• Role of MR imaging

MRI also plays a major role in diagnosis and follow-up. Changes in signal intensity and enhancement patterns reflect tumor biology and treatment response. The histopathologic characteristics of DTs correlate with MRI findings: hyperintense and enhancing areas indicate regions of high cellularity and proliferation, while hypointense and non-enhancing regions represent areas with low cellularity, primarily composed of collagen and fibrous tissue [12, 13]. An increase in collagenization within the tumor is associated with a decrease in cellularity, suggesting a reduction in tumor activity. This can be seen on MRI as a decrease in enhancement and T2 hyperintensity, which are imaging features indicating a positive response to therapy [14].

• Post-Cryoablation Imaging Findings

Cryoablation can reduce the active component of the tumor and increase necrosis and hyalinization. In early follow-up, significant edema may be noted in the cryoablation area and adjacent soft tissues (Figure 2).

Early imaging signs after the procedure include a non-enhancing ablation cavity with a high T2 signal, corresponding to coagulative necrosis. A thin inner rim of low T2 signal, indicative of hemosiderin (hemorrhagic congestion), can be better visualized on T2 gradient echo sequences. A thicker outer rim of intermediate/low T2 signal with post-contrast enhancement corresponds to granulation tissue/vascular fibrosis (Figures 3 and 4).

 In subsequent follow-ups, the non-enhancing ablation cavity shows a reduction in size and may show a marked decrease in T2 signal intensity, indicative of dense fibrosis. Additionally, a progressive reduction in post-contrast enhancement of the outer rim of granulation tissue/vascular fibrosis can be noted (Figures 5-6). Finally, a reduction in lesion volume is expected in successful cases (Figures 7-9), with a progressive replacement by adipose tissue observed in subcutaneous cases (Figure 10). If tumor recurrence occurs, it can be observed on post-contrast sequences as a nodular enhancing area (Figures 3 and 4).

The success rate of cryoablation is directly related to the ice ball encompassing the entire lesion during the procedure. However, cryoablation may induce the 'abscopal effect,' leading to the reduction or disappearance of the tumor in non-frozen sites (Figures 11 and 12). This phenomenon is of significant importance in tumor therapy, as it effectively reduces the likelihood of tumor recurrence [15].

Diffusion-Weighted Imaging (DWI) correlates with the content of the treated tissue, showing higher ADC values in cases of low cellularity and fibrous content. It can also be used to detect changes in the treated tissue post-cryotherapy. A recent article [16] reported low ADC values (mean 0.90 × 10³ mm2/s) in early follow-up of treated tissue after cryotherapy, possibly due to tumor necrosis. This should not be confused with recurrence, especially when no enhancement is present. Further studies are needed to confirm DWI patterns in treated tissue post-cryotherapy. However, we also observed this pattern in some of our cases, as described in Figure 13.