exposure. Conclusions: TD induced pleiotropic biological effects such as increased cytotoxicity and activation of distinct programmed cell death processes in colon cancer cells. The method of heat delivery was shown to play an important role on the outcome of the thermal treatment suggesting limitations for the use of the TD parameter as a dosimetric standard in the clinic.
Citation Format: Petros Mouratidis, Gail ter Haar. An investigation of thermal dose as a parameter to model the thermal effects of high intensity focused ultrasound in cancer therapy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3611. doi:10.1158/1538-7445.AM2015-3611