**4. Response evaluation**

### **4.1. Evaluation of response: Antiangiogenics and mTOR inhibitors**

We must consider several issues when assessing the therapeutic response of tumors. The morphological assessment with quantification of changes in size used in the RECIST criteria ("Criteria in Solid Tumors Response") has been our main concern when assessing tumor re‐ sponse [2]. This approach seems true for the use of cytotoxic drugs. However, this assess‐ ment is limited, since the macroscopic changes take time to become evident, often are not specific and do not provide information on the physiological and molecular component of tumors [42].

Advances in the field of oncology have led to the development of new drugs in renal cancer as sunitinib, sorafenib, pazopanib, tivoaznib, axitinib, temsirolimus, everolimus and bevaci‐ zumab [46]. These drugs (mainly cytostatic) cause little change in lesion size. Therefore, RE‐ CIST criteria are not entirely suitable for assessing tumor response, and proper techniques will vary according to the mechanism of action of the drug.

The recent emergence of techniques for the functional study of angiogenesis, such as perfu‐ sion CT or dynamic MRI allow obtaining quantitative parameters (blood volume, blood flow mean transit time, ktrans Ve, etc.) and would open a interesting field for assessing tu‐ mor response in a more objective [57] [58] [59]. This could open the door to the development of a strategy based on the image for the selection of patients to be treated with antiangiogen‐ ic therapies. However, each of these techniques has advantages and disadvantages. Thus, CT perfusion shows the drawback of radiation necessary for conducting the studies, where‐ as in the case of dynamic MRI the analysis of the results is much more complex.

The functional and molecular imaging techniques could offer clear opportunities in the study of renal tumors, but nevertheless, we must not forget that, for validation as bio‐ markers, would require completing a qualification and validation process, which would pass through standardization in the collection and analysis of the images and the correlation of the parameters obtained with patient outcomes. Once this is ach‐ ieved, functional-molecular techniques, especially perfusion CT, could become promis‐ ing tools in the selection of patients for targeted drug therapy and the assessment of the response [57] [58].
