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Ultrasound Imaging of Apoptosis in Tumor Response: Novel Preclinical Monitoring of Photodynamic Therapy Effects

Departments of ¹ Radiation Oncology and ² Medical Biophysics, Faculty of Medicine, University of Toronto; Departments of ³ Radiation Oncology and ⁴ Imaging Research, Sunnybrook Health Sciences Center; Department of ⁵ Physics and ⁶ Electrical and Computer Engineering, Ryerson University, Toronto, Ontario, Canada

Requests for reprints: Gregory J. Czarnota, Department of Radiation Oncology, University of Toronto and Sunnybrook Health Sciences Center, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 3M5. Phone: 416-480-5329; Fax: 416-480-6002; E-mail: Gregory.Czarnota{at}sunnybrook.ca.

Key Words: Ultrasound • Apoptosis • Photodynamic therapy • Spectroscopic imaging

High-frequency ultrasound is a novel method to detect apoptotic cell death based on changes in cell morphology that cause alterations in the viscoelastic and, consequently, the acoustic properties of cell ensembles and tissues. In this study, we evaluated the first preclinical tumor-based use of high-frequency ultrasound spectroscopy to noninvasively monitor tumor treatment by following xenograft malignant melanoma tumor responses to photodynamic therapy (PDT) in vivo. We observed a time-dependant increase in ultrasound backscatter variables after treatment. The observed increases in spectroscopic variables correlated with morphologic findings, indicating increases in apoptotic cell death, which peaked at 24 hours after PDT. We analyzed the changes in spectral slope and backscatter in relation to apoptosis and histologic variations in cell nuclear size. Changes in spectral slope strongly correlated with the changes in mean nuclear size over time, associated with apoptosis, after PDT (P < 0.05). At 48 hours, a decrease in ultrasound backscatter was observed, which could be explained by an increase in cell nuclear degradation. In summary, we show that high-frequency ultrasound spectroscopic variables can be used noninvasively to monitor response after treatment in a preclinical tumor cancer model. These findings provide a foundation for future investigations regarding the use of ultrasound to monitor and aid the customization of treatments noninvasively based on responses to specific interventions. [Cancer Res 2008;68(20):8590–6]

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