Percutaneous microwave ablation of cT1b renal cell carcinoma: Safety and oncologic efficacy in a large, single-center elderly and comorbid cohort

doi:10.21203/rs.3.rs-5296296/v1

Purpose: To evaluate safety and oncologic efficacy of percutaneous microwave ablation (MWA) for treating clinically localized T1b (cT1b) renal cell carcinoma (RCC).

Methods: This single-center retrospective study was performed under a waiver of informed consent. Seventy-four consecutive patients (49M/25F) with 76 cT1b RCC (median tumor diameter 4.5 cm) were treated with percutaneous MWA between 5/2012 and 8/2020. Patients were stratified into two groups by technique, depending on whether antennas were repositioned for additional ablation or not. Primary efficacy, complications, and local tumor progression (LTP) were compared using the Wilcoxon rank sum and Fisher’s exact tests. The Kaplan Meier method was used for survival analysis. Results: Patients were elderly (median age 69.5), obese (median BMI 34.5) and comorbid (Charlson Comorbidity Index = 4). Most tumors were low-grade (grade 1-2) (67/89, 88%) and clear cell RCC was the most common histology (62/76, 82%). A median of three MWA antennas were powered at 65W for 7 min for treatment. Renal masses were larger (4.6 vs 4.5 cm, p=0.01) and procedure times longer (100 min vs 80.5 min, p=0.04) for the antenna reposition cohort (n=34, 45%). Primary efficacy and high-grade complication rates were 93% and 8%, respectively. The local tumor progression rate (LTP), at a median follow-up was 28.2 months, was 16%. Primary efficacy, low and high-grade complications, change in estimated glomerular filtration rate and LTP were similar between cohorts (p=0.20-0.55).

Conclusion: Percutaneous MWA for cT1b RCC is safe in elderly and comorbid patients with acceptable oncologic efficacy. Repeat ablation is well-tolerated and can improve oncologic efficacy.

The incidence of clinically localized renal cell carcinoma (RCC), a common and lethal genitourinary malignancy, has increased over the last several decades due to ageing and obesity, among other risk factors. Detection rates have also increased with the widespread use of cross-sectional imaging. Surgery, ablation, and surveillance are established options for treating localized RCC. Treatment decisions are individualized, based upon tumor stage, size, complexiety, and histology (when available), as well as patients specific factors including age, BMI, comorbidities and patient preference (1–8).

The American Urological Association (AUA) and the National Comprehensive Cancer Network (NCCN) consider surgery (preferably partial nephrectomy), thermal ablation and active surveillance as treatment options for T1a RCC (≤ 4.0 cm). Radiofrequency ablation (RFA), microwave ablation (MWA) and cryoablation, the primary thermal ablation modalities, can offer oncologic efficacy comparable to surgery. Improved procedural expertise coupled with the larger ablation zones that can be generated with modern MWA and cryoablation devices has led to improved treatment outcomes in larger and more endophytic T1a RCC (4, 6–12).

Patients who present with T1b RCC (4.1–6.9 cm) and comorbid patients who progress while on active surveillance have fewer treatment options. Extirpation is generally recommended to mitigate the risk of metastatic disease; however, many patients are unfit for surgery, particularly those who may have been initially selected for active surveillance secondary to competing medical risks. Few studies have evaluated the efficacy of thermal ablation for these larger and often more complex tumors. Therefore, the purpose of this study was to evaluate the oncologic efficacy of microwave ablation in the treatment of clinically localized T1b RCC.

This single-center retrospective study was compliant with the Health Insurance Portability and Accountability Act. The requirement to obtain informed consent was waived by the institutional review board.

Patient Selection

Eighty-nine consecutive clinical stage T1b RCCs were treated in 87 patients during 76 treatment sessions from May 2012 to August 2020. Patients with oncocytomas (n = 7) and metastatic disease (n = 6) were excluded from analysis. The decision for each patient to undergo MWA was made in consensus by a team of subspecialty radiologists and urologists experienced in tumor ablation. The decision to treat was based upon age, comorbidities, tumor size, location and histology, and proximity of non-target anatomy. Percutaneous biopsy was performed and histology was determined for all patients prior to ablation. Renal artery embolization was not performed on any patient prior to ablation.

