ESTRO 2023 - Abstract Book

S1852

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ESTRO 2023

Figure 2 Multi-parametric scatter plots for the responder and a representative non-responder.

Conclusion Simultaneous PET and DW-MR enables sub-volume analysis of rectal cancers. Potentially this will improve the predictive value for treatment response to neoadjuvant chemoradiotherapy, as well as enabling dose painting treatment strategies. Further studies are required to determine the repeatability of simultaneous PET and DW-MR imaging and evaluate the predictive value in substantially larger patient cohorts.

PO-2073 Oxygen sensitivity of a silicone based magnetic resonance contrast agent

R. Cormack 1 , J. Tokuda 2 , G. Ekchian 3 , E. Kaza 4 , M. Dyer 1 , M. Cima 5

1 Brigham and Women's Hospital, Radiation Oncology, Boston, USA; 2 Brigham and Women's Hospital, Radiology, Boston, USA; 3 StratagenBio, -, Cambridge, USA; 4 Brigham and Women's Hospital, Radiation Oncolgy, Boston, USA; 5 MIT, Koch Institute, Cambridge, USA Purpose or Objective Hypoxic tumor cells are known to be less radiosensitive than well oxygenated cells, and hypoxia is a poor prognostic factor in prostate, cervical and head and neck solid tumors. Knowledge about tumor oxygen environment could allow radiation dose distributions to be planned with higher doses in the hypoxic regions to compensate for their relative radio resistance. A silicone based material has an MR signal that correlates to blood oxygen content in animal models. The material is biocompatible and, in a solid form, could be implanted in to measure oxygen levels at multiple locations in tumors or monitor oxygen levels over long term follow up. Such sensors could be deployed in brachytherapy procedures or as tumor fiducials in external beam radiation as part of current standard of care procedures. This work assesses the sensitivity of the MR T1 relaxation rate to oxygen levels in a solid form of the material. Materials and Methods Solid formulations of silicone material were placed in chambers to which a known mixture of gas could be supplied. The chambers were embedded in phantoms containing a solid paraffin phase change material (PCM) having a transition temperature of 37 C to ensure a constant temperature over the duration of a measurement (Figure 1). The phantoms were heated to the PCM’s melting point in a water bath while maintaining a steady flow of gas of known mixture of oxygen and nitrogen. The phantoms were transferred to an MR scanner for measurement of the T1 relaxation rate of the silicone material while continuing to maintain gas flow. The silicone material was scanned in a 3 tesla MR scanner. The phantoms were scanned with inversion recovery sequence for inversion times ranging from 200 to 2000 mS. The T1 relaxation rate of the material was determined from a 3 parameter exponential fit. Multiple experiments were performed varying the oxygen content from 0 to 50% to measure the oxygen dependence of the relaxation rate.