Between 2004 and 2018, a total of 337 renal masses were treated by RFA or MWA. Of these, 149 were excluded because of a benign, non-diagnostic, or no pathology result. As a result, 188 histologically proven RCCs in 164 unique patients treated in 167 treatments by means of RFA (n = 87) or MWA (n = 101), were included in the analyses. Pretreatment patient and tumor characteristics are shown in Table 1. In Fig. 1, CT scans of a treated RCC are shown.

A RCC treated by RFA. A pretreatment contrast-enhanced CT image with a RCC in the right kidney is shown (* in panel A). In panel B and C needle placement in consecutive images during RFA is shown. Three RFA needles are placed through the tumor. Note that the RFA procedure was performed with the patient in a prone position. Panel D shows an image directly after ablation. The ablation zone is marked by arrows. Ablation was considered successful; however, 1 year later, a local recurrence was noted (arrow in panel E, patient in supine position). The recurrence was successfully treated with a second RFA procedure

In the RFA group, tumors were significantly closer to the collecting system (< 4 mm; RFA 50% vs MWA 22%, p < 0.001) and the polar lines (crosses 2 polar lines; RFA 29% vs MWA 10%, p = 0.003) reflecting in a higher mRENAL nephrometry score (RFA median = 7 vs MWA median = 6, p < 0.001).

Ablations were performed under CT guidance in most of the procedures or in a hybrid setting consisting of a combination of ultrasound and CT. Epidural anesthesia was used in 36% of the RFA procedures and in 88% of the MWA procedures (p < 0.001), mainly in The Netherlands Cancer Institute where epidural anesthesia was standard practice for renal ablative therapies. In the other institutions, general anesthesia was mostly used.

Median follow-up was 4.26 (interquartile range 1.95–5.51) years in the RFA group and 1.62 (interquartile range 1.08–2.67) years in the MWA group. Primary efficacy rates yielded 92% (80/87 tumors) for the RFA group and 91% (92/101 tumors) for the MWA group. Sixteen patients showed residual disease on the first follow-up scan after 3 months (RFA (n = 7) and MWA (n = 9)). Nine patients developed recurrence after RFA (n = 7) and MWA (n = 2). In one patient, local progression was suspected on CT and this patient was treated by surgery, but no viable tumor cells were observed at the pathologic specimen.

Seventeen of the 25 residual/recurrent tumors were treated by ablation in a repeated procedure. The other 9 tumors were treated by means of surgery (n = 5), active surveillance (n = 3), and stereotactic body radiation (n = 1). The second ablation was successful in 14/17 tumors. Three tumors needed a third ablation, after which 1 patient (treated with MWA) needed surgery for completion therapy. This patient had a complete endophytic tumor with a close relation to the renal artery.

No difference was observed between RFA and MWA for change in kidney function using the estimated glomerular filtration rate (eGFR) (Table 2). Eight patients developed metastases (5 after RFA and 3 after MWA) during follow-up.

In total, 25 patients developed LTP (RFA n = 14, MWA n = 11). Time to LTP ranged from 1 month to 7.8 years for RFA and 1 week to 5.2 years for MWA.

There was no difference in the LTP-free interval between RFA and MWA in a univariable Cox regression model (Fig. 2A). Results from a multivariable Cox regression analysis (see Table 3) adjusting for mRENAL nephrometry score and stratified by center did not show a significant difference between ablation techniques (HR = 0.87, 95% CI 0.33–2.28, p = 0.78).

a Local control (LC) from date of ablation to tumor progression for the RFA and MWA group. b Local control (LC) from date of ablation to tumor progression divided by tumor size larger than 3 cm and smaller than 3 cm (pooled data of RFA and MWA groups). c Local control (LC) from date of ablation to tumor progression for distance to the collection system (> 7 mm, 4–7 mm, < 4 mm, pooled data of RFA and MWA groups). d Local control (LC) from date of ablation to tumor progression for exophytic or endophytic tumors (pooled data of RFA and MWA groups)

Univariable Cox regression analyses showed that increasing mRENAL nephrometry score, size (> 3 cm), nearness to the collecting system (< 7 mm), and endophytic location were associated with a worse LTP-free interval (Fig. 2B–D and Table 3). Similar conclusions were obtained in subgroup analyses for RFA and MWA patients separately, though for the MWA subgroup nearness to the collecting system 4–7 mm appeared to have poorer local control than < 7 mm. Given the small numbers in each of these subgroups and the similarities with overall results, these analyses are not displayed.

No evidence was found in a separate model adjusting for size and nearness of the collecting system (HR = 0.76, 95% CI 0.31–1.88, p = 0.55). In this model, increasing size remained associated with poorer LTP-free interval (HR = 3.81, 95% CI 1.53–9.44, p = 0.004) as was the case for nearness to the collecting system 4–7 mm and < 4 mm (HR > 5 in both cases) (Table 3). Results from a Cox regression model with inverse probability weighting using the propensity score showed similar results (HR = 0.99, 95% 0.35–2.81, p = 0.98). For a propensity score-matched dataset using one-to-one greedy nearest neighbor matching without replacement, conclusions were similar (HR = 0.82, 95% CI 0.16–4.31, p = 0.82). Matching induced balance in all baseline covariates (see Table 1), though it led to 41% of the data being unmatched (55 matched pairs) and thus a loss of precision and generalizability.

Given the difference in median follow-up time for both ablation techniques (4.26 years for RFA [interquartile range 1.95–5.51] and 1.62 years for MWA [interquartile range 1.08—2.67]), a sensitivity analysis was performed by truncating time to LTP at 6.52 years (the maximum follow-up in the arm with the shortest follow-up [MWA]) in multivariable analyses. No differences in LTP-free interval were found (results not shown).

After the 188 primary ablations, 28 (14.9%) complications occurred after 14 RFA procedures and 14 MWA procedures (Table 2). There were no peri-procedural deaths. There were four major adverse events (3%), 3 after RFA (1.6%) and 1 after MWA (0.53%). Damage to the pyelocaliceal system led to a significant stenosis in all four patients. In two patients, this stenosis caused total failure of the ablated kidney (1 RFA, 1 MWA) (grade IVa). In one patient, a fistula between a renal calix and the skin developed after RFA, complicated by abscess formation and sepsis for which the patient was admitted to the ICU (grade IIIb). The fourth patient needed double J placement (grade IIIa) for the treatment of the stenosis. Twenty-four (12.8%) minor adverse events occurred, 10 (5.3%) after RFA and 13 (7.0%) after MWA, and consisted of pain, hematoma, hematuria, or pneumothorax in most of the cases.