Evaluation of treatment response

CT is the standard approach for target localization and radiation-dose calculation. The IQon Spectral CT increases the value of CT in radiation treatment planning by functionally defining tissues and enabling clinicians to characterize differences, such as carbon/oxygen ratio, iodine density, electron density, and other features that might be exploited in clinical radiotherapy. The assessment of tissue hypoxia serves as a poignant example of the benefit of spectral CT. Adult and pediatric tumors have regions of hypoxia that are biologically distinct and prone to increased resistance to therapy. Targeting regions of hypoxia for increased treatment intensity could be a means to overcoming radiation resistance.

Imaging and quantification of the relative amounts of oxygen and other elements in tissues is a feature of spectral CT imaging that should be investigated further. Nontarget hypoxic normal tissues are prone to injury. Establishing normal ranges in these imaging parameters would help identify injured normal tissue that might be avoided during radiation-treatment planning. Quantitatively assessing the iodine concentration also holds promise to accurately assess tumor vascularity, hypoxia, and treatment response.

Spectral CT data are acquired in a dual-layer detector system under all conditions; these data make up an invaluable data set for retrospective research, enabling investigators to map treatment failures to quantitative imaging used to plan treatments. The dual-layer detector CT system can store spectral data for retrospective analysis; however, this is not possible for every type of DECT system. Some DECT scanners will not generate dual-energy data if used for conventional scans. Serial imaging, often performed during radiation therapy and necessitated by changes in patient anatomy or to account for alterations in the size and shape of the target, provides additional opportunities to obtain quantitative imaging data, including reassortment of underlying elements and radiobiological conditions, and to monitor treatment response.

DECT systems have been used to assess the response of solid tumors to radiation therapy and chemotherapy. In one study, iodine quantitation correlated with measurements obtained from FDG-PET/CT, suggesting that DECT-based iodine quantitation might substitute for assessing lung cancer response to treatment.28 A decrease in iodine density in the arterial phase has been used to assess the response to therapy in patients with cervical cancer.29 DECT was also used successfully in a murine soft-tissue sarcoma model; DECT imaging enabled investigators to quantify liposomal iodine and gold nanoparticle accumulation and assess vascular permeability after treatment with chemotherapy and variable doses of radiation therapy.30 An earlier study compared the abilities of SECT and DECT to derive the concentrations of oxygen and carbon in human tissues for ion therapy applications.31 DECT more accurately assigned concentrations than did SECT when noise and other parameters of uncertainty were not considered.

The blood volume of tumors can be more accurately assessed by average iodine density than by average CT value, and the former has been proposed as a noninvasive, quantitative method to assess radioresistance attributed to the presumed hypoxic cell fraction of a given tumor.32 The same group showed that in a series of patients with lung cancer, local tumor control rates are worse in patients with lower average iodine density, and higher average iodine density is significantly associated with local tumor control.

Effective atomic number (Z effective value), as a response parameter, was investigated in patients with rectal cancer treated with neoadjuvant therapy that included irradiation. Al-Najami I et al. showed that a reduction in the Z effective value after therapy was associated with better pathologic response.33