Prostate

In a preclinical study for prostate cancer, this technique was used as a potential predictive biomarker of radiation-therapy response. Dunning R3327-AT1 prostate tumors were surgically implanted subcutaneously in the flank of adult male syngeneic Copenhagen rats; nineteen days after implantation, OE-MRI was performed before each irradiation (2Fx15 Gy). The tumors were irradiated approximately 24 h after OE-MRI experiments. Before and during radiotherapy, the anesthetized animals inhaled either air or oxygen for at least 15 min. The OE-MRI and irradiation were repeated a week later, and the tumor growth delay was determined by the time required to achieve double-time volume (VDT) and quadruple time volume (VQT). Moreover, during hypofractionated radiotherapy, the BOLD and tissue oxygen-level dependent (TOLD) contrast, and the quantitative responses of relaxation rates (R1 and R2) were applied. The semi-quantitative parameters showed a significant correlation between the average TOLD and BOLD responses for single tumors before and after the irradiation. For the R1 and R2 rates, there were no significant differences before irradiation, but there was a significant difference between air and oxygen breathing. Tumors in oxygen-breathing animals expressed a more significant growth delay than tumors in the air group during irradiation. Therefore, the inhalation of oxygen during hypofractionated radiotherapy significantly improved radiation-therapy response, and OE-MRI may be added to routine clinical MRI for patient stratification and the personalization of radiotherapy treatment planning [135].

#### *4.6. Di*ff*usion-Weighted Imaging*

Diffusion-weighted imaging (DWI) is an MRI approach used to evaluate the diffusion rate of water molecules within tissues without the use of exogenous contrast agents [14]. The diffusion of water molecules and the degree of mobility are expressed quantitatively by the apparent diffusion coefficient (ADC), which can estimate the changes in cellularity and the integrity of cell membranes [99,136,137]. In particular, DWI is suited for the characterization of the tumor and for monitoring treatment response. Indeed, the variations in the ADC can demonstrate changes in the physiology of the tumors following therapeutic interventions, and therefore, ADC may be a potential biomarker of treatment efficacy [99,118,137–139]. The DWI has shown its value in tumor monitoring in both preclinical cancer models and clinical patients [101,138,140,141].

Preclinical assessments of early responses to therapy have been evaluated in breast and prostate cancer models, and in lymph node sites as a potential alternative method for detecting metastatic lesions. Primary applications of DWI are tumor detection and differentiation from non-tumor tissues, differentiation of malignant from benign lesions, and monitoring and prediction of treatment responses [137]. Hence, this technique may also be a noninvasive imaging biomarker of tumor aggressiveness for better stratification of patients with poor prognosis [138,139]. Indeed, this technique is being considered the standard of care for prostate and liver cancer, in which the different ADC measurements can predict tumor aggressiveness [14,93,139,142]. Additionally, in breast and thyroid cancers, ADC is considered an imaging biomarker able to differentiate malignant and benign lesions, and it has also been shown to predict the response to neoadjuvant chemotherapy in the former histotype [101,143]. Furthermore, DWI has been evaluated as a possible alternative to PET/CT for the detection of metastatic lesions [14,93,139,142].

#### 4.6.1. Thyroid

It has been shown that DWI and ADC maps, have the potential to distinguish malignant and benign nodules in the thyroid, since this technique can assess the different cellular architectures of tumors [144]. Differences in ADC were evaluated between benign and malignant nodules, correlating ADC to cytological results by fine-needle aspiration. The study included 36 patients with thyroid gland nodules and 24 healthy patients, all of whom were examined with DWI sequences. In the nodular patient group, there were 27 cases of benign nodules and nine cases diagnosed as thyroid gland malignancy; in total, 52 benign nodules and 16 malignant nodules were examined. The ADC values were significantly different between benign and malignant nodules, and from healthy thyroid tissues in the controls. In the benign group, the ADC value of thyroid nodules was increased while in the malignant group, it was reduced; in the healthy thyroid tissue, the ADC value was within the normal range. The reduction in the ADC observed in most malignant lesions was linked to the decrease in extracellular/extravascular space due to cellular proliferation, as confirmed by cytology. These results showed that DWI provides useful and promising results on the nature of a thyroid nodule, and it may have a role in the selection of nodules that should undergo needle aspiration cytology [143].

Similarly, DWI ADC mapping was performed on 14 patients with malignant thyroid nodules diagnosed by ultrasound and verified by biopsy. In these patients, 13 nodules were malignant, as shown with biopsy evidence, and five nodules were benign. Malignant nodules had significantly lower ADC values than the benign ones, confirming previous results [145].

In a preliminary study, the employment of DWI wasinvestigated using a readout-segmentedmultishot EPI (RESOLVE) sequence to differentiate between well-differentiated and undifferentiated subgroups of thyroid carcinomas. Moreover, the correlations between this technique and histopathologic data, such as the Ki-67 index and p53 expression, were evaluated. Indeed, Ki-67 represents a histopathologic parameter associated with cell proliferation, whereas p53 is considered a marker of tumor aggressiveness. Fourteen patients received preoperative MRI scans, including DWI-RESOLVE, and T1 and T2 conventional sequences before and after contrast-medium administration (gadopentetate dimeglumine). Four patients showed follicular thyroid carcinoma; four patients, papillary thyroid carcinoma; and six patients, undifferentiated thyroid carcinomas. The results showed that the mean ADC values were significantly lower in undifferentiated carcinomas compared with follicular and papillary carcinomas. A decrease in ADC mean values inversely correlated with an increase in Ki-67, while the increase in p53 expression was correlated to an increased in ADC mean values. Therefore, DWI seems able to distinguish between differentiated and undifferentiated thyroid carcinomas. This approach, once further validated, might help to preselect optimal therapeutic strategies in the presurgical phase [144].