Procedure

All procedures were performed in a CT suite (GE Optima 580) under general anesthesia. Ultrasound (US) (GE Logiq E9 or E10) and/or CT fluoroscopy was used for image-guidance. A 2.45 GHz, gas-cooled MWA device (Certus 140, NeuWave Medical) with 15- or 17-gauge antennas (PR or LK, NeuWave Medical) was used for all cases. There were no staged treatments and treatment intent was curative for all cases.

Hydrodisplacement was performed before and/or after antenna placement when non-target anatomy was within the expected zone of ablation. An 18- or 20-gauge coaxial needle was placed into the retroperitoneum, between the tumor and non-target anatomy, and normal saline was infused with a goal to achieve and maintain at least 1 cm of displacement.

Tumor size and location were used to determine number of antennas utilized. Following antenna placement or repositioning, focused unenhanced CT of the abdomen was performed to evaluate 1) the location of the antennas in the tumor and 2) the position of antennas relative to non-target anatomy. Antennas were placed with a spacing of 1–2 cm. When three antennas were used they were placed in a triangular array (Fig. 1). Ultrasound was used for real-time monitoring of the ablation zone. If the gas cloud failed to cover the entire tumor, the antennas were retracted and/or repositioned and additional ablation was performed (Fig. 2).

Contrast-enhanced CT (CECT) was obtained immediately after the ablation procedure to assess for residual viable tumor and complications, including active bleeding and urine leak. Immediate repeat MWA was performed if enhancing tumor was identified. Patients were admitted for observation according to the routine standard of care at our institution.

Data Collection

Clinical, imaging, pathologic and procedural data for each patient were collected from an institutional database by two authors (blinded). Clinical data collected included patient age, sex, BMI and Charlson comorbidity index (CCI).

Imaging and pathology data collected included tumor size and RENAL nephrometry score, tumor histology and grade. The RENAL nephrometry score was calculated on pre-procedure enhanced CT or MRI.

Procedural data collected included use and volume of hydrodisplacement, number and type (PR or LK) of MWA antennas, utilization of antenna repositioning for additional ablation, generator output and time, and duration of the procedure and hospitalization. The ablation procedure time was defined from the induction of anesthesia to extubation. The intervention time was defined from initial imaging to final post-procedure CT or US. Initial imaging was obtained after intubation and patient positioning but before sterile preparation and draping.

Complications within 30 days of ablation were classified according to the Clavien-Dindo system (13). Clinical follow-up and imaging with CECT or MRI were performed at target intervals of 6, 12, 18, and 24 months and annually thereafter. A fellowship-trained abdominal radiologist (blinded) experienced in oncologic imaging and tumor ablation (11 years) reviewed images for local oncologic efficacy, metastatic disease, and complications. Established criteria were used to assess primary efficacy and local tumor progression (LTP) (14). Primary efficacy is defined as the complete treatment of the target tumor following the initial ablation procedure. LTP is defined as the appearance of tumor foci (eccentric mass-like enhancement) along the edge of the ablation zone after at least one contrast-enhanced imaging follow-up study had confirmed complete treatment. Secondary efficacy is defined as complete treatment of residual or recurrent (LTP) tumor.

Statistical Analysis

Continuous features are summarized with medians and interquartile range (IQR). Categorical data are summarized with frequency counts and percentages. The cohort was stratified based upon tumor size when antennas were repositioned and compared with the Kruskal-Wallis or chi-squared tests. The Kaplan Meier method was used for survival analyses, which included local recurrence-free survival (RFS), cancer-specific (CSS) and overall survival (OS). The duration of follow-up for local RFS was defined from the date of the MWA to the identification of LTP or date of last imaging follow-up. The duration of follow-up for CSS and OS was defined from the date of the MWA (or first MWA procedure for the two patients with two T1b tumors treated in separate sessions) to the date of death or the date the patient was last known to be alive. p < 0.05 was considered significant for all statistical tests.

