Intraoperative MRI-derived volumetric ablation margins and initial correlation with local outcome after MRI-guided cryoablation of renal tumors

Abstract

Purpose: To assess volumetric ablation margins derived from intraoperative pre- and post-ablation MRI after magnetic resonance imaging (MRI)-guided percutaneous cryoablation of renal tumors and explore its correlation with local treatment success.

Methods: Retrospective analysis was performed on 30 patients (mean age 69y) who underwent percutaneous MRI-guided cryoablation between May 2014 and May 2020 for 32 renal tumors (size: 1.6-5.1 cm). Tumor and ice-ball volumes were segmented on intraprocedural pre- and post-ablation MR images using Software Assistant for Interventional Radiology (SAFIR) software. After MRI-MRI co-registration, the software automatically quantified the minimal treatment margin (MTM),defined as the smallest 3D distance between the tumor and ice-ball surface. Local tumor progression (LTP) after cryoablation was assessed on follow-up imaging.

Results: Median follow-up was 16 months (range: 1-58). Local control after cryoablation was achieved in 26 cases (81%) while LTP occurred in 6 (19%). The intended MTM of ≥5 mm was achieved in 3/32 (9%) cases. Median MTM was significantly smaller for cases with (- 7 mm; IQR:-10 to - 5) vs. without LTP (3 mm; IQR:2 to 4) (P < .001). All cases of LTP had a negative MTM. All negative treatment margins occurred in tumors > 3 cm.

Conclusions: Determination of volumetric ablation margins from intraoperative MRI was feasible and may be useful in predicting local outcome after MRI-guided renal cryoablation. In our preliminary data, an intraoperative MRI-derived minimal margin extending at least 1 mm beyond the MRI-visible tumor led to local control and this was more difficult to achieve in tumors > 3 cm. Ultimately, online margin analysis may be a valuable tool to intraoperatively assess therapy success, but larger prospective studies are needed to establish a reliable threshold for clinical use.

Keywords: Local recurrence; MRI; Percutaneous cryoablation; Renal tumor; Treatment margin.

Fig. 1

Schematic overview of patient selection

Fig. 2

Overview of Software Assistant for Interventional Radiology (SAFIR) layout. Available image series and tools are shown in bars on the left. Image views from left to right, top to bottom: (1) 3D view of annotated ice ball (blue) and tumor (green) overlap, (2) tumor divided in octants along the left-right, cranio-caudal and anterior-posterior axes showing color-coded subareas of total tumor volume with complete coverage including user-defined treatment margin (green), complete coverage but insufficient margin (yellow) and incomplete coverage (red)., (3) pre-ablation and (4) post-ablation T2-haste coronal MRI-images showing the tumor (green) and co-registered ice ball (blue) volumes. Side bar on the left shows calculated 3D ablation margin metrics

Fig. 3

Minimal treatment margin for cases with local tumor control and local tumor progression

Fig. 4

A Segmentation (orange outline) of a 3.5 cm diameter tumor located interpolar in the left kidney on pre-ablation axial T2-weighted MR imaging. B Segmentation (blue outline) of corresponding end ablation ice-ball volume on post-ablation axial T1-weighted MR imaging. C Overlay of pre-ablation and post-ablation images after co-registration (left) and resulting 3D margin calculation (right). Volumetric assessment shows complete tumor coverage (green) by the end-ablation ice-ablation ice-ball (blue) with MTM of 4 mm. D Ablation zone (white arrows) without tumor recurrence on follow-up axial T2-weighted MR imaging 2 months (left) and 16 months (right) after cryoablation

Fig. 5

A Segmentation of a 4.2 cm diameter tumor located in the lower pole of the right kidney (outlined in orange) on pre-ablation axial T2-weighted MR imaging. B Segmentation of the corresponding ice ball (outlined in blue) on post-ablation axial T1-weighted MR imaging. C Overlay of pre-ablation and post-ablation images after co-registration (left) and resulting 3D margin calculation (right). Volumetric assessment shows predicted incomplete ablation at medial side (red) with MTM of − 6 mm. D Follow-up axial T2-weighted MR imaging at 2 months (left) after cryoablation shows ablation zone (white arrows) with suspected incomplete coverage on medial side (red arrows). Six months follow-up MRI (right) confirms local tumor progression (red arrow) at the medial side

Fig. 6

Minimal treatment margin plotted against tumor size for cases of local control and local tumor progression