There was a total of 193 participants, 153 from site 1 and 40 from site 2. Two cases were excluded due to benign neoplasms. All CEUS studies were adequate for diagnosis. Figure 1 is a flow chart of enrollment. Age ranged from 33 to 90 years (average 68.0 years; IQR 58.0–73.0). Participants were 78 (40.4%) females and 115 (59.6%) males. 95 (49%) tumors were in the right kidney and 98 (51%) in the left kidney (p = 0.807).

Flow chart of the study enrollment

Tumors largest dimensions were 2.9 cm (IQR 2.2–3.7 cm: range 1.4–7.1 cm). Tumors were complex cystic 17(8.8%) or solid 176 (91.2%). Procedures included cryoablation 38 (19.7%), microwave ablation (MWA) 4 (2.1%), partial nephrectomy 135 (69.9%) and radiofrequency ablation (RFA) 16 (8.3%).

The time to follow-up with CEUS ranged from 1 to 336 months (average 35.1months; IQR 12.0, 70.0). Participant demographic, lesion size, solid or cystic, and length of follow-up for both sites are listed in Table 1. Complex cystic and solid renal masses were defined by the criteria of Silverman et al. [15].

Tumor histology included clear cell RCC (135, 69.9%), papillary RCC (27, 14.0%), chromophobe RCC (14, 7.3%), oncocytic RCC (8, 4.1%), multicystic RCC (3, 1.6%), eosinophilic RCC (1, 0.5%), RCC with rhabdoid features (1, 0.5%), and unknown due to FNA (4, 2.1%). Tumor types with histologic grades are listed in Table 2.

For all cases of CEUS positive the enhancement pattern was globular or round enhancement. For all procedures there was a total of 8.8% (17/193) positive residual or recurrence of tumor cases. This included 7.2% (11/153) for site 1 and 15% (6/40) for site 2. For combined sites for partial nephrectomy, the positive recurrence was 1.5% (2/135); RFA 43.7% (7/16); cryoablation 21.1% (8/38); MWA 0.0% (0/4). Based on follow-up procedures or continued enhancement over 1 year there was a 100% sensitivity (95% CI 81–100%) and 100% (95% CI 98–100%) specificity in detecting residual or recurrent tumor post nephron sparing procedures. Table 3 lists the results of the follow-up studies. Figure 2 is a 67-year-old female with residual tumor post cryoablation with concordant CECT and CEUS examinations. Figure 3 is a 60-year-old male with residual tumor post RFA where the CEUS is positive and the CECT is negative.

CEUS and CECT of a 67-year-old female with residual tumor post cryoablation for a clear cell renal carcinoma G2 pT1a. There is concordance with CECT (A, red arrows) and CEUS (B, measurement markers) with a 1.2 cm area of enhancement at the procedure site. The CEUS is a dual screen image with the contrast enhanced ultrasound on the left and a synthesized B-mode image on the right. The B-mode image is used to confirm localization and is not a diagnostic image. The patient was monitored and one year later the area of residual tumor had increased on CEUS to 1.8 cm (C, red arrows). The patient remains on surveillance monitoring

60-year-old male with residual tumor post RFA for a clear cell carcinoma G2 pT1a. one month post ablation. The CEUS (A) is positive with contrast within the ablation site (arrows) while the CECT (B) does not show any enhancement in the ablation site and is negative. The CEUS is a dual screen image with the contrast enhanced ultrasound on the left and a synthesized B-mode image on the right. The B-mode image is used to confirm localization and is not a diagnostic image. The patient is on surveillance monitoring and the enhancement on CEUS has not changed over a 1-year period

Of the 17 cases with abnormal post procedure enhancement, the two with partial nephrectomies had no further treatment; one is being monitored and the other expired with metastatic RCC. Of the 8 positive cryoablation cases 1 (12.5%) went on to partial nephrectomy, 2 (25.0%) remain on imaging surveillance monitoring, and 5 (62.5%) had repeat cryoablation with complete ablation documented on CEUS. Of the 7 positive cases post RFA, follow-up in 1 (14.3%) become positive after 6 years and 1 (14.3) became positive after 3 years of follow-up and both remain on monitoring, 3 (42.8%) remain on monitoring, and 1 (14.3%) had a repeat RFA with no residual tumor, 1 (14.3%) had a partial nephrectomy. Figure 4 depicts a 60-year-old male with a recurrence 6 years after partial nephrectomy. Figure 5 depicts a 78-year-old female with enhancing septations post RFA that persist for over 1 year. Table 4 lists the follow-up of the patients with positive residual tumor or recurrence.

60-year-old male with a recurrence 6 years after partial nephrectomy for an oncocytic RCC. The CEUS is a dual screen image with the contrast enhanced ultrasound on the left and a synthesized B-mode image on the right. The B-mode image is used to confirm localization and is not a diagnostic image. The initial CEUS post partial nephrectomy (A) demonstrates no enhancement at the surgical site (red arrows). On follow-up CEUS 6 years (B) later there is a 1.0 cm enhancing nodule (red arrows). CECT or CEMRI were not performed. The patient remains in monitoring

78-year-old female with enhancing septations in the ablation site post RFA of a clear cell carcinoma G1 pT1a. The CEUS is a dual screen image with the contrast enhanced ultrasound on the left and a synthesized B-mode image on the right. The B-mode image is used to confirm localization and is not a diagnostic image. On the initial post RFA CEUS (A) enhancing septations are noted (red arrows). Patient was followed and one year later (B) the enhancing septations (red arrows) are again noted and not significantly changed. The patient remains on monitoring. CECT or CEMRI were not performed

Of the 17 positive cases 88.2%. (15/17) were cases with residual tumor on the first post procedure CEUS and 11.8% (2/7) had recurrence after an initial negative study.

Of the 193 cases 47.7% (92/193) had CECT scans and 7.3% (14/193) had CEMRI post procedure. CECT follow-up occurred from 1 month to 313 months and CEMRI follow-up occurred from 10 months to 120 months. There were 53.9% (104/193) cases that only had CEUS follow-up.

For site 1 there was at least 1 post procedure CECT in 48% (74/153) participants and for site 2 45% (18/40). For site 1 there was at least 1 post procedure CEMRI in 1.3% (2/153) participants and 30% (12/40) for site 2.

Of the 92 cases with both CECT and CEUS there was concordance in 96.7% (89/92) with 3 cases of CEUS positive and CECT negative findings. There was concordance of CEMRI and CEUS in 100% (14/14) cases. There were no cases with a positive CECT or CEMRI and a negative CEUS.