Clinical Data

After excluding patients with oncocytomas or metastatic disease, 76 cT1b RCC were treated in 74 consecutive patients. The patient cohort was elderly [median age 69.5 years (IQR:65-76)], obese [median BMI 34.5 (IQR:29-41)] and predominantly male (n=49, 66%). The surgeon-reported performance status of most patients was good despite the high comorbidity score [median CCI 4 (IQR: 3-5)]. There was no difference in age, BMI, or comorbidities between the single and multi-position cohorts (p=0.26-0.98). The median eGFR for the study population was 63.5 mL/min/1.73m² (IQR: 52-74). Baseline renal function was similar for the single antenna position and reposition cohorts (eGFR: 62 mL/min/1.73m² vs 67 mL/min/1.73m², p=0.61). Clinical data is summarized in Table 1.

Imaging and Pathologic Data

The median tumor diameter for the study population was 4.5 cm (IQR: 4.3-5.1). Tumors that required antenna repositioning were larger [4.6 cm (IQR: 4.4-5.6) vs 4.5 cm (IQR: 4.2-4.9), p=0.01]. Clear cell RCC was the predominant subtype (n=62, 82%). Most RCC were low-grade (Grade 1: n=5, 7%; Grade 2: n=55, 72%). There was no difference in histologic subtype or tumor grade between the cohorts. Five RCC had aggressive features (necrosis, rhabdoid or sarcomatoid features), four in the single-position cohort and one in the reposition cohort.

The median RENAL nephrometry score for the study population was 8 (IQR: 7-9) with 25, 30 and 21 located anterior, posterior or neither (X), respectively. There were 8 (11%) low-complexity (score: 4-6), 57 (75%) moderate-complexity (score: 7-9) and 11 (14%) high-complexity (score: 10-12) tumors. The median RENAL score for the cohorts were similar [8 (IQR: 7-9) v 8 (IQR: 7-9), p=0.78). Imaging and pathologic data are summarized in Tables 1 and 3.

Procedural Data

The median number of antennas per procedure was 3 (IQR: 3-3). Two antennas were used in 12 (16%) procedures. Three antennas were used in all the remaining ablation procedures (84%). Median generator output power and time was 65 W (IQR: 65-65) and 7 min (IQR: 5-9.5). Antennas were repositioned in 34 (45%) ablation procedures because of incomplete tumor ablation, detected either on intraprocedural US and/or post procedure CECT (Figure 2). Median ablation time was longer [10 min (IQR: 7.5-13) vs 5 (IQR: 5-6.5), p<0.001)] for the antenna reposition cohort compared to the single position cohort.

Hydrodisplacement was performed during 34 procedures (45%); 21 times (50%, 21/42) in the single position cohort and 13 times (38%, 13/34) in the reposition cohort. The median volume of hydrodisplacement for the study population was 425 mL (IQR: 300-650). The median volume of hydrodisplacement for the cohorts were similar (425 mL v 475 mL, p=0.28).

Three ablation procedures were excluded from ablation procedure time analysis; one patient had an additional ablation procedure (hepatic hemangioma ablation) and the other two patients had an incomplete anesthesia record. All three of these patients were from the single position cohort. The median procedure time (induction to extubation) and intervention time (initial image to final image) for the remaining MWA procedures (n=73; 34 reposition and 39 single position) was 100 min (IQR: 63-128) and 80.5 min (IQR: 64-95), respectively. Procedure time [124.5 min (IQR: 82-160) v 100.5 min (IQR: 85-126), p=0.08) for the cohorts were similar while intervention time [100 min (IQR: 63-128) v 80.5 (IQR: 64-95), p=0.04) for the reposition cohort was longer. Procedural data are summarized in Tables 2 and 3.

Renal Function and Complications

As expected, there was a mild decline in eGFR after ablation. The median eGFR 6-months after ablation for the study population was 57 mL/min/1.73m²(IQR: 45-77), a 10.2% decline. The median eGFR after ablation for the reposition cohort was 59 mL/min/1.73m² (IQR: 44.5-80)], a 11.9% decline. The median eGFR after ablation for the single position cohort was 54 mL/min/1.73m²(IQR: 45-72.5), a 12.9% decline. The decline in renal function was similar for the cohorts (p=0.48).

There were 10 low-grade (Clavien-Dindo 1-2) and 6 high-grade (Clavien-Dindo 3-4) complications. The perioperative mortality rate (Clavien-Dindo 5) was zero. There were four urine leaks/urinomas (5.3%) identified at 6 month imaging follow-up, one in reposition cohort and three in the single position cohort. A ureteral stent was placed in one patient with a urinoma and coexisting stricture at the ureteropelvic junction. The other three patients were managed conservatively.

Grade 1 complications included acute kidney injury (n=3) and perinephric/subcapsular hematoma (n=1). Five patients required transfusions (n=4 in reposition cohort) and an iatrogenic pneumothorax was aspirated without chest tube placement in two patients (Grade 2) (both in single position cohort). Embolization (Grade 3a) for active bleeding/pseudoaneurysm was required for two patients (n=1 in reposition cohort); both of these patients resumed anticoagulation on ablation day 0. Antibiotics were administered and a ureteral stent was placed for management of an ablation-related colorenal fistula (Grade 3b) (single position cohort). Grade 4 complications included aspiration and respiratory arrest immediately after extubation, stroke and sepsis (unrelated to the ablation procedure). The 30-day readmission rate was 8% (6/76).

Oncologic Efficacy

Technical success was achieved in all 76 cases (100%). The rate of primary efficacy for the study population was 93% (71/76). The primary efficacy rate was similar for the reposition and single position cohorts [91% (31/34) vs 95% (40/42), p=0.48]. Successful repeat ablation was performed in three of the five patients with residual tumor, conferring a secondary efficacy of 97%. The other two patients with residual enhancing tumor remain in active surveillance. These two patients (1 in reposition cohort and 1 in the single position cohort) were excluded from local tumor progression (LTP) assessment. Four patients, all in the single-position cohort, were lost to follow-up and excluded from LTP and survival analysis.

Eleven patients experienced LTP (16%, 11/70) at a median imaging follow-up of 28.2 months (IQR: 21.1-39.4). Seven patients in the reposition cohort (21%, 7/34) and 4 patients in the single-position cohort (11%, 4/37) experienced LTP (p=0.20). Successful repeat ablation was performed in four of the eleven patients with LTP, conferring a secondary LTP rate of 10% (7/70). The other seven patients with LTP remain in active surveillance. To date, no patient with aggressive RCC features has experienced LTP, metastatic disease or died of RCC. Kaplan Meier estimates of recurrence-free and overall survival are plotted in Figures 3 and 4.

One patient (1.4%, 1/74) with a 4.2 cm low grade ccRCC developed metastatic RCC (pulmonary metastasis) and remains alive. No patients died of RCC. Twenty-three patients (31%) died of causes unrelated to RCC at a median of 2.2 years after ablation.

The management of patients with biologically heterogeneous small renal masses (SRM) is increasingly complex, especially in the elderly and comorbid. Twenty to forty percent of SRMs are benign and most (70–80%) malignant SRM are low-grade with a very low rate of metastasis (< 2% for RCC less than 4 cm). Further, death from competing risks is substantially higher than cancer-related death from cT1a RCC regardless of age or comorbidities (15–20). As a result, active surveillance (AS) has emerged as a safe alternative management strategy to active treatment with a low rate (1–6%) of metastatic progression alongside an other-cause mortality rate of up to 45% (21–22). Delayed intervention is triggered to mitigate the risk for metastatic disease in patients who progress (accelerated growth rate reaching 3 cm, stage migration or symptoms) on AS (23–34). Partial or radical nephrectomy are standard of care for delayed intervention and offer a high rate of durable oncologic efficacy (98%) with an acceptable safety profile (25). However, most elderly and comorbid patients on AS requiring delayed intervention are not suitable surgical candidates. For example, patients with a CCI > 2 have a 6-fold risk for postoperative complications compared to patients with a CCI of 0 (26). Thermal ablation is an alternative active treatment option endorsed by the American Urological Association (AUA) for patients with small renal masses (< 3 cm) and is effective and safe even for elderly and comorbid patients (4, 7–12). Unfortunately, limited data is available to evaluate the oncologic efficacy and safety of percutaneous MWA for cT1b RCC.

In this single-center, retrospective study, we evaluated the oncologic efficacy of percutaneous MWA for cT1b RCC in an elderly and comorbid (median CCI: 4) cohort. The primary and secondary efficacy rates were 93% and 97%. The local tumor progression rate, at a median of 2.4 years, was 16%. Repeat ablation was performed for four of the eleven patients who experienced LTP, conferring a secondary LTP rate of 10%. Thus, the overall oncologic efficacy rate, after a single MWA session, was approximately 80% and the secondary oncologic efficacy rate, when accounting for a repeat ablation session for 7 of the 16 patients with residual or recurrent tumor, was approximately 90%. The nine patients with residual or recurrent tumor who did not undergo an additional ablation remain in active surveillance without evidence of metastatic disease. While one patient with low-grade clear cell RCC developed metastasis (pulmonary), no patients have died from RCC. Twenty-three patients (31%) died of causes unrelated to RCC at a median of 2.2 years after ablation.

The primary alternative to ablation for cT1b renal masses is surgery, either radical or partial nephrectomy; thus, comparison of the complications between surgery and ablation is critically important. Partial nephrectomy complications have been reported in the range of 8–40% with a 15% rate of blood transfusion for tumors cT1b tumors (27–28). Radical nephrectomy tends to be associated with a reduced complication rate (3–25%), but results in a significant decline in post-operative renal function (28). We report an overall complication rate of 21%, a high-grade complication rate of 8%, a blood transfusion rate of 7%, and a 30-day readmission rate of 8% in a study population with a median CCI of 4. Additionally, the decline in eGFR after ablation in our study population (-10.2%) was similar to reported eGFR decline after partial nephrectomy for cT1b tumors (3–17%) (28).

Compared to cT1a, percutaneous MWA of cT1b RCC 1) is more time consuming, 2) is associated with a higher rate of complications and 3) has a lower rate of oncologic efficacy. Prior studies have shown that performing hydrodisplacement and placing additional ablation applicators increase procedure time (4). We found that tumors 5.3 cm and larger required antenna repositioning for additional ablation, significantly increasing intervention time. The overall and high-grade complication rates in this study were 21% and 8% compared to 10–15% and 0–3% for large cT1a cohorts treated with MWA or cryoablation (29). The overall complication rate after MWA and cryoablation for T1b RCC is similar with bleeding representing the most common complication across both modalities (30). Importantly, prophylactic embolization, commonly used to reduce the risk for significant bleeding after cryoablation for cT1b RCC, was not used prior to MWA in our cohort (31–32). The primary efficacy and durable local control (100% – LTP rate) of MWA for cT1b RCC in our study was 92% and 84%, respectively. This compares to a primary efficacy rate and durable local control of 99% and 96%, respectively, in a large cohort of cT1a RCC treated with MWA (4). Importantly, repeat ablation for residual and/or recurrent tumor is safe, well-tolerated and improved the oncologic efficacy from 79–88% in our study. Finally, none of the patients with residual or recurrent RCC after ablation developed metastatic disease.

Retrospective study designs introduce selection bias. Careful patient selection is important when considering percutaneous thermal ablation as a treatment option for patients with cT1b RCC. Ablation patients are often older and have more comorbidities which may limit comparisons with surgical cohorts. All patients had a diagnostic biopsy prior to the ablation procedure which may contribute to a higher ratio of low-grade RCC in this study, compared to surgical series. However, tumor grade is difficult to determine at biopsy due to tumor heterogeneity (33). Single-center studies can be biased by local expertise. Outcomes reported in this study may not be generalizable to other centers with less experience and/or alternate MWA technology. Longer follow-up is necessary to assess durable oncologic efficacy and cancer-specific survival.

In conclusion, percutaneous MWA for cT1b RCC is safe in elderly and comorbid patients with acceptable oncologic efficacy. Repeat ablation is well-tolerated and can improve oncologic efficacy.

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Table 1: Patient (n=74) and tumor (n=76) characteristics for the entire cohort.

Age	Years	69.5	(65-76)
BODY MASS INDEX	kg/m²	34.5	(29-41)
Sex	Female	25	(34%)
	Male	49	(66%)
Charlson comorbidity index		4	(3-5)
Renal Function	eGFR	63.5	(52-74)
	before ablation
Tumor diameter	cm	4.5	(4.3-5.1)
Renal NEPHROMETRY SCORE		8	(7-9)
*low complexity*	4-6	8	(11%)
*moderate complexity*	7-9	57	(75%)
*High complexity*	10-12	11	(14%)
Tumor location
*Anterior*	A	25	(33%)
*posterior*	P	30	(39%)
*neither*	X	21	(28%)
Laterality
*Left*		37	(49%)
*Right*		39	(51%)
Tumor histology
*Clear cell*	ccRCC	62	(82%)
*Papillary*	pRCC	10	(13%)
*chromophobe*	cRCC	4	(5%)
Tumor Grade
*Low*	1-2	60	(79%)
*High*	3-4	7	(9%)
*Unknown*		9	(12%)

Quantitative variables are summarized by median (interquartile range; IQR); Categorical variables are summarized by N (%).

Table 2: Procedural characteristics for the entire cohort (n=76).

antennas		3	(3-3)
Two		12	(16%)
three		64	(84%)
Antenna reposition	No	42	(55%)
	Yes	34	(45%)
Hydrodisplacement	No	42	(55%)
	Yes	34	(45%)
*volume*	mL	475	(300-725)
Generator Output power	W	65	(65-65)
Generator Time	Minutes	7	(5-9.5)
Intervention time	Minutes	80.5	(64-95)
Procedure Time	Minutes	100	(63-128)

Renal Function	eGFR	57	(45-77)
	after ablation
Complications	No	63	(83%)
	Yes	16	(21%)
Complication grade
*Low*	1-2	10	(13%)
*High*	3-4*	6	(8%)
Primary efficacy		71	(93%)
Imaging follow-up	Months	28.2	(15.6-51.7)
Local Tumor progression		11	(16%)
Metastatic progression		1	(1%)

Quantitative variables are summarized by median (Inter-quartile range; IQR); Categorical variables are summarized by N (%). ^ eGFR obtained 6 months post ablation. *There were no Clavien-Dindo grade 5 (death) complications.

Table 3: Patient, tumor and procedural characteristics stratified based upon antenna repositioning.

		Antenna reposition
				p-value
Tumor diameter	cm	4.6 (4.4-5.6)	4.5 (4.2-4.9)	0.01
Renal Nephrometry score		8 (7-9)	8 (7-9)	0.78
Hydodisplacement Performed	%	38	50	0.36
*volume*	mL	425	475
Generator time	minutes	10 (7.5-13)	5 (5-6.5)	<0.001
Intervention Time	minutes	100 (63-128)	80.5 (64-95)	0.04
Procedure time	minutes	124.5 (82-160)	100.5 (85-126)	0.08
Renal function	D% eGFR	-11.9 (-14.3-6.6)	-12.9 (-15.4-2.3)	0.48
Complications		9 (27%)	7 (17%)	0.18
*High Grade*		4 (12%)	2 (5%)
Primary efficacy	%	91	95	0.48
local Tumor progression	%	21	11	0.20

*p-value is based on Wilcoxon rank sum test for quantitative variables and on Fisher’s exact test for categorical variables.

Competing interest reported. Competing Interests: Authors SAW, JLH, and TJZ are consultants for Ethicon Inc. The other authors, have no relevant financial or non-financial interests to disclose.

Percutaneous microwave ablation of cT1b renal cell carcinoma: Safety and oncologic efficacy in a large, single-center elderly and comorbid cohort

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Abstract

Figures

INTRODUCTION

MATERIALS AND METHODS

Patient Selection

Procedure

Data Collection

Statistical Analysis

RESULTS

DISCUSSION

References

Tables

Additional Declarations

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